US20070165472A1 - Method and apparatus for evaluating and optimizing a signaling system - Google Patents

Abstract

A method and apparatus for evaluating and optimizing a signaling system is described. A pattern of test information is generated in a transmit circuit of the system and is transmitted to a receive circuit. A similar pattern of information is generated in the receive circuit and used as a reference. The receive circuit compares the patterns. Any differences between the patterns are observable. In one embodiment, a linear feedback shift register (LFSR) is implemented to produce patterns. An embodiment of the present disclosure may be practiced with various types of signaling systems, including those with single-ended signals and those with differential signals. An embodiment of the present disclosure may be applied to systems communicating a single bit of information on a single conductor at a given time and to systems communicating multiple bits of information on a single conductor simultaneously.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This patent application is a continuation of U.S. patent application Ser. No. 11/559,111, filed Nov. 13, 2006, which is a continuation of U.S. patent application Ser. No. 09/976,170, filed Oct. 12, 2001, now U.S. Pat. No. 7,137,048, which is a continuation-in-part of U.S. patent application Ser. No. 09/776,550, filed Feb. 2, 2001, now U.S. Pat. No. 6,873,939, each of which is hereby incorporated by reference herein in its entirety.
FIELD OF THE DISCLOSURE
• The present disclosure relates generally to communication systems and, more specifically, to in-situ testing of communications systems.
BACKGROUND OF THE DISCLOSURE
• For information communication and processing systems to operate reliably, it is important to be able to test these systems and measure various performance characteristics that pertain to them. Classically, it has been very difficult to observe the fidelity of a signaling system from a transmit circuit to a receive circuit, including a medium through which the transmit circuit is coupled to the receive circuit. It has been especially difficult to obtain in-situ measurements of the operation of the system. Rather, external test equipment is typically introduced into the system for the purpose of obtaining measurements. It is common for an external signal generator, for example one capable of producing test signals with ultrafast or adjustable transition times, and an external measurement device, such as an oscilloscope, to be connected to a system under test. However, since such external test equipment has characteristics different from system under test, measurements derived using the external test equipment may not accurately reflect the actual performance of the system under test.
• While it was possible to obtain meaningful information from simpler systems of the past using external test equipment, the increasing complexity and operating frequencies of modern systems introduce additional complications that impair the effectiveness of testing using external test equipment. For example, much higher frequencies and controlled impedances make it much harder to introduce external test equipment without distorting the signals being measured and, therefore, affecting the measurements themselves. Moreover, connection and disconnection of the external test equipment requires time, effort, and, potentially, additional design considerations, such as the provision of test points within a system. Also, external test equipment does not allow testing to be performed from the perspective of the actual receive circuit within the system. Thus, such testing cannot definitively provide information as to what the receive circuit actually receives. Therefore, traditional testing techniques fail to provide complete and accurate information about the system under test. Thus, a technique is needed to provide complete and accurate information about a system under test and to enable in-situ testing of the system.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a block diagram illustrating a signaling system in accordance with an embodiment of the present disclosure.
• FIG. 2 is a logic diagram illustrating a prior art transmit circuit.
• FIG. 3 is a logic diagram illustrating a prior art receive circuit.
• FIG. 4 is a logic diagram illustrating a transmit circuit capable of operating in a pseudo-random bit sequence (PRBS) test mode in accordance with an embodiment of the present disclosure.
• FIG. 5 is a logic diagram illustrating a receive circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.
• FIG. 6 is a logic diagram illustrating a transmit circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.
• FIG. 7 is a logic diagram illustrating a receive circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.
• FIG. 8 is a logic diagram illustrating a prior art quad signaling level transmit circuit.
• FIG. 9 is a logic diagram illustrating a quad signaling level transmit circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.
• FIG. 10 is a logic diagram illustrating a quad signaling level transmit circuit capable of operating in a PRBS mode and a roll test mode in accordance with an embodiment of the present disclosure.
• FIG. 11 is a logic diagram illustrating a prior art quad signaling level receive circuit.
• FIG. 12 is a logic diagram illustrating a quad signaling level receive circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.
• FIG. 13 is a logic diagram illustrating a quad signaling level receive circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.
• FIG. 14 is a timing diagram illustrating signals in accordance with an embodiment of the present disclosure.
• FIG. 15 is a block diagram illustrating a differential receiver that may be used in conjunction with and embodiment of the present disclosure.
• FIG. 16 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure.
• FIG. 17 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure.
• FIG. 18 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure.
• FIG. 19 is a schematic diagram illustrating an example of an input stage of an offsetable differential receiver in accordance with an embodiment of the present disclosure.
• FIG. 20 is a schematic diagram illustrating an example of an input stage of an offsetable differential receiver in accordance with an embodiment of the present disclosure.
• FIG. 21 is a two-dimensional graphical diagram illustrating a relationship between a waveform of a signal received at the receive circuit and variations occurring in the interpretation of data represented by the signal in accordance with an embodiment of the present disclosure.
• FIG. 22 is a two-dimensional graphical diagram illustrating the accumulation of locations of regions where variations are observed in accordance with an embodiment of the present disclosure.
• FIG. 23 is a two-dimensional graphical diagram illustrating the accumulation of locations of regions where variations are observed based on sampling pertaining to a differential signal in accordance with an embodiment of the present disclosure.
• FIGS. 24A and 24B are a flow diagram illustrating a method in accordance with an embodiment of the present disclosure.
• FIG. 25 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 26 is a waveform diagram illustrating an example of a display of samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 27 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 28 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 29 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 30 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure.
• FIG. 31 is a block diagram illustrating a system in accordance with an embodiment of the present disclosure.
• FIG. 32 is a flow diagram illustrating a method in accordance with an embodiment of the present disclosure.
• FIG. 33 is a waveform diagram illustrating optimization of crosstalk cancellation resulting from iterative application of the method illustrated inFIG. 32 in accordance with an embodiment of the present disclosure.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
• A method and apparatus for evaluating and optimizing a signaling system is described. Evaluation is accomplished using the same circuits actually involved in normal operation of the signaling system. Such circuits are adapted to provide testing capability with minimal additional complexity. Thus, capability for in-situ testing of a signaling system is provided, and information may be obtained from the actual perspective of a receive circuit in the system. Both the need for the introduction of external test equipment and the inaccuracy caused by its introduction are avoided. An embodiment of the present disclosure may be implemented to provide a built-in self-test (BIST) capability within an operational system. Such capability can be very beneficial, especially where access to internal components of a system would otherwise be difficult. For example, in addition to being applicable to signaling systems where the transmit circuit is located separately from the receive circuit, an embodiment of the present disclosure may be applied to a relatively closed system, for example, a system fabricated as an integrated circuit. Thus, even when an integrated circuit is packaged, extensive internal evaluation and optimization capability may be provided in accordance with the present disclosure.
• A pattern of test information is generated in a transmit circuit of the system and is transmitted to a receive circuit. A similar pattern of information is generated in the receive circuit and used as a reference. The receive circuit receives the pattern of test information generated in the transmit circuit and compares it to the pattern of information generated in the receive circuit. Any differences between the patterns are observable. Preferably, the patterns are repeating patterns that allow many iterations of testing to be performed. Thus, even events that occur infrequently within the system may be observed.
• In one embodiment, a linear feedback shift register (LFSR) is implemented to produce patterns. Information obtained from testing may be used to assess the effects of various system parameters, including but not limited to output current, crosstalk cancellation coefficients, and self-equalization coefficients, and system parameters may be adjusted to optimize system performance. Embodiments of the present disclosure may be used in assessing a wide variety of phenomena, including, but not limited to, overshoot, undershoot, edge rates, crosstalk, duty cycle error (including the cumulative duty cycle error across the transmit circuit, the medium, and the receive circuit), impedance continuity/discontinuity, and the effectiveness of different receive and transmit effects. An embodiment of the present disclosure may be practiced with various types of signaling systems, including those with single-ended signals and those with differential signals. An embodiment of the present disclosure may be applied to systems communicating a single bit of information on a single conductor at a given time and to systems communicating multiple bits of information on a single conductor simultaneously.
• Several embodiments of the present disclosure described herein are particularly useful for obtaining data expressing attributes of waveforms communicated from the transmit circuit to the receive circuit. As a brief example that is described in greater detail with reference to the drawings, the following process is performed: a transmit repeating pattern is initially transmitted to the receive circuit, one or more parameters affecting reception of the transmit repeating pattern in the receive circuit are set, the receive circuit is placed in a mode wherein the receive circuit generates a receive repeating pattern, the transmit repeating pattern is repeatedly transmitted to the receive circuit, any variation in the relationship between the transmit repeating pattern as received by the receive circuit and the receive repeating pattern is detected, the values of the parameters in effect during such variation are stored, and the process (or a portion thereof) is repeated for different values of the parameters.
• Variations detected in such a process occur when the relationship between the transmit repeating pattern as received by the receive circuit and the receive repeating pattern does not remain fixed over multiple iterations of the patterns. Since, logically, the relationship between the patterns is known to be fixed, such variations indicate failures involving uncertainty in the communication of the transmit repeating pattern to the receive circuit. Such failures can occur when the parameters affecting reception of the transmit repeating pattern in the receive circuit are set to values that border on ranges of values of the parameters that provide for accurate communication of the transmit repeating pattern to the receive circuit.
• When the parameters are set to values within the range of values that provide for accurate communication of the transmit repeating pattern to the receive circuit, accurate communication is reliably provided. Thus, no variations are detected in the relationship between the transmit repeating pattern as received by the receive circuit and the receive repeating pattern. When the parameters are set to values well outside the range of values that provide for accurate communication of the transmit repeating pattern to the receive circuit, the transmit repeating pattern is consistently misinterpreted by the receive circuit, thereby yielding a relationship between the transmit repeating pattern as received by the receive circuit and the receive repeating pattern that does not vary. Thus, no variation is detected. Then, the relationship between the transmit repeating pattern and the receive repeating pattern may be said to exhibit repeatability. However, when the parameters are set to critical values bordering the ranges of values that provide for accurate communication, the transmit repeating pattern will sometimes be accurately interpreted by the receive circuit, but, at other times, the transmit repeating pattern will be inaccurately interpreted by the receive circuit. Thus, variation will occur and will be detected. Then, the relationship between the transmit repeating pattern and the receive repeating pattern may be said to exhibit non-repeatability. Such a phenomenon can occur when parameters such as a timing margin (e.g., setup and hold times) or a voltage margin are set to values insufficient to provide reliably accurate communication. A parameter affecting the voltage margin may be set by setting a voltage reference used for the comparison of voltages of a signal representing the transmit repeating pattern being received at the receive circuit.
• By performing such testing over multiple iterations over large numbers of combinations of parameter values, the subset of those combinations where variations occur can be identified. If each parameter is represented by a dimension in an n-dimensional space (for n parameters), the subset of the combinations where variations occur can be mapped into that n-dimensional space. For example, using two parameters (e.g., timing margin and voltage margin), a two-dimensional mapping can be provided. For typical receive circuit components, variations tend to be observed across contiguous combinations of parameter values. Thus, such a two-dimensional mapping typically exhibits one or more lines or continuous curves formed from adjacent points representing combinations of parameter values yielding variations. The thickness or width of such lines or curves depends on the number of critical values of parameters for which variations are observed, especially when several values of one parameter yield variation for a fixed value of another parameter.
