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 | |||
| Type | Serial communicationbus | ||
|---|---|---|---|
| Production history | |||
| Designer | NXP Semiconductors (thenPhilips Semiconductors) | ||
| Designed | 1982; 43 years ago (1982) | ||
| Data | |||
| Data signal | Open-collector oropen-drain | ||
| Width | 1-bit (SDA) with separate clock (SCL) | ||
| Bitrate | 0.1, 0.4, 1.0, 3.4 or5.0 Mbit/s depending on mode | ||
| Protocol | Serial,half-duplex | ||
| Pinout | |||
| SDA | Data Pin | ||
| SCL | Clock Pin | ||
I2C (Inter-Integrated Circuit; pronounced as “eye-squared-see” or “eye-two-see”), alternatively known asI2C orIIC, is asynchronous,multi-master/multi-slave,single-ended,serial communicationbus invented in 1982 byPhilips Semiconductors. It is widely used for attaching lower-speed peripheralintegrated circuits (ICs) to processors andmicrocontrollers in short-distance, intra-board communication.
I2C bus can be found in a wide range of electronics applications where simplicity and low manufacturing cost are more important than speed.PC components and systems which involve I2C areserial presence detect (SPD)EEPROMs ondual in-line memory modules (DIMMs),Extended Display Identification Data (EDID) for monitors viaVGA,DVI andHDMI connectors, accessingNVRAM chips, etc. Common I2C applications include reading hardware monitors, sensors,real-time clocks, controlling actuators, accessing low-speedDACs andADCs, controlling simpleLCD orOLED displays, changingcomputer display settings (e.g. backlight, contrast, hue, color balance) viaDisplay Data Channel, and changing speaker volume.
A particular strength of I2C is the capability of amicrocontroller to control a network of device chips with just twogeneral-purpose I/O pins and software. Many other bus technologies used in similar applications, such asSerial Peripheral Interface Bus (SPI), require more pins and signals to connect multiple devices.
System Management Bus (SMBus), defined by Intel and Duracell in 1994, is a subset of I2C, defining a stricter usage. One purpose of SMBus is to promote robustness and interoperability. Accordingly, modern I2C systems incorporate some policies and rules from SMBus, sometimes supporting both I2C and SMBus, requiring only minimal reconfiguration either by commanding or output pin use. System management for PC systems uses SMBus whose pins are allocated in both conventionalPCI andPCI Express connectors.
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I2C uses only twosignals: serial data line (SDA) and serial clock line (SCL). Both are bidirectional andpulled up withresistors.[2] Typicalvoltages used are +5 V or +3.3 V, although systems with other voltages are permitted.
The I2Creference design has a 7-bitaddress space, with a rarely used 10-bit extension.[3] Common I2C bus speeds are the100 kbit/sstandard mode and the400 kbit/sfast mode. There is also a10 kbit/slow-speed mode, but arbitrarily low clock frequencies are also allowed. Later revisions of I2C can host more nodes and run at faster speeds (400 kbit/sfast mode,1 Mbit/sfast mode plus,3.4 Mbit/shigh-speed mode, and5 Mbit/sultra-fast mode). These speeds are more widely used on embedded systems than on PCs.
Note that the bit rates are quoted for the transfers between controller (master) and target (slave) without clock stretching or other hardware overhead. Protocol overheads include a target address and perhaps a register address within the target device, as well as per-byte ACK/NACK bits. Thus the actual transfer rate of user data is lower than those peak bit rates alone would imply. For example, if each interaction with a target inefficiently allows only 1 byte of data to be transferred, the data rate will be less than half the peak bit rate.
The number of nodes which can exist on a given I2C bus is limited by the address space and also by the total buscapacitance of 400 pF, which restricts practical communication distances to a few meters. The relatively high impedance and low noise immunity requires a common ground potential, which again restricts practical use to communication within the same PC board or small system of boards.