• For relatively thin or narrow lines or curves, the mapping approximates a trace of the transmit repeating pattern as received at the receive circuit of the sort that one having experience using an oscilloscope would perceive as familiar. Thus, embodiments of the present disclosure may be used to display an eye diagram of multiple iterations of the transmit repeating pattern, or, by altering the timing of the transmit repeating pattern and the receive repeating pattern as multiples or sub-multiples of one another, the trace can be “unfolded” to yield a display without overlap of different aspects of the waveform represented by the mapping, thereby allowing finer details of the waveform to be observed. Therefore, embodiments of the present disclosure may be used to provide an in-situ “virtual oscilloscope” capability for observing representations of signals while avoiding external influences on system behavior typically introduced when traditional test instruments are coupled to a system under test.
• FIG. 1 is a block diagram illustrating a signaling system in accordance with an embodiment of the present disclosure. The system comprises transmitcircuit101, medium102, and receivecircuit103. Transmitcircuit101 comprisesshift register105 andtest loop106. Transmitcircuit101 can operate in a normal mode or a test mode. In the normal mode,shift register105 receives data via adata input104, which may be a parallel or serial data input.Shift register105 shifts the bits of the data to the right, providing each bit as a output at the interface withmedium102.
• The data are transmitted acrossmedium102 to receivecircuit103. In addition to providing output tomedium102,shift register105 also provides its output to testloop106.Test loop106 provides aninput107 toshift register105. In the test mode,shift register105 andtest loop106 function to generate a test pattern for transmission overmedium102. The test pattern is preferably, although not necessarily a repeating pattern.Test loop106 may be a simple loop, such as a loop of wire, or it may include other combinational and/or sequential logic. For example, it may include logic gates such as AND, OR, NAND, NOR, XOR, and/or XNOR gates. Such gates may be used to implement an LFSR. An LFSR is capable of producing maximal-length repeating patterns with a minimum of additional complexity. The LFSR can produce pseudo-random bit sequences (PRBS), which may be beneficially applied to testing the system under a wide variety of logical conditions. When an LFSR is implemented,test loop106 serves as a transmit linear feedback logic gate andshift register105 serves as a transmit shift register. Coupling119, which couplesshift register105,test loop106, and, optionally,transmitter115 ormedium102, serves as transmit shift register output. Coupling107 serves as a transmit shift register input when the transmit circuit is operating in a test mode. When an LFSR is implemented, additional transmit shift register outputs can be provided fromshift register105 to additional transmit linear feedback logic inputs oftest loop106.
• Receivecircuit103 includesshift register108 andtest loop111. In the normal mode, receivecircuit103 receives data from the transmitcircuit101 viamedium102, which may, as one example, be an electrical conductor coupling the transmitcircuit101 to the receivecircuit103. The bits of the data are shifted throughshift register108 and provided atdata output109. When an LFSR is implemented,test loop111 serves as a receive linear feedback logic gate andshift register108 serves as a receive shift register. Coupling110, which couplesshift register108 to testloop111 serves as receive shift register output. Coupling112 serves as a receive shift register input when the receive circuit is operating in a test mode. When an LFSR is implemented, additional receive shift register outputs can be provided fromshift register108 to additional receive linear feedback logic inputs oftest loop111.
• As appropriate, the transmitcircuit101 may include atransmitter115 coupled tomedium102 and the receivecircuit103 may include areceiver116 coupled to the medium102. In that case, an output ofshift register105 is coupled to an input oftransmitter115 and to an input oftest loop106. The output oftransmitter115 is coupled tomedium102.Medium102 is coupled to an input ofreceiver116. A receive clock signal is provided atinput117 ofreceiver116. A voltage reference signal is provided atinput118 ofreceiver116. Both the receive clock signal atinput117 and the voltage reference signal may be varied over a wide range to allow testing of the system under a wide variety of conditions.
• Transmitter115 may be any suitable transmitter. An example includes, but is not limited to, a driver circuit for driving signals ontomedium102. The driver circuit may provide desirable characteristics, for example, a controlled source impedance and well-defined transition times.Transmitter115 may provide a single-ended signal or a differential signal overmedium102.Transmitter115 may be able to communicate one or more bits of information over a single conductor simultaneously.
• Likewise,receiver116 may be any suitable receiver. An example includes, but is not limited to, a receiver for determining digital signaling levels of signals present onmedium102.Receiver116 may provide desirable characteristics, for example, one or more voltage or current level thresholds, controlled hysteresis, and controlled timing.Receiver116 may be configured to receive a single-ended signal or a differential signal frommedium102.Receiver116 may be able to receive one or more bits of information over a single conductor simultaneously.
• While subsequent Figures, for example,FIGS. 4, 6,9, and10 are illustrated without a transmitter such astransmitter115, the embodiments of these Figures may be implemented with such a transmitter. While subsequent Figures, for example,FIGS. 5, 7,12, and13 are illustrated without a receiver such asreceiver116, the embodiments of these Figures may be implemented with such a receiver.
• Shift register108 is coupled totest loop111 viacoupling110. The output of thetest loop111 is coupled back to an input ofshift register108 and also to an input ofcomparison circuit113, which may, as one example, be implemented using an XOR logic gate. Another input of thecomparison circuit113 is coupled tomedium102 and receives the test pattern transmitted by transmitcircuit101 overmedium102.Comparison circuit113 provides acomparison output114. In the test mode,shift register108 andtest loop111 operate to produce a pattern identical to or deterministically related to the pattern produced by transmitcircuit101.Comparison circuit113 compares the pattern generated in the transmitcircuit101 to the pattern generated in the receivecircuit103 to determine for each bit whether the patterns match. To establish a relationship between the pattern produced by the transmitcircuit101 and the pattern produced by the receivecircuit103, information to synchronize the pattern of the receivecircuit103 with the pattern of the transmitcircuit101 may be communicated from the transmitcircuit101 to the receivecircuit103, or elements of the receive circuit may be preloaded with such information.
• The system may be operated in roll test mode. In the roll test mode, the patterns need not necessarily match, as long as they are repeatable. In the roll test mode, the system operates as a “repeatability detector.” The roll test mode generates repeating patterns, and, upon comparison, any variation from the repeating patterns is detected. Thus, the pattern generated by the transmit circuit and the pattern generated by the receive circuit need not be the same. In fact, the receive circuit is not preloaded with information to synchronize its pattern with that of the transmit circuit. Rather, the receive circuit is seeded by the transmit circuit.
• In the roll test mode, the timing ofreceiver116 may be adjusted, for example, swept over a range, by varying the receive clock signal atnode117, and the symbol thresholds ofreceiver116 may be adjusted, for example, swept over a range, by varying the voltage reference signal atnode118. While the voltage reference signal is stated in terms of voltage, it may be implemented to allow adjustment of other electrical parameters relating to reception of signals atreceiver116.
• Shift registers with which embodiments of the present disclosure may be implemented, for example,shift registers105 and108, preferably comprise a plurality of registers coupled to one another. As an example, such registers are preferably implemented using D flip flops with the Q output of one D flip flop coupled to the D input of a subsequent D flip flop. The D flip flops of a shift register are preferably clocked in unison with one another.
• Several signaling systems such as that illustrated inFIG. 1 may be used together. For example, a transmit circuit of a first signaling system may be used to produce a pattern, and a receive circuit of a second signaling system may be used to observe any influence of the pattern of the first signaling system on the second signaling system. Such a technique is useful, for example, to observe crosstalk that may occur between the signaling systems. Since crosstalk can occur without an intentional connection between the signaling systems, no such connection needs to be made between the signaling systems, and the media of the signaling systems may be electrically insulated from one another.
• In the embodiment illustrated inFIG. 1,shift register105 andtest loop106 form a transmit repeating pattern generator capable of producing a repeating pattern signal.Data input104 serves as a transmit data input, andmedium102 provides a transmit data output for the transmitcircuit101 and a receive data input for the receivecircuit103. The transmitcircuit101 produces a transmit data output signal at the transmit data output based on a transmit data input signal obtained from the transmit data input when the transmit circuit is operating in a normal mode. When the transmitcircuit101 is operating in a test mode, the transmitcircuit101 produces a transmit data output signal at the transmit data output based on repeating pattern signal.
• The receivecircuit103 is operably coupled to the transmitcircuit101 viamedium102. The receivecircuit103 receives the transmit data output signal from the transmit circuit at the receive data input. Theshift register108 and thetest loop111 form a receive repeating pattern generator capable of producing a receive repeating pattern, which may or may not be the same pattern as the repeating pattern signal produced by the transmit circuit.Data output109 serves as a receive data output. When the receivecircuit103 is operating in the normal mode, the receivecircuit103 produces a receive data output signal at the receive data output based on the transmit data output signal. When the receivecircuit103 is operating in the test mode, the receivecircuit103 produces a comparison signal based on comparison dependent on the transmit data output signal and the receive repeating pattern signal.
• At this point it should be noted that the transmit repeating pattern may be received in a test receiver (not shown) separate from the receivecircuit103 when the transmitcircuit101 is operating in a test mode. Also, the transmit repeating pattern may be transmitted from a test transmitter (not shown) separate from the transmitcircuit101 when the receivecircuit103 is operating in a test mode.
• Theshift register105 of the transmitcircuit101 may be thought of as a transmit data storage element. Alternatively, the transmit data storage element may be implemented using another structure capable of storing data and allowing the sequential transmission of the data, for example, when the transmitcircuit101 is operated in a normal mode. In the test mode, the transmit data storage element is capable of providing a repeating pattern signal, wherein the transmitcircuit101 sequentially transmits the transmit data output signal based on the repeating pattern signal. The repeating pattern signal may represent a sequence of data bits, with the transmit data storage element storing each of the data bits, or the repeating pattern signal may have a data length greater than the data capacity of the transmit data storage element. For example, a LFSR may be used to produce a repeating pattern signal having a data length (e.g., a number of bits produced as the repeating pattern signal before the repeating pattern signal begins repeating) much greater than the data capacity of the transmit data storage element (e.g., the number of bits that can be stored in the transmit data storage element).
• The transmit data storage element may be divided into transmit data storage sub-elements during operation in the normal mode. For example, odd-numbered bits may be handled by one sub-element (e.g., one pipeline structure), while even-numbered bits may be handled by another sub-element (e.g., another pipeline structure). Thus, distinct data are passed through each of the plurality of sub-elements when the transmit circuit is operating in the normal mode. Since such sub-elements may be rather short, they might not yield a repeating pattern signal having a sufficiently high level of desired entropy in the test mode. Thus, the transmit data storage sub-elements may be united into a single unit in the form of the transmit data storage element for providing the repeating pattern signal when the transmit circuit is operating in the test mode. Thus, a much “richer” repeating pattern signal exhibiting substantially higher entropy may be provided. Alternatively, if higher entropy is not needed, the longer data length of the repeating pattern signal made possible by such union may be used to specify a longer specific bit sequence for the repeating pattern signal.
• The data storage element may be loaded from the transmit data input to initialize the test mode from a source other than the transmit data input, for example, the transmit data storage element may be loaded via a parallel transmit load input.
• In the receivecircuit103, theshift register108 may be thought of as a receive data storage element. Alternatively, the receive data storage element may be implemented using another structure capable of storing information relating to either a receive data input signal or a repeating pattern signal. In the normal mode, the receive data storage element outputs a receive data output signal based on a receive data input signal received at the receive data input. In the test mode, the receive data storage element provides the repeating pattern signal. The repeating pattern signal may represent a sequence of data bits, with the receive data storage element storing each of the data bits, or the repeating pattern signal may have a data length greater than the data capacity of the receive data storage element. For example, a LFSR may be used to produce a repeating pattern signal having a data length (e.g., a number of bits produced as the repeating pattern signal before the repeating pattern signal begins repeating) much greater than the data capacity of the receive data storage element (e.g., the number of bits that can be stored in the receive data storage element).
• Thecomparison circuit113 serves as a comparison element, performing a comparison of a relationship between the repeating pattern signal and the receive data input signal received at the receive data input to produce a comparison output signal based on the comparison when the receivecircuit103 is operating in the test mode.
• As with the transmit data storage element, the receive data storage element may be divided into receive data storage sub-elements (e.g., pipeline structures) during operation in the normal mode. Thus, distinct data are passed through each of the plurality of sub-elements when the receive circuit is operating in the normal mode. The receive data storage sub-elements may be united into a single unit in the form of the receive data storage element for providing the repeating pattern signal when the receive circuit is operating in the test mode.
• The receive data storage element may be loaded from the receive data input to initialize the test mode from a source other than the receive data input, for example, the receive data storage element may be loaded via a parallel receive load input.
• An embodiment of the present disclosure may be implemented in a manner so as not to be incompatible with an existing transmit circuit, for example, the transmit circuit ofFIG. 2, and/or an existing receive circuit, for example, the receive circuit ofFIG. 3. Such implementation of an embodiment of the present disclosure can be used to overcome the disadvantages of the existing circuits.
• FIG. 14 is a timing diagram illustrating signals in accordance with an embodiment of the present disclosure. A transmitcircuit load signal1401, a transmit circuittest mode signal1402, a receive circuittest mode signal1405, and acomparison output signal1406 are illustrated. A plurality of transmitcircuit data1403 and a plurality of receivecircuit data1404 are also illustrated.
• A PRBS test mode is entered upon theassertion1409 of the transmit circuittest mode signal1402. Data are loaded into a shift register in the transmit circuit on the risingedge1407 ofpulse1408 of the transmitcircuit load signal1401. Sufficient transmitcircuit data1403 to initialize the transmit circuit and the receive circuit to like states is communicated from the transmit circuit to the receive circuit, where it appears as receivecircuit data1404.Bits1413,1414,1415,1416,1417,1418, and1419 of transmitcircuit data1403 are communicated to providebits1420,1421,1422,1423,1424,1425, and1426 of receivecircuit data1404 respectively. These data are communicated betweentime1411 andtime1412, duringperiod1410, and serves to seed the receive circuit with appropriate data.
• Once sufficient data has been communicated between the transmit circuit and the receive circuit, receive circuittest mode signal1405 is asserted atassertion1427. Then, betweentime1429 andtime1430, duringperiod1428, testing may be performed using the seeded receive circuit data. If, during the testing, an element of transmit circuit data being transmitted to the receive circuit does not match a corresponding element of receive circuit data,comparison output signal1406 is asserted, such as occurs atassertions1433 and1434, which occur duringperiod1432 aftertime1431. When testing is completed, the system may be returned to its normal mode by deasserting the transmit circuittest mode signal1402 and the receive circuittest mode signal1405.
• In a transmit circuit capable of a PRBS test mode, a PRBS test signal of the transmit circuit may be asserted to serve as the transmit circuittest mode signal1402. In a transmit circuit capable of a roll test mode, a roll test signal of the transmit circuit may be asserted to serve as the transmit circuittest mode signal1402. In a receive circuit capable of a PRBS test mode, a PRBS test signal of the receive circuit may be asserted to serve as the receive circuittest mode signal1405. In a receive circuit capable of a roll test mode, a roll test signal of the receive circuit may be asserted to serve as the receive circuittest mode signal1405.
• FIG. 15 is a block diagram illustrating a differential receiver that may be used in conjunction with an embodiment of the present disclosure.Differential receiver1501 comprises anon-inverting input1502, an invertinginput1503, an offsetinput1504, and anoutput1505. Offsetinput1504 may be implemented in various ways, for example, as a single-ended input or a differential input.
• FIG. 16 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure. The differential signal comprises asignal1601 and itscomplementary signal1602.
• FIG. 17 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure. The differential signal comprises asignal1701 and itscomplementary signal1702. Thesignal1701 and itscomplementary signal1702 have been shifted slightly relative to each other, for example through the use of offsetinput1504 ofFIG. 15.
• FIG. 18 is a waveform diagram illustrating a differential signal that may be used in conjunction with an embodiment of the present disclosure. The differential signal comprises asignal1801 and itscomplementary signal1802. Thesignal1801 and itscomplementary signal1802 have been shifted substantially relative to each other, for example through the use of offsetinput1504 ofFIG. 15.
• FIG. 19 is a schematic diagram illustrating an example of an input stage of an offsetable differential receiver in accordance with an embodiment of the present disclosure. A differential input signal is coupled to aninput1901 at a gate of afirst input transistor1903 and to aninput1902 at a gate of asecond input transistor1904. A source of thefirst input transistor1903 and a source of thesecond input transistor1904 are coupled to afirst terminal1911 ofcurrent source1912. Asecond terminal1913 ofcurrent source1912 is coupled to ground.
• A drain of thefirst input transistor1903 is coupled to the drain oftransistor1917, to a first terminal ofresistor1907, and to a first output. A drain of thesecond input transistor1904 is coupled to the drain oftransistor1918, to a first terminal ofresistor1908, and to a second output. The second end of the first resistor is coupled to avoltage reference1905. The second end of the second resistor is coupled to avoltage reference1906. The sources oftransistors1917 and1918 are coupled to afirst terminal1919 of a variablecurrent source1920. Thesecond terminal1921 of variablecurrent source1920 is coupled to ground.
• A shiftinginput signal1914 is applied to the gate oftransistor1917. The shiftinginput signal1914 is inverted byinverter1915 and applied to thegate terminal1916 oftransistor1918.
• FIG. 20 is a schematic diagram illustrating an example of an input stage of an offsetable differential receiver in accordance with an embodiment of the present disclosure. A differential input signal is coupled to aninput2001 at a gate of afirst input transistor2003 and to aninput2002 at a gate of asecond input transistor2004. A source of thefirst input transistor2003 and a source of thesecond input transistor2004 are coupled to afirst terminal2011 ofcurrent source2012. Asecond terminal2013 ofcurrent source2012 is coupled to ground.
• A drain of thefirst input transistor2003 is coupled to the drain oftransistor2017, to a first terminal ofresistor2007, and to a first output. A drain of thesecond input transistor2004 is coupled to the drain oftransistor2018, to a first terminal ofresistor2008, and to a second output. The second end of the first resistor is coupled to avoltage reference2005. The second end of the second resistor is coupled to avoltage reference2006. The sources oftransistors2017 and2018 are coupled to afirst terminal2019 of acurrent source2020. Thesecond terminal2021 ofcurrent source2020 is coupled to ground. A shiftinginput signal2014 is applied to the gate oftransistor2017. A fixedDC voltage2016 is applied to the gate terminal oftransistor2018. Alternatively, a variable signal, such as a variable voltage, may be applied to the gate terminal oftransistor2018. As one example, the variable signal at the gate terminal oftransistor2018 may vary complementary to the shiftinginput signal2014. This would allow, for example, a differential signal to be used to control the offset. Alternatively, the signal at the gate terminal oftransistor2018 may vary independent of the shiftinginput signal2014.
• FIG. 21 is a two-dimensional graphical diagram illustrating a relationship between a waveform of a signal received at the receive circuit and variations occurring in the interpretation of data represented by the signal in accordance with an embodiment of the present disclosure.Waveform2101 plots a portion of the signal received at the receive circuit against atime axis2102 and avoltage axis2103. If the parameters of the system are adjusted so that sampling occurs attime2122 with a sampling time requirementΔT spanning range2104 with a voltage threshold set atvoltage reference2123 with a voltage overdrive requirement ofΔV spanning range2105, then ifwaveform2101 passes through regions2106-2109, repeatability of data extracted from the signal within those regions is not guaranteed, so variations can be expected to occur between iterations ofwaveform2101 passing through these regions. However, ifwaveform2101 passes through regions2110-2121, accurate data can be reliably extracted fromwaveform2101. Thus, repeatability of the data occurs within these regions, and variations are not detected within these regions.
• By observing the variations occurring within regions2106-2109, information representative of the locations of regions2106-2109 can be stored. Then, the positions oftime2122 andvoltage reference2123 within the plane formed bytime axis2102 andvoltage axis2103 are adjusted, and variations in extracted data are observed for samples taken within the adjusted ranges2104 and2105 corresponding to the adjusted positions oftime2122 andvoltage reference2123. Information representative of locations of regions where variation is observed for these adjusted positions is then stored. By cumulatively storing this information over several iterations of this process, a representation ofwaveform2101 can be displayed based on the information. To make the representation of waveform2101 a closer approximation of theactual waveform2101, interpolation between the locations where variations are observed can be performed. Such interpolation can be explicitly performed or allowed to occur during visualization of the representation ofwaveform2101, utilizing the same visual effects that allow images of discrete elements, such as dot matrix displays or bit-mapped images, to appear as though the discrete elements are merged into a larger element.
• When ranges2104 and2105 are very small, variations may be observed in only a single region for each iteration of positions oftime2122 andvoltage reference2123. In that case, the accumulation of locations of regions where variations are observed yields a thin and precise representation ofwaveform2101. However, when ranges2104 and2105 are larger, variations may be observed over the several regions lying within theranges2104 and2105. Thus, a thicker representation ofwaveform2101 results, allowing the broader area of the plane over which variations in extracted data occur to be observed.
• FIG. 22 is a two-dimensional graphical diagram illustrating the accumulation of locations of regions where variations are observed in accordance with an embodiment of the present disclosure. As an example,waveform2201 is illustrated as a triangle wave, but it should be understood thatwaveform2201 may be of any arbitrary shape.Waveform2202 is illustrated as being sampled according to parameters that affect the extraction of data fromwaveform2201. In the example at the top ofFIG. 22,waveform2201 is illustrated as being sampled according to avoltage reference2202 andtimes2206,2207, and2208, yieldingpoints2218,2219, and2220, respectively, representative of the locations of regions where variations in extracted data occur, as discussed above in detail with reference toFIG. 21. It should be understood that sampling can also occur at any other times alongwaveform2201, but that the lack of variation of data of such samples allows those samples to be disregarded when it is desired to identify locations corresponding to regions where variations in the data occur. Also, by samplingwaveform2202 according to a DC voltage reference, awaveform2201 derived from a single-ended signal (e.g., a signal that may be communicated over a single conductor with reference to a reference potential, such as ground) may be sampled. However, it should be noted that embodiments of the present disclosure may be used for waveforms other than those derived from a single-ended signal, for example, those derived from a differential signal, as described below with reference toFIG. 23.
• In the next example ofFIG. 22,waveform2201 is illustrated as being sampled according to avoltage reference2203 andtimes2209,2210, and2211, yieldingpoints2221,2222, and2223, respectively, representative of the locations of regions where variations in extracted data occur for these adjusted parameter values. In the next example,waveform2201 is illustrated as being sampled according to avoltage reference2204 andtimes2212,2213,2214, yieldingpoints2224,2225, and2226, respectively. In the next example,waveform2201 is illustrated as being sampled according to avoltage reference2205 andtimes2215,2216, and2217, yieldingpoints2227,2228, and2229.
• By accumulating points2218-2229 and plotting them according to the voltage references and times pertaining to their respective sampling, anapproximation2230 ofwaveform2201 can be obtained, as illustrated near the bottom ofFIG. 22. Continuity of theapproximation2230 ofwaveform2201 can be obtained by interpolating between points2218-2229, as illustrated by the dashed line ofapproximation2230, or by allowing such continuity to be perceived when the plotted accumulation of points2218-2229 is visualized.
• FIG. 23 is a two-dimensional graphical diagram illustrating the accumulation of locations of regions where variations are observed based on sampling pertaining to a differential signal in accordance with an embodiment of the present disclosure. A differential signal involves a complimentary relationship between a first signal on a first conductor to a second signal on a second conductor. As withFIG. 22, an exemplary triangle waveform is illustrated inFIG. 23, but it should be understood that the technique described with reference toFIG. 23 may be applied to any arbitrary waveform.
• In the example illustrated at the top ofFIG. 23, the first signal is represented bywaveform2301. The second signal is represented bywaveform2304. In this example,waveform2304 is used as a reference to whichwaveform2301 is compared. Bysampling waveform2301 relative towaveform2304 attimes2310,2313, and2316 in accordance with the technique described above with reference toFIG. 21, points2319,2322, and2325, respectively, are identified. By shiftingwaveform2304 by an offset2347, as illustrated bywaveform2305, andsampling waveform2301 attimes2311,2312, and2317, points2320,2321, and2326, respectively, are identified. By shiftingwaveform2304 by an offset2348, as illustrated bywaveform2303, andsampling waveform2301 attimes2309,2314, and2315, points2318,2323, and2324, respectively, are identified.
• In addition to identifying points2318-2326 by samplingwaveform2301 relative to waveforms2303-2305, another set of points2327-2335 can be identified bysampling waveform2302 usingwaveform2307 as a reference. In this case, the first signal is represented bywaveform2307, and the second signal is represented bywaveform2302. Bysampling waveform2302 relative towaveform2307 attimes2310,2313, and2316, points2328,2331, and2234, respectively, are identified. By shiftingwaveform2307 by an offset2349, as illustrated bywaveform2306, andsampling waveform2302 attimes2309,2314, and2315, points2327,2332, and2333, respectively, are identified. By shiftingwaveform2307 by an offset2350, as illustrated bywaveform2308, andsampling waveform2302 attimes2311,2312, and2317, points2329,2330, and2335, respectively, are identified.
• Thus, according to the times and offset shifts (or lack thereof) used for identifying points2318-2335, points2318-2335 can be plotted to yieldapproximations2336 and2337 of the waveforms corresponding to the first signal and the second signal, respectively, as illustrated inFIG. 23. To obtain an approximation of the differential waveform corresponding to the differential signal, theapproximation2337 corresponding to the second signal is subtracted from theapproximation2336 corresponding to the first signal, thereby yieldingapproximation2351 corresponding to the differential signal. As can be seen, points2338-2346 result from subtracting the locations of points2327-2335 from the locations of points2318-2326. Depending on the resolution of points2338-2346, the apparent continuity ofapproximation2351 may be obtained by interpolating between points2338-2346 or by allowing such continuity to be perceived when the plotted accumulation of points2338-2346 is visualized.