| Mode[2] | Maximum speed | Maximum capacitance | Drive | Direction |
|---|---|---|---|---|
| Standard mode (Sm) | 100 kbit/s | 400pF | Open drain* | Bidirectional |
| Fast mode (Fm) | 400 kbit/s | 400 pF | Open drain* | Bidirectional |
| Fast mode plus (Fm+) | 1 Mbit/s | 550 pF | Open drain* | Bidirectional |
| High-speed mode (Hs) | 1.7 Mbit/s | 400 pF | Open drain* | Bidirectional |
| High-speed mode (Hs) | 3.4 Mbit/s | 100 pF | Open drain* | Bidirectional |
| Ultra-fast mode (UFm) | 5 Mbit/s | ? | Push–pull | Unidirectional |
The aforementioned reference design is a bus with aclock (SCL) and data (SDA) lines with 7-bit addressing. The bus has two roles for nodes, either controller (master) or target (slave):
The bus is a multi-controller bus, which means that any number of controller nodes can be present. Additionally, controller and target roles may be changed between messages (after a STOP is sent).
There may be four potential modes of operation for a given bus device, although most devices only use a single role and its two modes:
In addition to 0 and 1 data bits, the I2C bus allows special START and STOP signals which act as message delimiters and are distinct from the data bits. (This is in contrast to thestart bits andstop bits used inasynchronous serial communication, which are distinguished from data bits only by their timing.)
The controller is initially in controller transmit mode by sending a START followed by the 7-bit address of the target it wishes to communicate with, which is finally followed by a single bit representing whether it wishes to write (0) to or read (1) from the target.
If the target exists on the bus then it will respond with anACK bit (active low for acknowledged) for that address. The controller then continues in either transmit or receive mode (according to the read/write bit it sent), and the target continues in the complementary mode (receive or transmit, respectively).
The address and the data bytes are sentmost significant bit first. The start condition is indicated by a high-to-low transition of SDA with SCL high; the stop condition is indicated by a low-to-high transition of SDA with SCL high. All other transitions of SDA take place with SCL low.
If the controller wishes to write to the target, then it repeatedly sends a byte with the target sending an ACK bit. (In this situation, the controller is in controller transmit mode, and the target is in target receive mode.)
If the controller wishes to read from the target, then it repeatedly receives a byte from the target, the controller sending an ACK bit after every byte except the last one. (In this situation, the controller is in controller receive mode, and the target is in target transmit mode.)
An I2C transaction may consist of multiple messages. The controller terminates a message with a STOP condition if this is the end of the transaction or it may send another START condition to retain control of the bus for another message (a "combined format" transaction).
I2C defines basic types of transactions, each of which begins with a START and ends with a STOP:
In a combined transaction, each read or write begins with a START and the target address. The START conditions after the first are also calledrepeated START bits. Repeated STARTs are not preceded by STOP conditions, which is how targets know that the next message is part of the same transaction.
Any given target will only respond to certain messages, as specified in its product documentation.
Pure I2C systems support arbitrary message structures.SMBus is restricted to nine of those structures, such asread word N andwrite word N, involving a single target.PMBus extends SMBus with aGroup protocol, allowing multiple such SMBus transactions to be sent in one combined message. The terminating STOP indicates when those grouped actions should take effect. For example, one PMBus operation might reconfigure three power supplies (using three different I2C target addresses), and their new configurations would take effect at the same time: when they receive that STOP.
With only a few exceptions, neither I2C nor SMBus define message semantics, such as the meaning of data bytes in messages. Message semantics are otherwise product-specific. Those exceptions include messages addressed to the I2Cgeneral call address (0x00) or to the SMBusAlert Response Address; and messages involved in the SMBusAddress Resolution Protocol (ARP) for dynamic address allocation and management.
In practice, most targets adopt request-response control models, where one or more bytes following a write command are treated as a command or address. Those bytes determine how subsequent written bytes are treated or how the target responds on subsequent reads. Most SMBus operations involve single-byte commands.
One specific example is the 24C32 typeEEPROM, which uses two request bytes that are called Address High and Address Low. (Accordingly, these EEPROMs are not usable by pure SMBus hosts, which support only single-byte commands or addresses.) These bytes are used for addressing bytes within the 32 kbit (or 4 kB) EEPROM address space. The same two-byte addressing is also used by larger EEPROMs, like the 24C512 which stores 512 kbits (or 64 kB). Writing data to and reading from these EEPROMs uses a simple protocol: the address is written, and then data is transferred until the end of the message. The data transfer part of the protocol can cause trouble on the SMBus, since the data bytes are not preceded by a count, and more than 32 bytes can be transferred at once. I2C EEPROMs smaller than 32 kbit, like the 2 kbit 24C02, are often used on the SMBus with inefficient single-byte data transfers to overcome this problem.
A single message writes to the EEPROM. After the START, the controller sends the chip's bus address with the direction bit clear (write), then sends the two-byte address of data within the EEPROM and then sends data bytes to be written starting at that address, followed by a STOP. When writing multiple bytes, all the bytes must be in the same 32-byte page. While it is busy saving those bytes to memory, the EEPROM will not respond to further I2C requests. (That is another incompatibility with SMBus: SMBus devices must always respond to their bus addresses.)
To read starting at a particular address in the EEPROM, a combined message is used. After a START, the controller first writes that chip's bus address with the direction bit clear (write) and then the two bytes of EEPROM data address. It then sends a (repeated) START and the EEPROM's bus address with the direction bit set (read). The EEPROM will then respond with the data bytes beginning at the specified EEPROM data address — a combined message: first a write, then a read. The controller issues an ACK after each read byte except the last byte, and then issues a STOP. The EEPROM increments the address after each data byte transferred; multi-byte reads can retrieve the entire contents of the EEPROM using one combined message.
At thephysical layer, both SCL and SDA lines are anopen-drain (MOSFET) oropen-collector (BJT) bus design, thus apull-up resistor is needed for each line. A logic "0" is output by pulling the line to ground, and a logic "1" is output by letting the line float (outputhigh impedance) so that the pull-up resistor pulls it high. A line is never actively driven high. This wiring allows multiple nodes to connect to the bus without short circuits from signal contention. High-speed systems (and some others) may use acurrent source instead of a resistor to pull-up only SCL or both SCL and SDA, to accommodate higher bus capacitance and enable faster rise times.
An important consequence of this is that multiple nodes may be driving the lines simultaneously. Ifany node is driving the line low, it will be low. Nodes that are trying to transmit a logical one (i.e. letting the line float high) can detect this and conclude that another node is active at the same time.
When used on SCL, this is calledclock stretching and is a flow-control mechanism for targets. When used on SDA, this is calledarbitration and ensures that there is only one transmitter at a time.
When idle, both lines are high. To start a transaction, SDA is pulled low while SCL remains high. It is illegal[2]: 14 to transmit a stop marker by releasing SDA to float high again (although such a "void message" is usually harmless), so the next step is to pull SCL low.
Except for the start and stop signals, the SDA line only changes while the clock is low; transmitting a data bit consists of pulsing the clock line high while holding the data line steady at the desired level.
While SCL is low, the transmitter (initially the controller) sets SDA to the desired value and (after a small delay to let the value propagate) lets SCL float high. The controller then waits for SCL to actually go high; this will be delayed by the finite rise time of the SCL signal (theRC time constant of thepull-up resistor and theparasitic capacitance of the bus) and may be additionally delayed by a target's clock stretching.
Once SCL is high, the controller waits a minimum time (4 μs for standard-speed I2C) to ensure that the receiver has seen the bit, then pulls it low again. This completes transmission of one bit.
After every 8 data bits in one direction, an "acknowledge" bit is transmitted in the other direction. The transmitter and receiver switch roles for one bit, and the original receiver transmits a single "0" bit (ACK) back. If the transmitter sees a "1" bit (NACK) instead, it learns that:
Only the SDA line changes direction during acknowledge bits; the SCL is always controlled by the controller.
After the acknowledge bit, the clock line is low and the controller may do one of three things:
One of the more significant features of the I2C protocol is clock stretching. An addressed target device may hold the clock line (SCL) low after receiving (or sending) a byte, indicating that it is not yet ready to process more data. The controller that is communicating with the target may not finish the transmission of the current bit, but must wait until the clock line actually goes high. If the target is clock-stretching, the clock line will still be low (because the connections areopen-drain). The same is true if a second, slower, controller tries to drive the clock at the same time. (If there is more than one controller, all but one of them will normally lose arbitration.)
The controller must wait until it observes the clock line going high, and an additional minimal time (4 μs for standard100 kbit/s I2C) before pulling the clock low again.