• FIGS. 24A and 24B are a flow diagram illustrating a method in accordance with an embodiment of the present disclosure. The method begins instep2401. Instep2402, one or more parameters affecting reception of a transmit repeating pattern at a receive circuit are set. For example, a timing parameter, such as a timing parameter of a transmit circuit and/or a timing parameter of the receive circuit, and/or an amplitude parameter, such as a voltage parameter influencing voltages of logic levels for transmitting a transmit repeating pattern to the receive circuit and/or a voltage parameter influencing the ability to distinguish such logic levels in the receive circuit, may be set. Instep2403, the transmit repeating pattern is generated in a transmit circuit.Step2403 may includestep2404. Instep2404, a shift register may be utilized to generate the transmit repeating pattern.Step2404 may includestep2405. Instep2405, a linear feedback shift register (LFSR) may be utilized to generate the transmit repeating pattern.
• Fromstep2403, the method continues to step2406. Instep2406, a transmit repeating pattern is transmitted to the receive circuit.Step2406 may includesteps2407,2408, and/or2409. Instep2407, the transmit repeating pattern is transmitted as a signal referenced to a ground. Instep2408, the transmit repeating pattern is transmitted as a differential signal over a pair of conductors. Instep2409, the transmit repeating pattern is transmitted by encoding two bits of information on a single conductor simultaneously. Fromstep2406, the method continues to step2410. Instep2410, a receive repeating pattern is generated in the receive circuit. Fromstep2410, the method continues to step2411. Instep2411, the transmit repeating pattern is compared to the receive repeating pattern to obtain a comparison. Fromstep2411, the method continues to step2412. Instep2412, one or more of the one or more parameters affecting reception of the transmit repeating pattern at the receive circuit are adjusted. For example, parameters affecting the relative position of the transmit repeating pattern with respect to a voltage reference or timing reference of the receive circuit may be adjusted. Such adjustments may be made in the transmit circuit, the receive circuit, or both. Fromstep2412, the method continues to step2413, where a decision is made as to whether or not the process is done. If the process is done, the method continues to step2414, where it ends. If the process is not done, the method returns to step2403 and performs one or more additional iterations, allowing the effects of adjustments performed instep2412 to be assessed and further optimization to occur. Such iterations may continue to occur without limit. As an example, the iterations may continue with the adjustments performed over one or more ranges of the one or more parameters to assess characteristics over the one or more ranges of the one or more parameters. For example, by adjusting a voltage and/or timing offset parameter of the receive circuit, information can be obtained in the reiteration ofstep2411. Such information can be expressed in a coordinate system, such as a Cartesian coordinate system, and used to plot a waveform representing the transmit repeating pattern as received at the receive circuit, for example, to provide in-situ “virtual oscilloscope” capability. A representation of a waveform may be constructed based on the comparison performed instep2411.
• While a transmit clock for a transmit circuit and a receive clock for a receive circuit may operate at the same frequency (or approximately the same frequency), the transmit clock and the receive clock may be set to operate at frequencies that are multiple or submultiples of one another (or at frequencies that approximate such frequencies). If the frequencies are equal, an eye diagram such as that illustrated inFIG. 25 can result. The eye diagram results from the effective “folding” of the representation of the waveform, which results in overlapping display of samples obtained from different cycles of the waveform. While such overlapping is useful in some circumstances, allowing observation of changes in the waveform between different cycles, such overlapping can sometimes obscure details of the representation of the waveform that are meaningful.
• It is possible to effectively “unfold” the representation of the waveform so as to allow observation of the waveform (or a portion thereof) in detail. Such “unfolding” may be achieved by controlling the frequency relationship between the transmit clock and the receive clock. For example, by setting the receive clock to operate slower than the transmit clock, the representation of the waveform may be displayed in greater detail.
• Evaluation of a signaling system using repeating patterns may be performed over one medium, for example, one conductor, with the results of the evaluation used to adjust one or more parameters affecting communication over that one medium. Alternatively, if several media, for example, several conductors, may be characterized as providing similar performance and are similarly affected by changes to the parameters that relate to them, for example, in a closely-coupled bus system, evaluation may be performed on one medium, with the results of the evaluation applied to the adjustment of one or more parameters affecting some or all of the several media.
• As an example, an embodiment of the present disclosure may be applied to a memory device or a memory system. Media such as a data line, an address line, and/or a control line may be evaluated. Based on the results of such evaluation, parameters affecting that data line, address line, and/or control line and/or others similar to them may be adjusted. Thus, a common medium may be used for evaluation and subject to effects of parameter adjustment. Alternatively, a medium may be subject to effects of parameter adjustment based on evaluation involving another medium, namely an analysis medium. Thus, for example, a parameter may be adjusted that affects reception of a second receive data input signal, the second receive data input signal being distinct from a receive data input signal, while that parameter may or may not affect reception of the receive data input signal.
• As one example of an embodiment of the present disclosure, multiple evaluations may be performed for one transmit circuit coupled to multiple receive circuits, potentially yielding multiple adjustments of one or more parameters. Likewise, multiple evaluations may be performed for one receive circuit coupled to multiple transmit circuit, potentially yielding multiple adjustments of one or more parameters. As an example, in a memory system comprising multiple memory devices, a memory controller may perform separate evaluations for some or all of the multiple memory devices and use separate parameters to optimize communication with the multiple memory devices.
• Evaluation of a signaling system may be performed at many different times. For example, evaluation may performed during a manufacturing process, at system start-up, when a communication failure is detected, or during normal operation of a signaling system. Evaluation may be performed occasionally between periods of communication of user data between the transmit circuit and the receive circuit. As an example of evaluation at system start-up, evaluation may be performed before the system is operating normally and ready to communicate user data.
• While the transmit circuit and the receive circuit may be contained within a signaling system being evaluated, either the transmit circuit or the receive circuit may be provided externally. For example, a transmit circuit may be evaluated using an external receive circuit, or a receive circuit may be evaluated using an external transmit circuit. In one example, such external circuits may be provided in a manufacturing environment to evaluate signaling systems during their manufacturing process.
• FIG. 25 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure. In this diagram, the transmit clock is operated at a slower frequency than the receive clock (in this example, one fourth the receive clock frequency). In this example, a transmit repeating pattern of1111000011110000 is used. As can be seen, multiple cycles overlap, resulting inedges2501 and2502, which represent rising edges for some cycles and falling edges for other cycles. As can be seen fromlevels2503 and2504, for some cycles, no transition occurred at the time corresponding to edge2501. As can be seen fromlevels2506 and2507, for some cycles, no transition occurred at the time corresponding to edge2502. While such information is useful under some circumstances, it can obscure desired details under other circumstances.
• FIG. 26 is a waveform diagram illustrating an example of a display of samples of a waveform in accordance with an embodiment of the present disclosure. In this diagram, the transmit clock is operated at a higher frequency than the receive clock (in this example, four times the receive clock frequency). In this example, a transmit repeating pattern of1111000011110000 is used. As can be seen, greater detail of the waveform can be observed, as risingedge2601 is clearly visible, not overlapping with a falling edge, and fallingedge2603 is clearly visible, not overlapping with a rising edge.High level2602 is clearly visible, not overlapping with cycles lacking risingedge2601. Consequently, ringingeffect2604 can be observed in detail. Likewise, ringingeffect2605 can be observed in detail.
• A display of samples of a waveform such as that illustrated inFIG. 26 is useful for observing the step response of a signaling system. The step response is a characterization of how a system is affected by a signal having a rapid transition from one level to another. In the example ofFIG. 26, a rapid transition from a low level to ahigh level2602 occurs at risingedge2601. The ringingeffect2604 is associated with the step response of the system. Thus, to observe a step response of a system, a signal having a rapid transition from one level to another may be applied to the system and the resulting system behavior observed. Embodiments of the present disclosure may be used to observe such system behavior. Awareness of the step response of a system can be readily used for system optimization.
• FIG. 27 is a waveform diagram illustrating an example of a display of samples of a waveform in accordance with an embodiment of the present disclosure. This example may also be applied to determination of a step response. In this diagram, the transmit clock is operated at a higher frequency than the receive clock (in this example, four times the receive clock frequency). In this example, a simplified transmit repeating pattern of111111110000000 is used. As can be seen, reducing the receive clock frequency can be used to allow display of the waveform over multiple symbol times. In this example, risingedge2701 leads tohigh level2702, allowing detailed observation of ringingeffect2703. Also, fallingedge2704 leads tolow level2705, allowing detailed observation of ringingeffect2706. While risingedge2701 overlaps with fallingedge2704, the greatly expanded detail with which the waveform is displayed due to combinations of a simplified pattern and clocking at a lower frequency avoids the problems of the risingedge2701 and the fallingedge2704 obscuring one another.
• In accordance with an embodiment of the present disclosure, an iterative process may be applied to adjust system parameters so as to minimize theringing effects2703 and2706 illustrated inFIG. 27. By reducing the frequency of the receive circuit clock relative to the transmit circuit clock, multiple symbols may be observed. Ringingpatterns2703 and2706 may be observed regardless of whether they are one symbol or multiple symbols away from a causative transition. For example, in a transmission line environment, a transition may cause a disturbance, such as theringing pattern2703 or2706, that propagates along a transmission line until it is reflected at some point along the transmission line and is observed at some later time relative to the time at which the causative transition occurred. By properly adjusting the timing relationships affecting the portion of the signal being observed, such disturbances can be observed regardless of their temporal position relative to the causative transition.
• FIG. 28 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure. One embodiment of the present disclosure may be used to provide information similar to that which is traditionally obtainable through the use of an oscilloscope. In the test mode, comparison can be made between a pattern generated in the transmit circuit and a pattern generated in the receive circuit. This testing can be reiterated for different receiver timing and overdrive conditions while the signal representing the pattern is being compared. When the time and overdrive condition of the receiver is varied to the region where the pattern comparisons yield inconsistent results, the failing region corresponds to a metastable region of the receiver. When these metastable regions are plotted, they outline the signal waveform that is being received at the receiver, including the signal uncertainty (jitter) and the receiver timing and overdrive deadband requirement.
• Hence, the outline of these metastable regions represent the signal as seen by the receiver with its own receiving characteristics. With this capability of visualizing the signal, various effects on the signal can be checked out. For example, an output current level, crosstalk, attenuation, etc. In case of unwanted signal integrity behaviors, different compensation techniques can be used to reduce or eliminate those behaviors. The term “metastable” as used herein refers to a region wherein the receiver is unable to reliably identify a level of an incoming signal. Thus, for the same level of the incoming signal, the receiver will, on different occasions, identify that level to be different levels. Thus, for repetitions of a given incoming signal, the receiver will not provide a repeatable output within the metastable region. Thus, the metastable region may also be referred to as a region of unrepeatability. Consequently, to identify such a region, the transmit circuit and the receive circuit may be configured to operate using patterns having lengths or periods that bear a multiple and submultiple relationship to each other, and any lack of repeatability of the receiver may be observed. As an example, the length or period of a transmit repeating pattern may be a multiple of the length or period of a receive repeating pattern, and the length or period of the receive repeating pattern may be a submultiple of the length or period of the transmit repeating pattern. As another example, the length or period of a receive repeating pattern may be a multiple of the length or period of a transmit repeating pattern, and the length or period of the transmit repeating pattern may be a submultiple of the length or period of the receive repeating pattern. A transmit clock rate of a transmit repeating pattern and a receive clock rate of a receive repeating pattern may bear a multiple and submultiple relationship to one another. One subset of all possible multiple and submultiple relationships is a one-to-one relationship.
• The example ofFIG. 28 illustrates samples of waveforms exhibiting the effects of crosstalk induced by other nearby conductors, as can be seen, for example atlocations2801,2802,2805, and2804.