Although the controller may also hold the SCL line low for as long as it desires (this is not allowed since Rev. 6 of the protocol – subsection 3.1.1), the term "clock stretching" is normally used only when targets do it. Although in theory any clock pulse may be stretched, generally it is the intervals before or after the acknowledgment bit which are used. For example, if the target is amicrocontroller, its I2C interface could stretch the clock after each byte, until the software decides whether to send a positive acknowledgment or a NACK.
Clock stretching is the only time in I2C where the target drives SCL. Many targets do not need to clock stretch and thus treat SCL as strictly an input with no circuitry to drive it. Some controllers, such as those found inside customASICs may not support clock stretching; often these devices will be labeled as a "two-wire interface" and not I2C.
To maximize busthroughput,SMBus places limits on how far clocks may be stretched. Hosts and targets adhering to those limits cannot block access to the bus for more than a short time, which is not a guarantee made by pure I2C systems.
Every controller monitors the bus for start and stop bits and does not start a message while another controller is keeping the bus busy. However, two controllers may start transmission at about the same time; in this case, arbitration occurs. Target transmit mode can also be arbitrated, when a controller addresses multiple targets, but this is less common. In contrast to protocols (such asEthernet) that use random back-off delays before issuing a retry, I2C has a deterministic arbitration policy. Each transmitter checks the level of the data line (SDA) and compares it with the levels it expects; if they do not match, that transmitter has lost arbitration and drops out of this protocol interaction.
If one transmitter sets SDA to 1 (not driving a signal) and a second transmitter sets it to 0 (pull to ground), the result is that the line is low. The first transmitter then observes that the level of the line is different from that expected and concludes that another node is transmitting. The first node to notice such a difference is the one that loses arbitration: it stops driving SDA. If it is a controller, it also stops driving SCL and waits for a STOP; then it may try to reissue its entire message. In the meantime, the other node has not noticed any difference between the expected and actual levels on SDA and therefore continues transmission. It can do so without problems because so far the signal has been exactly as it expected; no other transmitter has disturbed its message.
If the two controllers are sending a message to two different targets, the one sending the lower target address always "wins" arbitration in the address stage. Since the two controllers may send messages to the same target address, and addresses sometimes refer to multiple targets, arbitration must sometimes continue into the data stages.
Arbitration occurs very rarely, but is necessary for proper multi-controller support. As with clock stretching, not all devices support arbitration. Those that do, generally label themselves as supporting "multi-controller" communication.
One case which must be handled carefully in multi-controller I2C implementations is that of the controllers talking to each other. One controller may lose arbitration to an incoming message, and must change its role from controller to target in time to acknowledge its own address.
In the extremely rare case that two controllers simultaneously send identical messages, both will regard the communication as successful, but the target will only see one message. For this reason, when a target can be accessed by multiple controllers, every command recognized by the target either must beidempotent or must be guaranteed never to be issued by two controllers at the same time. (For example, a command which is issued by only one controller need not be idempotent, nor is it necessary for a specific command to be idempotent when some mutual exclusion mechanism ensures that only one controller can be caused to issue that command at any given time.)
While I2C only arbitrates between controllers,SMBus uses arbitration in three additional contexts, where multiple targets respond to the controller, and one gets its message through.
PMBus version 1.3 extends the SMBus alert response protocol in its "zone read" protocol.[5] Targets may be grouped into "zones", and all targets in a zone may be addressed to respond, with their responses masked (omitting unwanted information), inverted (so wanted information is sent as 0 bits, which win arbitration), or reordered (so the most significant information is sent first). Arbitration ensures that the highest priority response is the one first returned to the controller.
PMBus reserves I2C addresses 0x28 and 0x37 for zone reads and writes, respectively.
There are several possible operating modes for I2C communication. All are compatible in that the100 kbit/sstandard mode may always be used, but combining devices of different capabilities on the same bus can cause issues, as follows:
Some of the vendors provide a so-called non-standardTurbo mode with a speed up to1.4 Mbit/s.
In all modes, the clock frequency is controlled by the controller(s), and a longer-than-normal bus may be operated at a slower-than-nominal speed byunderclocking.
I2C is popular for interfacing peripheral circuits to prototyping systems, such as theArduino andRaspberry Pi. I2C does not employ a standardized connector, however, board designers have created various wiring schemes for I2C interconnections. To minimize the possible damage due to plugging 0.1-inch headers in backwards, some developers have suggested using alternating signal and power connections of the following wiring schemes: (GND, SCL, VCC, SDA) or (VCC, SDA, GND, SCL).[6]
The vast majority of applications use I2C in the way it was originally designed—peripheral ICs directly wired to a processor on the same printed circuit board, and therefore over relatively short distances of less than 1 foot (30 cm), without a connector. However using a differential driver, an alternate version of I2C can communicate up to 20 meters (possibly over 100 meters) overCAT5 or other cable.[7][8]
Several standard connectors carry I2C signals. For example, theUEXT connector carries I2C;the 10-pin iPack connector carries I2C;[9] the6P6CLego Mindstorms NXT connector carries I2C;[10][11][12][13] a few people use the 8P8C connectors and CAT5 cable normally used forEthernet physical layer to instead carry differential-encoded I2C signals[14] or boosted single-ended I2C signals;[15] and everyHDMI and mostDVI andVGA connectors carryDDC2 data over I2C.
When there are many I2C devices in a system, there can be a need to include busbuffers ormultiplexers to split large bus segments into smaller ones. This can be necessary to keep the capacitance of a bus segment below the allowable value or to allow multiple devices with the same address to be separated by a multiplexer. Many types of multiplexers and buffers exist and all must take into account the fact that I2C lines are specified to be bidirectional. Multiplexers can be implemented with analog switches, which can tie one segment to another. Analog switches maintain the bidirectional nature of the lines but do not isolate the capacitance of one segment from another or provide buffering capability.
Buffers can be used to isolate capacitance on one segment from another and/or allow I2C to be sent over longer cables or traces. Buffers for bi-directional lines such as I2C must use one of several schemes for preventing latch-up. I2C is open-drain, so buffers must drive a low on one side when they see a low on the other. One method for preventing latch-up is for a buffer to have carefully selected input and output levels such that the output level of its driver is higher than its input threshold, preventing it from triggering itself. For example, a buffer may have an input threshold of 0.4 V for detecting a low, but an output low level of 0.5 V. This method requires that all other devices on the bus have thresholds which are compatible and often means that multiple buffers implementing this scheme cannot be put in series with one another.
Alternatively, other types of buffers exist that implement current amplifiers or keep track of the state (i.e. which side drove the bus low) to prevent latch-up. The state method typically means that an unintended pulse is created during a hand-off when one side is driving the bus low, then the other drives it low, then the first side releases (this is common during an I2C acknowledgement).
When having a single controller, it is possible to have multiple I2C buses share the same SCL line.[16][17] The packets on each bus are either sent one after the other or at the same time. This is possible, because the communication on each bus can be subdivided in alternating short periods with high SCL followed by short periods with low SCL. And the clock can be stretched, if one bus needs more time in one state.
Advantages are using targets devices with the same address at the same time and saving connections or a faster throughput by using several data lines at the same time.
These tables show the various atomic states and bit operations that may occur during an I2C message.
| Type | Inactive bus (N) | Start (S) | Idle (i) | Stop (P) | Clock stretching (CS) |
|---|---|---|---|---|---|
| Note | Free to claim arbitration | Bus claiming (controller) | Bus claimed (controller) | Bus freeing (controller) | Paused by target |
| SDA | Passive pullup | Falling edge (controller) | Held low (controller) | Rising edge (controller) | Don't care |
| SCL | Passive pullup | Passive pullup | Passive pullup | Passive pullup | Held low (target) |
| Type | Sending one data bit (1) (0) (SDA is set/sampled after SCL to avoid false state detection) | Receiver reply with ACK bit (Byte received from sender) | Receiver reply with NACK bit (Byte not received from sender) | |||
|---|---|---|---|---|---|---|