• FIG. 29 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure. The example ofFIG. 29 shows results obtained using “step response” types of waveforms.Locations2901 and2902 illustrate effects of an impedance discontinuity in the medium between the transmit circuit and the receive circuit.
• FIG. 30 is a waveform diagram illustrating an example of a persistent display of overlapping samples of a waveform in accordance with an embodiment of the present disclosure. The example ofFIG. 30 illustrated a 4-level pulse amplitude modulation (4-PAM) generated using patterns representing a stair-step-type signal. Differences between the risingedges3001,3002,3003,3004,3005, and3006 and their respective fallingedges3007,3008,3009,3010,3011, and3012 are observable.
• Thus, in view ofFIGS. 25-30, it can be seen that an embodiment of the present disclosure provides a powerful tool for in-situ characterization and optimization of signaling systems. Characteristics that could not be determined using traditional test equipment are readily ascertainable in accordance with an embodiment of the present disclosure.
• FIG. 31 is a block diagram illustrating a system in accordance with an embodiment of the present disclosure. The system comprises transmitcircuit3101, medium3102, and receivecircuit3103. Transmitcircuit3101 comprises ashift register3105 having aparallel load input3104 to load data fromregister3121. A transmit circuit load signal coupled to an input ofregister3121 atnode3122 is used to control the loading of data. Afeedback loop3106 couples a serial data output ofshift register3105 to a serial data input ofshift register3105. The serial data output ofshift register3105 is also coupled to an input oftransmitter3115. An output oftransmitter3115 is coupled to medium3102.
• Receiver circuit3103 comprisesreceiver3116,multiplexer3119,shift register3108, andXOR gate3113.Medium3102 is coupled to an input ofreceiver3116. A receive circuit timing signal is coupled to an input ofreceiver3116 atnode3117. A voltage reference signal is coupled to an input ofreceiver3116 atnode3118. An output ofreceiver3116 is coupled to an input ofmultiplexer3119 and to an input ofXOR gate3113. A fill pipe signal is coupled to a selection input ofmultiplexer3119 atnode3120. An output ofmultiplexer3119 is coupled to a serial data input ofshift register3108. A serial data output ofshift register3108 is coupled to an input ofmultiplexer3119 and to an input ofXOR gate3113 via line3111. An output ofXOR gate3113 provides an error output atnode3114.
• FIG. 32 is a flow diagram illustrating a method in accordance with an embodiment of the present disclosure. The method begins instep3201 and continues to step3202. Instep3202, mapping of a system waveform is performed, for example, according to some or all ofsteps2402 through2412 ofFIG. 24. Fromstep3202, the method proceeds to step3203. Instep3203, a system performance characteristic is evaluated based on information obtained instep3202. For example, a system performance characteristic, such as a voltage margin, a timing margin, a value of a voltage level, or timing of an edge, may be evaluated. Instep3204, one or more system parameters are adjusted. These system parameters may include, for example, an output current, a crosstalk cancellation coefficient, a self-equalization coefficient, a receive circuit timing signal, and a voltage reference.
• Fromstep3204, the method continues to step3205. Instep3205, mapping of a system waveform is performed, for example, in a manner as described in relation to step3202. Fromstep3205, the method continues to step3206. Instep3206, a system performance characteristic is evaluated, for example, in a manner as described in relation to step3203. Instep3207, the results of the evaluation ofstep3203 and the evaluation ofstep3206 are compared. Instep3208, a determination is made based on the comparison of3207. If the results of the evaluation ofstep3206 are better (e.g., closer to a desired performance level of a system performance characteristic) than the results of the evaluation ofstep3203, the method returns to step3204 for further adjustment of one or more system parameters using the same direction or sign of adjustment. If, however, the results of the evaluation ofstep3206 are not better than the results of the evaluation ofstep3203, the method continues to step3209. Instep3209, the direction or sign of the system parameter adjustment to be performed is changed. Fromstep3209, the method returns to step3204 for further adjustment of one or more system parameters in accordance with the change of direction or sign provided instep3209.
• Iterations of this method may continue as long as desired, for example, until a desired performance level of a system performance characteristic is obtained. As another example, iterations may continue until an optimal level of a system performance characteristic is reached. Multiple iterations of the method can be performed to allow adjustment of each system performance characteristic to its optimal level. For example, one system performance characteristic can be optimized, then a second system performance characteristic can be optimized, thereby providing sequential optimization of multiple system performance characteristics. Once a desired or optimal performance level of the system is obtained, the process illustrated inFIG. 32 may end.
• The method ofFIG. 32 may be performed until the system reaches a generally steady state condition. Such steady state conditions may be defined at a plurality of levels. For example, at a broader level, the steady state condition may be more approximate, while, at a narrower level, the steady state condition may be more precise. As an example, a steady state condition may be identified whenstep3209 occurs, especially if it occurs several times over a relatively small range of adjustment of the relevant system parameter. Adjustment back and forth within a limited range may be observed as “dithering” and may be used to indicate completion of the method ofFIG. 32. If hysteresis occurs during adjustment, such that no specific value for the system parameter is identified for the steady state condition, a value can be interpolated within the range of adjustment observed. For example, a value in the middle of the range may be selected. When the method ofFIG. 32 is completed for one system parameter, it may be repeated for a different system parameter. Alternatively, multiple system parameters may be adjusted simultaneously.
• FIG. 33 is a waveform diagram illustrating optimization of crosstalk cancellation resulting from iterative application of the method illustrated inFIG. 32 in accordance with an embodiment of the present disclosure. Waveforms3301,3302,3303,3304, and3305, representing a non-transitioning portion of a signal subject to iterative application of the method described in relation toFIG. 32, are illustrated in relation to ahorizontal axis3313 and avertical axis3314. Thehorizontal axis3313 may represent, for example, time, while thevertical axis3314 may represent, for example, an amplitude, such as a voltage.
• Waveform3306, representing a transitioning portion of an adjacent signal, is illustrated in relation to thehorizontal axis3313 and avertical axis3315, which may represent, for example, an amplitude, such as a voltage. Waveforms3301,3302,3303,3304, and3305exhibit disturbances3308,3309,3310,3311, and3312, respectively, which result from the influence of the sharply risingedge3307 ofwaveform3306, which causes a crosstalk phenomenon.
• With a first set of values of system parameters, thedisturbance3308 occurs whenwaveform3301 is influenced bywaveform3306. By adjusting system parameters, the amplitude of the disturbance can be reduced. Thus, with a second set of values of system parameters, thedisturbance3309 ofwaveform3302 is of a lesser amplitude thandisturbance3308 ofwaveform3301. Yet, the crosstalk phenomenon is still undercompensated. A third set of values of system parameters yields a further reduced amplitude ofdisturbance3310 ofwaveform3303. A fourth set of values of system parameters results inwaveform3304 being almost immune to the crosstalk phenomenon, exhibitingonly disturbance3311 of very slight amplitude. A fifth set of values of system parameters overcompensates for the influence ofwaveform3306, resulting indisturbance3312 of an oppositepolarity affecting waveform3305.
• Therefore, using by detecting undercompensation and overcompensation, the iterative application of the method described in reference toFIG. 32 can be used to find optimal values for system parameters, for example, the system parameters that result inwaveform3304. If desired, additional iterations could be performed using sets of values of system parameters between those ofwaveforms3303 and3305 to more finely adjust the optimal values.
• As one skilled in the art can readily appreciate, there are myriad system parameters that can be optimized in accordance with an embodiment of the present disclosure. Some examples of parameters that can be adjusted in this manner include equalization coefficients, crosstalk cancellation coefficients, output drive levels, termination settings, transmit and receive clock offsets, input receiver windows, as well as many others. A termination setting is a parameter that affects a termination impedance of a transmission line. A transmit clock offset is a parameter that affects the temporal position of a clock signal in a transmit circuit or the temporal position of a signal transmitted by a transmit circuit. A receive clock offset is a parameter that affects the temporal position of a clock signal in a receive circuit, the temporal position of a signal used by the receive circuit for receiving a transmitted signal, or the temporal position of a sampling time during which a transmitted signal is sampled by the receive circuit. An input receiver window is a parameter affecting the differentiation of different logic levels in the receive circuit. An output drive level is a parameter affecting the representation of different logic levels at the transmit circuit. A crosstalk cancellation coefficient is a parameter affecting immunity to crosstalk caused by signals on other conductors. An equalization coefficient is a parameter that may be adjusted to effect equalization adjustment.
• FIG. 2 is a logic diagram illustrating a prior art transmit circuit.Node201 is coupled to a serial data input of a shift register comprising flip-flops215,217,219, and221. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode203. A transmit clock signal is provided to a clock input of the shift register atnode205.Nodes207,209,211, and213 are coupled to parallel load data inputs of flip-flops215,217,219, and221, respectively. The serial data output of the shift register at the output of flip-flop221 is coupled tonode223, which is coupled to an input ofmultiplexer226.
• Node202 is coupled to a serial data input of a shift register comprising flip-flops216,218,220, and222. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode204. A transmit clock signal is provided to a clock input of the shift register atnode206.Nodes208,210,212, and214 are coupled to parallel load data inputs of flip-flops216,218,220, and222, respectively. The serial data output of the shift register at the output of flip-flop222 is coupled tonode224, which is coupled to an input ofmultiplexer226.
• A transmit clock signal is provided to a input ofmultiplexer226 atnode225. The output ofmultiplexer226 is coupled tonode230, which is coupled to an input ofoutput driver227. An output ofoutput driver227 is coupled tonode228, which is coupled to pad229.
• FIG. 3 is a logic diagram illustrating a prior art receive circuit.Pad301 is coupled tonode302, which is coupled to an input of evenreceiver305 and to an input ofodd receiver306. A receive clock signal is provided to an input of evenreceiver305 atnode303 and to an input ofodd receiver306 atnode304. An output of evenreceiver305 atnode307 is coupled to an input of a shift register comprising flip-flops313,315,317, and319. A receive clock is provided to a clock input of the shift register atnode309. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode311. Parallel data are provided atparallel data outputs321,323,325, and327 of flip-flops313,315,317, and319, respectively.
• An output ofodd receiver306 atnode308 is coupled to an input of a shift register comprising flip-flops314,316,318, and320. A receive clock is provided to a clock input of the shift register atnode310. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode312. Parallel data are provided atparallel data outputs322,324,326, and328 of flip-flops314,316,318, and320, respectively.
• FIG. 4 is a logic diagram illustrating a transmit circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.Node401 is coupled to a serial data input of a shift register comprising flip-flops415,417,419, and421. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode403. A transmit clock signal is provided to a clock input of the shift register atnode405.Nodes407,409,411, and413 are coupled to parallel load data inputs of flip-flops415,417,419, and421, respectively.
• The serial data output of the shift register at the output of flip-flop421 is coupled tonode423, which is coupled to an input ofmultiplexer426.Node423 is also coupled to an input ofXOR gate433. A signal atnode431 taken from the data output of flip-flop417 is coupled to an input ofXOR gate433. The output ofXOR gate433 is coupled to an input ofmultiplexer446 vianode435. A fixed logic zero signal is coupled to an input ofmultiplexer446 atnode442. A PRBS test signal is coupled to a selection input ofmultiplexer446 vianode444. The output ofmultiplexer446 is coupled tonode402.
• Node402 is coupled to a serial data input of a shift register comprising flip-flops416,418,420, and422. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode404. A transmit clock signal is provided to a clock input of the shift register atnode406.Nodes408,410,412, and414 are coupled to parallel load data inputs ofshift registers416,418,420, and422, respectively.
• Node424 is taken from a serial data output of the shift register at the output of flip-flop422 and is coupled to an input ofmultiplexer426 and to an input ofXOR gate434. A signal atnode432 taken from the data output of flip-flip418 is coupled to an input ofXOR gate434. The output ofXOR gate434 appears atnode436, which is coupled to an input of flip-flop438.
• A transmit clock signal is provided to a clock input of flip-flop438 vianode437. An output of flip-flop438 is coupled to an input ofmultiplexer445 vianode439. A fixed logic zero input is coupled to an input ofmultiplexer445 atnode441. A PRBS test signal is coupled to a selection input ofmultiplexer445 vianode443. An output ofmultiplexer445 is coupled tonode401.
• A transmit clock signal is provided to multiplexer426 via node425. The output of multiplexer of426 is coupled vianode430 to an input ofoutput driver427.Output driver427 provides an output atnode428, which is coupled to pad429.
• FIG. 5 is a logic diagram illustrating a receive circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.Pad501 is coupled tonode502, which is coupled to evenreceiver505 andodd receiver506. A receive clock signal is provided to evenreceiver505 atnode503 and toodd receiver506 atnode504.
• An output of evenreceiver505 is coupled to an input ofmultiplexer543 and to an input ofXOR gate547 vianode507. The output ofmultiplexer543 is coupled to an input ofXOR gate547 and to a serial data input of a shift register comprising flip-flops513,515,517, and519 vianode545. An output ofXOR gate547 is coupled to an input of ORgate551 atnode549.
• A receive clock signal is provided to a clock input of the shift register vianode509. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode511. Parallel data are provided atparallel data outputs521,523,525, and527 of flip-flops513,515,517, and519, respectively.