| Bit setup (Bs) | Ready to sample (Bx) | Bit setup (Bs) | ACK (A) | Bit setup (Bs) | NACK (A') | |
| Note | Sender set bit (controller/target) | Receiver sample bit (controller/target) | Sender transmitter hi-Z | Sender sees SDA is low | Sender transmitter hi-Z | Sender sees SDA is high |
| SDA | Set bit (after SCL falls) | Capture bit (after SCL rises) | Held low by receiver (after SCL falls) | Driven high (or passive high) by receiver (after SCL falls) | ||
| SCL | Falling edge (controller) | Rising edge (controller) | Falling edge (controller) | Rising edge (controller) | Falling edge (controller) | Rising edge (controller) |
| Type | Setting up for a (Sr) signal after an ACK/NACK | Repeated start (Sr) | |||
|---|---|---|---|---|---|
| Note | Start here from ACK | Avoiding stop (P) state | Start here from NACK | Same as start (S) signal | |
| SDA | Was held low for ACK | Rising edge | Passive high | Passive high | Falling edge (controller) |
| SCL | Falling edge (controller) | Held low | Rising edge (controller) | Passive high | Passive pullup |
| Field: | S | I2C address field | R/W' | A | I2C message sequences... | P | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Type | Start | Byte 1 | ACK | Byte X, etc. Rest of the read or write message goes here | Stop | |||||||
| Bit position in byte X | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||||
| 7-bit address pos | 7 | 6 | 5 | 4 | 3 | 2 | 1 | |||||
| Note | MSB | LSB | 1 = Read | |||||||||
| 0 = Write | ||||||||||||
| Field: | S | 10-bit mode indicator | Upper addr | R/W' | A | Lower address field | A | I2C message sequences | P | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Type | Start | Byte 1 | ACK | Byte 2 | ACK | Byte X etc. Rest of the read or write message goes here | Stop | ||||||||||||||
| Bit position in byte X | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |||||
| Bit value | 1 | 1 | 1 | 1 | 0 | X | X | X | X | X | X | X | X | X | X | X | |||||
| 10-bit address pos | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | |||||||||||
| Note | Indicates 10-bit mode | MSB | 1 = Read | LSB | |||||||||||||||||
| 0 = Write | |||||||||||||||||||||
Two groups of 8 addresses each are reserved for special functions:
0000 000 to0000 1111111 000 to1111 111| Reserved address index | 8-bit byte | Description | ||
|---|---|---|---|---|
| 7-bit address | R/W value | |||
| MSB (4-bit) | LSB (3-bit) | 1-bit | ||
| 1 | 0000 | 000 | 0 | General call |
| 2 | 0000 | 000 | 1 | Start byte |
| 3 | 0000 | 001 | X | CBUS address |
| 4 | 0000 | 010 | X | Reserved for different bus format |
| 5 | 0000 | 011 | X | Reserved for future purpose |
| 6 | 0000 | 1XX | X | HS-mode controller code |
| 7 | 1111 | 1XX | 1 | Device ID |
| 8 | 1111 | 0XX | X | 10-bit target (slave) addressing |
In addition, the remaining 112 addresses are designated for specific classes of device, and some of them are further reserved by either related standards or common usage.
SMBus reserves some additional addresses. In particular,0001 000 is reserved for the SMBus host, which may be used by controller-capable devices,0001 100 is the "SMBus alert response address" which is polled by the host after an out-of-band interrupt, and1100 001 is the default address which is initially used by devices capable of dynamic address assignment.
| MSB (4-bit) | Typical usage[18][19][20][21][22] |
|---|---|
| 0001 | Digital receivers,SMBus |
| 0010 | TV video line decoders,IPMB |
| 0011 | AVcodecs |
| 0100 | Video encoders,GPIO expanders |
| 0101 | ACCESS.bus,PMBus |
| 0110 | VESADDC,PMBus |
| 0111 | Display controller |
| 1000 | TV signal processing, audio processing,SMBus |
| 1001 | AV switching,ADCs andDACs,IPMB,SMBus |
| 1010 | Storage memory,real-time clock |
| 1011 | AV processors |
| 1100 | PLLs and tuners, modulators and demodulators,SMBus |
| 1101 | AV processors and decoders, audio power amplifiers,SMBus |
| 1110 | AVcolour space converters |
Although MSB1111 is reserved for Device ID and 10-bit target (slave) addressing, it is also used by VESADDC display dependent devices such aspointing devices.[21]
An I2Ctransaction consists of one or moremessages. Each message begins with a start symbol, and the transaction ends with a stop symbol. Start symbols after the first, which begin a message but not a transaction, are referred to asrepeated start symbols.
Each message is a read or a write. A transaction consisting of a single message is called either a read or a write transaction. A transaction consisting of multiple messages is called a combined transaction. The most common form of the latter is a write message providing intra-device address information, followed by a read message.