• An output of flip-flop519 is coupled to an input ofXOR gate533 vianode529. An output of flip-flop515 is coupled to an input ofXOR gate533 vianode531. An output ofXOR gate533 is coupled to an input ofmultiplexer544 vianode535. A PRBS test signal is applied to aselection input542 ofmultiplexer544.
• An output ofodd receiver506 is coupled to an input ofmultiplexer544 and to an input ofXOR gate548 vianode508. The output ofmultiplexer544 is coupled to an input ofXOR gate548 and to the serial data input of a shift register comprising flip-flops514,516,518, and520 vianode546. An output ofXOR gate548 is coupled to an input of ORgate551 atnode550.
• A receive clock signal is provided to a clock input of the shift register vianode510. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode512. Parallel data are provided atparallel data outputs522,524,526, and528 of flip-flops514,516,518, and520, respectively.
• An output of flip-flop520 is coupled to an input ofXOR gate534 vianode530. An output of flip-flop516 is coupled to an input ofXOR gate534 vianode532. An output ofXOR gate534 is coupled to an input of flip-flop538 vianode536. A receive clock signal is applied to aclock input537 of flip-flop538. An output of flip-flop538 is coupled to an input ofmultiplexer543 vianode539. A PRBS test signal is applied to aselection input541 ofmultiplexer543.
• An output of ORgate551 at node552 is coupled to an input of flip-flop555. A receive clock signal is provided to a clock input of flip-flop555 atnode553. A PRBS test signal is applied to an input of flip-flop555 atnode554. An error flag output of flip-flop555 is provided atnode556.
• FIG. 6 is a logic diagram illustrating a transmit circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.Node601 is coupled to a serial data input of a shift register comprising flip-flops615,617,619, and621. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode603. A transmit clock signal is provided to a clock input of the shift register atnode605.Nodes607,609,611, and613 are coupled to parallel load data inputs of flip-flops615,617,619, and621, respectively.
• The serial data output of the shift register at the output of flip-flop621 is coupled tonode623, which is coupled to an input ofmultiplexer626.Node623 is also coupled to an input ofXOR gate633 and to an input ofmultiplexer645. A signal atnode631 taken from the data output of flip-flop617 is coupled to an input ofXOR gate633. The output ofXOR gate633 is coupled to an input ofmultiplexer646 vianode635. A fixed logic zero signal is coupled to an input ofmultiplexer646 atnode642. A PRBS test signal is coupled to a selection input ofmultiplexer646 vianode644. A roll test signal is coupled to an input ofmultiplexer646 vianode648. The output ofmultiplexer646 is coupled tonode602.
• Node602 is coupled to a serial data input of a shift register comprising flip-flops616,618,620, and622. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode604. A transmit clock signal is provided to a clock input of the shift register atnode606.Nodes608,610,612, and614 are coupled to parallel load data inputs ofshift registers616,618,620, and622, respectively.
• Node624 is taken from a serial data output of the shift register at the output of flip-flop622 and is coupled to an input ofmultiplexer626, an input ofXOR gate634, and an input ofmultiplexer646. A signal atnode632 taken from the data output of flip-flip618 is coupled to an input ofXOR gate634. The output ofXOR gate634 appears atnode636, which is coupled to an input of flip-flop638.
• A transmit clock signal is provided to a clock input of flip-flop638 vianode637. An output of flip-flop638 is coupled to an input ofmultiplexer645 vianode639. A fixed logic zero input is coupled to an input ofmultiplexer645 atnode641. A PRBS test signal is coupled to a selection input ofmultiplexer645 vianode643. A roll test signal is coupled to a selection input ofmultiplexer645 vianode647. An output ofmultiplexer645 is coupled tonode601.
• A transmit clock signal is provided to multiplexer626 vianode625. The output of multiplexer of626 is coupled vianode630 to an input ofoutput driver627.Output driver627 provides an output atnode628, which is coupled to pad629.
• FIG. 7 is a logic diagram illustrating a receive circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.Pad701 is coupled tonode702, which is coupled to evenreceiver705 andodd receiver706. A receive clock signal is provided to evenreceiver705 atnode703 and toodd receiver706 atnode704.
• An output of evenreceiver705 is coupled to an input ofmultiplexer743 and to an input ofXOR gate747 vianode707. The output ofmultiplexer743 is coupled to an input ofXOR gate747 and to a serial data input of a shift register comprising flip-flops713,715,717, and719 vianode745. An output ofXOR gate747 is coupled to an input of ORgate751 atnode749.
• A receive clock signal is provided to a clock input of the shift register vianode709. An unload signal for providing parallel data outputs from the shift register is applied to the shift register via node711. Parallel data are provided atparallel data outputs721,723,725, and727 of flip-flops713,715,717, and719, respectively.
• An output of flip-flop719 is coupled to an input ofXOR gate733 and to an input ofmultiplexer743 vianode729. An output of flip-flop715 is coupled to an input ofXOR gate733 vianode731. An output ofXOR gate733 is coupled to an input ofmultiplexer744 vianode735. A PRBS test signal is applied to aselection input742 ofmultiplexer744. A roll test signal is applied to aselection input788 ofmultiplexer744.
• An output ofodd receiver706 is coupled to an input ofmultiplexer744 and to an input ofXOR gate748 vianode708. The output ofmultiplexer744 is coupled to an input ofXOR gate748 and to the serial data input of a shift register comprising flip-flops714,716,718, and720 vianode746. An output ofXOR gate748 is coupled to an input of ORgate751 atnode750.
• A receive clock signal is provided to a clock input of the shift register vianode710. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode712. Parallel data are provided atparallel data outputs722,724,726, and728 of flip-flops714,716,718, and720, respectively.
• An output of flip-flop720 is coupled to an input ofXOR gate734 and to an input ofmultiplexer744 via node730. An output of flip-flop716 is coupled to an input ofXOR gate734 vianode732. An output ofXOR gate734 is coupled to an input of flip-flop738 vianode736. A receive clock signal is applied to aclock input737 of flip-flop738. An output of flip-flop738 is coupled to an input ofmultiplexer743 vianode739. A PRBS test signal is applied to aselection input741 ofmultiplexer743. A roll test signal is applied to aselection input787 ofmultiplexer743.
• An output of ORgate751 atnode752 is coupled to an input of flip-flop755. A receive clock signal is provided to a clock input of flip-flop755 atnode753. A PRBS test or roll test signal is applied to an input of flip-flop755 atnode754. An error flag output of flip-flop755 is provided atnode756.
• FIG. 8 is a logic diagram illustrating a prior art quad signaling level transmit circuit. A fixed logic zero signal is coupled atnode801 to an input of a shift register comprising flip-flops815,817,819, and821. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode803. A transmit clock signal is provided to a clock input of the shift register atnode805.Nodes807,809,811, and813 are coupled to parallel load data inputs of flip-flops815,817,819, and821, respectively. A serial data output of the shift register at the output of flip-flop821 is coupled to an input of multiplexer826 at node823.
• A fixed logic zero signal is coupled atnode802 to an input of a shift register comprising flip-flops816,818,820, and822. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode804. A transmit clock signal is provided to a clock input of the shift register atnode806.Nodes808,810,812, and814 are coupled to parallel load data inputs of flip-flops816,818,820, and822, respectively. A serial data output of the shift register at the output of flip-flop822 is coupled to an input of multiplexer826 atnode824.
• A fixed logic zero signal is coupled at node851 to an input of a shift register comprising flip-flops865,867,869, and871. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode853. A transmit clock signal is provided to a clock input of the shift register atnode855.Nodes857,859,861, and863 are coupled to parallel load data inputs of flip-flops865,867,869, and871, respectively. A serial data output of the shift register at the output of flip-flop871 is coupled to an input ofmultiplexer876 atnode873.
• A fixed logic zero signal is coupled atnode852 to an input of a shift register comprising flip-flops866,868,870, and872. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode854. A transmit clock signal is provided to a clock input of the shift register at node856.Nodes858,860,862, and864 are coupled to parallel load data inputs of flip-flops866,868,870, and872, respectively. A serial data output of the shift register at the output of flip-flop872 is coupled to an input ofmultiplexer876 atnode874.
• A transmit clock signal is provided to multiplexer826 vianode825. The output of multiplexer of826 is coupled vianode830 to an input ofoutput driver827. A transmit clock signal is coupled to an input of multiplexer of876 vianode875. The output ofmultiplexer876 is coupled tooutput driver827 vianode880.Output driver827 provides an output atnode828, which is coupled to pad829.
• FIG. 9 is a logic diagram illustrating a quad signaling level transmit circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.Node901 is coupled to a serial data input of a shift register comprising flip-flops915,917,919, and921. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode903. A transmit clock signal is provided to a clock input of the shift register atnode905.Nodes907,909,911, and913 are coupled to parallel load data inputs of flip-flops915,917,919, and921, respectively.
• The serial data output of the shift register at the output of flip-flop921 is coupled tonode923, which is coupled to an input ofmultiplexer926.Node923 is also coupled to an input ofXOR gate933. A signal at node931 taken from the data output of flip-flop915 is coupled to an input ofXOR gate933. The output ofXOR gate933 is coupled to an input ofmultiplexer946 vianode935. A fixed logic zero signal is coupled to an input ofmultiplexer946 atnode942. A PRBS test signal is coupled to a selection input ofmultiplexer946 via node944. The output ofmultiplexer946 is coupled tonode902.
• Node902 is coupled to a serial data input of a shift register comprising flip-flops916,918,920, and922. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode904. A transmit clock signal is provided to a clock input of the shift register atnode906.Nodes908,910,912, and914 are coupled to parallel load data inputs ofshift registers916,918,920, and922, respectively.
• Node924 is taken from a serial data output of the shift register at the output of flip-flop922 and is coupled to an input ofmultiplexer926 and to an input ofXOR gate934. A signal atnode932 taken from the data output of flip-flip916 is coupled to an input ofXOR gate934. The output ofXOR gate934 appears atnode936, which is coupled to an input ofmultiplexer995. A fixed logic zero signal is coupled to an input ofmultiplexer995 vianode991. A PRBS test signal is coupled to a selection input ofmultiplexer995 vianode993. An output ofmultiplexer995 is coupled tonode951.
• Node951 is coupled to a serial data input of a shift register comprising flip-flops965,967,969, and971. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode953. A transmit clock signal is provided to a clock input of the shift register atnode955.Nodes957,959,961, and963 are coupled to parallel load data inputs of flip-flops965,967,969, and971, respectively.
• Node973 is taken from the serial data output of the shift register at the output of flip-flop971 and is coupled to an input ofmultiplexer976 and to an input ofXOR gate983.Node981 provides a signal taken from the data output of flip-flop of965 and provides it to an input ofXOR gate983. The output ofXOR gate983 appears at node985 and is coupled to an input of multiplexer of996. A fixed logic zero signal is coupled to an input ofmultiplexer996 viainput992. A PRBS test signal is coupled to a selection input ofmultiplexer996 vianode994. The output ofmultiplexer996 is coupled tonode952.
• Node952 is coupled to a serial data input of a shift register comprising flip-flops966,968,970, and972. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode954. A transmit clock signal is provided to a clock input of the shift register atnode956.Nodes958,960,962, and964 are coupled to parallel load data inputs of flip-flops966,968,970, and972, respectively.
• Node974 provides a signal taken from the serial data output of the shift register at the output of flip-flop972 to an input ofmultiplexer976 and to an input ofXOR gate984.Node982 is taken from a data output of flip-flop966 and coupled to an input ofXOR gate984.XOR gate984 provides an output atnode986, which is coupled to an input of flip-flop938.
• A transmit clock signal is provided to a clock input of flip-flop938 vianode937. A PRBS test input signal is provided to an inverted input of flip-flop938 vianode949. An output of flip-flop938 is coupled to an input ofmultiplexer945 vianode939. A fixed logic zero input is coupled to an input ofmultiplexer945 atnode941. A PRBS test signal is coupled to a selection input ofmultiplexer945 vianode943. An output ofmultiplexer945 is coupled tonode901.
• A transmit clock signal is provided to multiplexer926 vianode925. The output of multiplexer of926 is coupled vianode930 to an input ofoutput driver927. A transmit clock signal is coupled to an input of multiplexer of976 vianode975. The output ofmultiplexer976 is coupled tooutput driver927 vianode980.Output driver927 provides an output atnode928, which is coupled to pad929.
• FIG. 10 is a logic diagram illustrating a quad signaling level transmit circuit capable of operating in a PRBS mode and a roll test mode in accordance with an embodiment of the present disclosure.Node1001 is coupled to a serial data input of a shift register comprising flip-flops1015,1017,1019, and1021. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode1003. A transmit clock signal is provided to a clock input of the shift register atnode1005.Nodes1007,1009,1011, and1013 are coupled to parallel load data inputs of flip-flops1015,1017,1019, and1021, respectively.
• The serial data output of the shift register at the output of flip-flop1021 is coupled tonode1023, which is coupled to an input ofmultiplexer1026.Node1023 is also coupled to an input ofXOR gate1033 and to an input ofmultiplexer1045. A signal atnode1031 taken from the data output of flip-flop1015 is coupled to an input ofXOR gate1033. The output ofXOR gate1033 is coupled to an input ofmultiplexer1046 vianode1035. A fixed logic zero signal is coupled to an input ofmultiplexer1046 atnode1042. A PRBS test signal is coupled to a selection input ofmultiplexer1046 vianode1044. A roll test signal is coupled to an input ofmultiplexer1046 vianode1048. The output ofmultiplexer1046 is coupled to node1002.
• Node1002 is coupled to a serial data input of a shift register comprising flip-flops1016,1018,1020, and1022. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode1004. A transmit clock signal is provided to a clock input of the shift register atnode1006.Nodes1008,1010,1012, and1014 are coupled to parallel load data inputs ofshift registers1016,1018,1020, and1022, respectively.
• Node1024 is taken from a serial data output of the shift register at the output of flip-flop1022 and is coupled to an input ofmultiplexer1026, an input ofXOR gate1034, and an input ofmultiplexer1046. A signal at node1032 taken from the data output of flip-flip1016 is coupled to an input ofXOR gate1034. The output ofXOR gate1034 appears atnode1036, which is coupled to an input ofmultiplexer1095. A fixed logic zero signal is coupled to an input ofmultiplexer1095 via node1091. A PRBS test signal is coupled to a selection input ofmultiplexer1095 vianode1093. A roll test signal is coupled to a selection input of multiplexer of1095 vianode1097. An output ofmultiplexer1095 is coupled tonode1051.
• Node1051 is coupled to a serial data input of a shift register comprising flip-flops1065,1067,1069, and1071. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode1053. A transmit clock signal is provided to a clock input of the shift register atnode1055.Nodes1057,1059,1061, and1063 are coupled to parallel load data inputs of flip-flops1065,1067,1069, and1071, respectively.
• Node1073 is taken from the serial data output of the shift register at the output of flip-flop1071 and is coupled to an input ofmultiplexer1076, to an input ofXOR gate1083, and to an input ofmultiplexer1095.Node1081 provides a signal taken from the data output of flip-flop of1065 and provides it to an input ofXOR gate1083. The output ofXOR gate1083 appears atnode1085 and is coupled to an input of multiplexer of1096. A fixed logic zero signal is coupled to an input ofmultiplexer1096 viainput1092. A PRBS test signal is coupled to a selection input ofmultiplexer1096 vianode1094. A roll test signal is coupled to a selection input ofmultiplexer1096 vianode1098. The output ofmultiplexer1096 is coupled tonode1052.
• Node1052 is coupled to a serial data input of a shift register comprising flip-flops1066,1068,1070, and1072. A load signal for performing a parallel data load of the shift register is provided to the shift register atnode1054. A transmit clock signal is provided to a clock input of the shift register atnode1056.Nodes1058,1060,1062, and1064 are coupled to parallel load data inputs of flip-flops1066,1068,1070, and1072, respectively.
• Node1074 provides a signal taken from the serial data output of the shift register at the output of flip-flop1072 to an input ofmultiplexer1076, to an input ofXOR gate1084, and to an input ofmultiplexer1096.Node1082 is taken from a data output of flip-flop1066 and coupled to an input ofXOR gate1084.XOR gate1084 provides an output atnode1086, which is coupled to an input of flip-flop1038.
• A transmit clock signal is provided to a clock input of flip-flop1038 vianode1037. A PRBS test input signal is provided to an input of flip-flop1038 vianode1049. An output of flip-flop1038 is coupled to an input ofmultiplexer1045 vianode1039. A fixed logic zero input is coupled to an input ofmultiplexer1045 atnode1041. A PRBS test signal is coupled to a selection input ofmultiplexer1045 vianode1043. A roll test signal is coupled to a selection input ofmultiplexer1045 vianode1047. An output ofmultiplexer1045 is coupled tonode1001.
• A transmit clock signal is provided tomultiplexer1026 vianode1025. The output of multiplexer of1026 is coupled vianode1030 to an input ofoutput driver1027. A transmit clock signal is coupled to an input of multiplexer of1076 vianode1075. The output ofmultiplexer1076 is coupled tooutput driver1027 vianode1080.Output driver1027 provides an output atnode1028, which is coupled topad1029.
• FIG. 11 is a logic diagram illustrating a prior art quad signaling level receive circuit.Pad1101 is coupled tonode1102, which is coupled to even receiver1105 and toodd receiver1106. A receive clock signal is provided to even receiver1105 at node1103 and toodd receiver1106 at node1104.
• The most significant bits (MSB) from even receiver1105 are passed to an input of a shift register comprising flip-flops1163,1165,1167, and1169 vianode1157. A receive clock signal is provided to a clock input of the shift register vianode1159. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1161. Parallel data are provided atparallel data outputs1171,1173,1175, and1177 of flip-flops1163,1165,1167, and1169, respectively.
• The least significant bits (LSB) from even receiver1105 are passed to an input of a shift register comprising flip-flops1113,1115,1117, and1119 vianode1107. A receive clock signal is provided to a clock input of the shift register vianode1109. An unload signal for providing parallel data outputs from the shift register is applied to the shift register via node1111. Parallel data are provided atparallel data outputs1121,1123,1125, and1127 of flip-flops1113,1115,1117, and1119, respectively.
• The most significant bits (MSB) fromodd receiver1106 are passed to an input of a shift register comprising flip-flops1164,1166,1168, and1170 vianode1158. A receive clock signal is provided to a clock input of the shift register vianode1160. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1162. Parallel data are provided atparallel data outputs1172,1174,1176, and1178 of flip-flops1164,1166,1168, and1170, respectively.
• The least significant bits (LSB) from evenreceiver1106 are passed to an input of a shift register comprising flip-flops1114,1116,1118, and1120 vianode1108. A receive clock signal is provided to a clock input of the shift register vianode1110. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1112. Parallel data are provided atparallel data outputs1122,1124,1126, and1128 of flip-flops1114,1116,1118, and1120, respectively.
• FIG. 12 is a logic diagram illustrating a quad signaling level receive circuit capable of operating in a PRBS test mode in accordance with an embodiment of the present disclosure.Pad1201 is coupled tonode1202, which is coupled to most significant bits (MSB)receiver1205 and least significant bits (LSB)receiver1206. A receive clock signal is provided toMSB receiver1205 atnode1203 and toLSB receiver1206 atnode1204.
• Even-numbered bits fromMSB receiver1205 are passed to an input of multiplexer1293 and to an input ofXOR gate1297 vianode1257. The output of multiplexer1293 is coupled to an input ofXOR gate1297 and to a serial data input of a shift register comprising flip-flops1263,1265,1267, and1269 vianode1295. An output ofXOR gate1297 is provided at node1299. A receive clock signal is provided to a clock input of the shift register vianode1259. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1261. Parallel data are provided atparallel data outputs1271,1273,1275, and1277 of flip-flops1263,1265,1267, and1269, respectively.
• An output of flip-flop1269 is coupled to an input ofXOR gate1283 vianode1279. An output of flip-flop1263 is coupled to an input ofXOR gate1283 vianode1281. An output ofXOR gate1283 is coupled to an input ofmultiplexer1243 vianode1285. A PRBS test signal is applied to aselection input1241 ofmultiplexer1243.
• Odd-numbered bits fromMSB receiver1205 are passed to an input ofmultiplexer1243 and to an input ofXOR gate1247 vianode1207. The output ofmultiplexer1243 is coupled to an input ofXOR gate1247 and to the serial data input of a shift register comprising flip-flops1213,1215,1217, and1219 vianode1245. An output ofXOR gate1247 is provided at node1249. A receive clock signal is provided to a clock input of the shift register vianode1209. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1211. Parallel data are provided atparallel data outputs1221,1223,1225, and1227 of flip-flops1213,1215,1217, and1219, respectively.
• An output of flip-flop1219 is coupled to an input ofXOR gate1233 via node1229. An output of flip-flop1213 is coupled to an input ofXOR gate1233 vianode1231. An output ofXOR gate1233 is coupled to an input ofmultiplexer1294 vianode1235. A PRBS test signal is applied to aselection input1292 ofmultiplexer1294.
• Even-numbered bits fromLSB receiver1206 are passed to an input ofmultiplexer1294 and to an input ofXOR gate1298 vianode1258. The output ofmultiplexer1294 is coupled to an input ofXOR gate1298 and to the serial data input of a shift register comprising flip-flops1264,1266,1268, and1270 vianode1296. An output ofXOR gate1298 is provided atnode1200. A receive clock signal is provided to a clock input of the shift register vianode1260. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1262. Parallel data are provided atparallel data outputs1272,1274,1276, and1278 of flip-flops1264,1266,1268, and1270, respectively.
• An output of flip-flop1270 is coupled to an input ofXOR gate1284 via node1280. An output of flip-flop1264 is coupled to an input ofXOR gate1284 vianode1282. An output ofXOR gate1284 is coupled to an input of multiplexer1244 via node1286. A PRBS test signal is applied to aselection input1242 of multiplexer1244.
• Odd-numbered bits fromLSB receiver1206 are passed to an input of multiplexer1244 and to an input ofXOR gate1248 vianode1208. The output of multiplexer1244 is coupled to an input ofXOR gate1248 and to the serial data input of a shift register comprising flip-flops1214,1216,1218, and1220 via node1246. An output ofXOR gate1248 is provided at node1250. A receive clock signal is provided to a clock input of the shift register via node1210. An unload signal for providing parallel data outputs from the shift register is applied to the shift register via node1212. Parallel data are provided atparallel data outputs1222,1224,1226, and1228 of flip-flops1214,1216,1218, and1220, respectively.
• An output of flip-flop1220 is coupled to an input ofXOR gate1234 via node1230. An output of flip-flop1214 is coupled to an input ofXOR gate1234 vianode1232. An output ofXOR gate1234 is coupled to an input of flip-flop1238 vianode1236. A receive clock signal is applied to aclock input1237 of flip-flop1238. An output of flip-flop1238 is coupled to an input of multiplexer1293 vianode1239. A PRBS test signal is applied to aselection input1291 of multiplexer1293.
• FIG. 13 is a logic diagram illustrating a quad signaling level receive circuit capable of operating in a PRBS test mode and a roll test mode in accordance with an embodiment of the present disclosure.Pad1301 is coupled tonode1302, which is coupled to most significant bits (MSB)receiver1305 and least significant bits (LSB)receiver1306. A receive clock signal is provided toMSB receiver1305 atnode1303 and toLSB receiver1306 atnode1304.
• Even-numbered bits fromMSB receiver1305 are passed to an input ofmultiplexer1393 and to an input ofXOR gate1397 vianode1357. The output ofmultiplexer1393 is coupled to an input ofXOR gate1397 and to a serial data input of a shift register comprising flip-flops1363,1365,1367, and1369 vianode1395. An output ofXOR gate1397 is provided at node1399. A receive clock signal is provided to a clock input of the shift register vianode1359. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1361. Parallel data are provided atparallel data outputs1371,1373,1375, and1377 of flip-flops1363,1365,1367, and1369, respectively.
• An output of flip-flop1369 is coupled to an input ofXOR gate1383 and to an input ofmultiplexer1393 vianode1379. An output of flip-flop1363 is coupled to an input ofXOR gate1383 vianode1381. An output ofXOR gate1383 is coupled to an input ofmultiplexer1343 vianode1385. A PRBS test signal is applied to aselection input1341 ofmultiplexer1343. A roll test signal is applied to aselection input1387 ofmultiplexer1343.
• Odd-numbered bits fromMSB receiver1305 are passed to an input ofmultiplexer1343 and to an input ofXOR gate1347 vianode1307. The output ofmultiplexer1343 is coupled to an input ofXOR gate1347 and to the serial data input of a shift register comprising flip-flops1313,1315,1317, and1319 vianode1345. An output ofXOR gate1347 is provided atnode1349. A receive clock signal is provided to a clock input of the shift register via node1309. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1311. Parallel data are provided atparallel data outputs1321,1323,1325, and1327 of flip-flops1313,1315,1317, and1319, respectively.
• An output of flip-flop1319 is coupled to an input ofXOR gate1333 and to an input ofmultiplexer1343 vianode1329. An output of flip-flop1313 is coupled to an input ofXOR gate1333 via node1331. An output ofXOR gate1333 is coupled to an input ofmultiplexer1394 vianode1335. A PRBS test signal is applied to aselection input1392 ofmultiplexer1394. A roll test signal is applied to aselection input1390 ofmultiplexer1394.
• Even-numbered bits fromLSB receiver1306 are passed to an input ofmultiplexer1394 and to an input ofXOR gate1398 vianode1358. The output ofmultiplexer1394 is coupled to an input ofXOR gate1398 and to the serial data input of a shift register comprising flip-flops1364,1366,1368, and1370 vianode1396. An output ofXOR gate1398 is provided atnode1300. A receive clock signal is provided to a clock input of the shift register via node1360. An unload signal for providing parallel data outputs from the shift register is applied to the shift register via node1362. Parallel data are provided atparallel data outputs1372,1374,1376, and1378 of flip-flops1364,1366,1368, and1370, respectively.
• An output of flip-flop1370 is coupled to an input ofXOR gate1384 and to an input ofmultiplexer1394 vianode1380. An output of flip-flop1364 is coupled to an input ofXOR gate1384 vianode1382. An output ofXOR gate1384 is coupled to an input ofmultiplexer1344 vianode1386. A PRBS test signal is applied to a selection input1342 ofmultiplexer1344. A roll test signal is applied to aselection input1388 ofmultiplexer1344.
• Odd-numbered bits fromLSB receiver1306 are passed to an input ofmultiplexer1344 and to an input ofXOR gate1348 vianode1308. The output ofmultiplexer1344 is coupled to an input ofXOR gate1348 and to the serial data input of a shift register comprising flip-flops1314,1316,1318, and1320 via node1346. An output ofXOR gate1348 is provided atnode1350. A receive clock signal is provided to a clock input of the shift register vianode1310. An unload signal for providing parallel data outputs from the shift register is applied to the shift register vianode1312. Parallel data are provided atparallel data outputs1322,1324,1326, and1328 of flip-flops1314,1316,1318, and1320, respectively.
• An output of flip-flop1320 is coupled to an input ofXOR gate1334 and to an input ofmultiplexer1344 vianode1330. An output of flip-flop1314 is coupled to an input ofXOR gate1334 via node1332. An output ofXOR gate1334 is coupled to an input of flip-flop1338 vianode1336. A receive clock signal is applied to aclock input1337 of flip-flop1338. An output of flip-flop1338 is coupled to an input ofmultiplexer1393 via node1339. A PRBS test signal is applied to aselection input1391 ofmultiplexer1393. A roll test signal is applied to aselection input1389 ofmultiplexer1393.
• While embodiments of the present disclosure have been described in reference to signaling systems generally, it should be understood that the present disclosure may be applied to various types of signaling systems in various contexts. As an example, the present disclosure may be implemented in a signaling system existing within a memory system. For example, an embodiment of the present disclosure may be provided where either or both of the transmit circuit and the receive circuit are incorporated in either or both of a memory controller and a memory device. Thus, signaling relating to memory operations within the memory system may be evaluated and optimized. The term signaling is understood to be broadly applicable. Even within the specific context of a memory system, signaling is understood to refer to any type of signals that may exist, for example, address signals, control signals, and/or data signals. The present disclosure may be applied to evaluate and optimize either or both of memory read operations and memory write operations.
• Accordingly, a method and apparatus for evaluating and optimizing a signaling system has been described. It should be understood that the implementation of other variations and modifications of the present disclosure in its various aspects will be apparent to those of ordinary skill in the art, and that the present disclosure is not limited by the specific embodiments described. It is therefore contemplated to cover by the present disclosure, any and all modifications, variations, or equivalents that fall within the spirit and scope of the basic underlying principles disclosed and claimed herein.

Claims (29)

1. A circuit comprising:

a semiconductor chip;

a transmit circuit disposed on the semiconductor chip, the transmit circuit to generate an output signal using a parameter having a value, the output signal having a first sequence of symbols;

a receive circuit disposed on the semiconductor chip, the receive circuit to receive the first sequence of symbols and to generate a first received sequence of symbols; and

a comparison circuit disposed on the semiconductor ship, the comparison circuit to compare the first sequence of symbols with the first received sequence of symbols to generate a comparison result, wherein the value of the parameter is adjustable.

2. The circuit ofclaim 1, wherein each symbol comprises a single bit.

3. The circuit ofclaim 1, wherein the circuit outputs the comparison result.

4. The circuit ofclaim 1, wherein the circuit further outputs the value of the parameter.

5. The circuit ofclaim 1, wherein the parameter is a timing parameter.

6. The circuit ofclaim 1, wherein the parameter is selected from the group consisting of an output current reference, a crosstalk cancellation coefficient, and a self-equalization coefficient.

7. A circuit comprising:

a semiconductor chip;

a transmit circuit disposed on the semiconductor chip, the transmit circuit to generate a first sequence of symbols while using a parameter set to a first value and to generate a second sequence of symbols while using the parameter set to a second value;

a receive circuit disposed on the semiconductor chip, the receive circuit to receive the first and second sequences of symbols and to generate respective first and second received sequences of symbols; and

a comparison circuit disposed on the semiconductor chip, the comparison circuit to compare the first sequence of symbols with the first received sequence of symbols to generate a first comparison result and to compare the second sequence of bits with the second received sequence of symbols to generate a second comparison result.

8. The circuit ofclaim 7, wherein the circuit outputs the first and second comparison results.

9. The circuit ofclaim 8, wherein the circuit further outputs the first and second values of the parameter.

10. The circuit ofclaim 7, wherein the parameter is a timing parameter.

11. A circuit comprising:

a semiconductor chip;

a transmit circuit disposed on the semiconductor chip, the transmit circuit to generate an output signal, the output signal having a first sequence of symbols;

a receive circuit disposed on the semiconductor chip, the receive circuit to receive the first sequence of symbols and generate a first received sequence of symbols using a parameter having a value; and

a comparison circuit disposed on the semiconductor chip, the comparison circuit to compare the first received sequence of symbols with a second sequence of symbols to generate a comparison result; wherein the value of the parameter is adjustable.

12. The circuit ofclaim 11, wherein the parameter is a timing parameter.

13. The circuit ofclaim 11, wherein the parameter is a voltage reference parameter.

14. The circuit ofclaim 11, wherein the second sequence of symbols is related to the first sequence of symbols.

15. The circuit ofclaim 14, wherein the second sequence of symbols is derived from the first sequence of symbols.

16. The circuit ofclaim 11, wherein the circuit outputs the comparison result.

17. The circuit ofclaim 16, wherein the circuits further outputs the value of the parameter.

18. A circuit comprising:

a semiconductor chip;

a transmit circuit disposed on the semiconductor chip, the transmit circuit to generate a first output signal having first and second sequences of symbols;

a receive circuit disposed on the semiconductor chip, the receive circuit to receive the first sequence of symbols and to generate, while using a parameter that is set to a first value, a first received sequence of symbols and to receive the second sequence of symbols and to generate, while the parameter is set to a second value, a second received sequence of symbols; and

a comparison circuit disposed on the semiconductor chip, the comparison circuit to compare the first received sequence of symbols with a third sequence of symbols to generate a first comparison result and to compare the second received sequence of symbols with a forth sequence of symbols to generate a second comparison result.

19. The circuit ofclaim 18, wherein the third sequence of symbols is related to the first sequence of symbols and the fourth sequence of symbols is related to the second sequence of symbols.

20. The circuit ofclaim 18, wherein the parameter is a timing parameter.

21. The circuit ofclaim 18, wherein the parameter is a voltage reference parameter.

22. The circuit ofclaim 18, wherein the circuit outputs the first and second comparison results.

23. The circuit ofclaim 22, wherein the circuit further outputs the first and second values of the parameter.

24. The circuit ofclaim 23, wherein the third sequence of symbols is derived from the first sequence of symbols and the fourth sequence of symbols is derived from the second sequence of symbols.

25. A circuit comprising:

a semiconductor chip;

a transmit circuit disposed on the semiconductor chip, the transmit circuit to generate a first output signal having first and second sequences of symbols;

a receive circuit disposed on the semiconductor chip, the receive circuit to receive the first sequence of symbols and to generate, while using a parameter that is set to a first value, a first received sequence of symbols, and to receive the second sequence of symbols and to generate, while the parameter is set to a second value, a second received sequence of symbols; and

a comparison circuit disposed on the semiconductor chip, the comparison circuit to compare the first received sequence of symbols with the first sequence of symbols to generate a first comparison result and to compare the second received sequence of symbols with the second sequence of symbols to generate a second comparison result.

26. The circuit ofclaim 25, wherein the parameter is a timing parameter.

27. The circuit ofclaim 25, wherein the parameter is a voltage reference parameter.

28. The circuit ofclaim 25, wherein the circuit outputs the first and second comparison results.

29. The circuit ofclaim 28, wherein the circuit further outputs the first and second values of the parameter.

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