Many I2C devices do not distinguish between a combined transaction and the same messages sent as separate transactions, but not all. The device ID protocol requires a single transaction; targets are forbidden from responding if they observe a stop symbol. Configuration, calibration or self-test modes which cause the target to respond unusually are also often automatically terminated at the end of a transaction.
To avoid false marker detection, there is a minimum delay between the SCL falling edge and changing SDA, and between changing SDA and the SCL rising edge. The minimum delay time is dependent upon the data transfer rate in use. Note that an I2C message containingn data bits (including acknowledgements) containsn + 1 clock pulses.
I2C lends itself to a "bus driver" software design. Software for attached devices is written to call a "bus driver" that handles the actual low-level I2C hardware. This permits the driver code for attached devices to port easily to other hardware, including a bit-banging design.
Below is an example ofbit-banging the I2C protocol as an I2C controller (master). The example is written inpseudoC. It illustrates all of the I2C features described before (clock stretching, arbitration, start/stop bit, ack/nack).[23]
// Hardware-specific support functions that MUST be customized:#define I2CSPEED 100voidI2C_delay(void);boolread_SCL(void);// Return current level of SCL line, 0 or 1boolread_SDA(void);// Return current level of SDA line, 0 or 1voidset_SCL(void);// Do not drive SCL (set pin high-impedance)voidclear_SCL(void);// Actively drive SCL signal lowvoidset_SDA(void);// Do not drive SDA (set pin high-impedance)voidclear_SDA(void);// Actively drive SDA signal lowvoidarbitration_lost(void);boolstarted=false;// global datavoidi2c_start_cond(void){if(started){// if started, do a restart condition// set SDA to 1set_SDA();I2C_delay();set_SCL();while(read_SCL()==0){// Clock stretching// You should add timeout to this loop}// Repeated start setup time, minimum 4.7usI2C_delay();}if(read_SDA()==0){arbitration_lost();}// SCL is high, set SDA from 1 to 0.clear_SDA();I2C_delay();clear_SCL();started=true;}voidi2c_stop_cond(void){// set SDA to 0clear_SDA();I2C_delay();set_SCL();// Clock stretchingwhile(read_SCL()==0){// add timeout to this loop.}// Stop bit setup time, minimum 4usI2C_delay();// SCL is high, set SDA from 0 to 1set_SDA();I2C_delay();if(read_SDA()==0){arbitration_lost();}started=false;}// Write a bit to I2C busvoidi2c_write_bit(boolbit){if(bit){set_SDA();}else{clear_SDA();}// SDA change propagation delayI2C_delay();// Set SCL high to indicate a new valid SDA value is availableset_SCL();// Wait for SDA value to be read by target, minimum of 4us for standard modeI2C_delay();while(read_SCL()==0){// Clock stretching// You should add timeout to this loop}// SCL is high, now data is valid// If SDA is high, check that nobody else is driving SDAif(bit&&(read_SDA()==0)){arbitration_lost();}// Clear the SCL to low in preparation for next changeclear_SCL();}// Read a bit from I2C busbooli2c_read_bit(void){boolbit;// Let the target drive dataset_SDA();// Wait for SDA value to be written by target, minimum of 4us for standard modeI2C_delay();// Set SCL high to indicate a new valid SDA value is availableset_SCL();while(read_SCL()==0){// Clock stretching// You should add timeout to this loop}// Wait for SDA value to be written by target, minimum of 4us for standard modeI2C_delay();// SCL is high, read out bitbit=read_SDA();// Set SCL low in preparation for next operationclear_SCL();returnbit;}// Write a byte to I2C bus. Return 0 if ack by the target.booli2c_write_byte(boolsend_start,boolsend_stop,unsignedcharbyte){unsignedbit;boolnack;if(send_start){i2c_start_cond();}for(bit=0;bit<8;++bit){i2c_write_bit((byte&0x80)!=0);byte<<=1;}nack=i2c_read_bit();if(send_stop){i2c_stop_cond();}returnnack;}// Read a byte from I2C busunsignedchari2c_read_byte(boolnack,boolsend_stop){unsignedcharbyte=0;unsignedcharbit;for(bit=0;bit<8;++bit){byte=(byte<<1)|i2c_read_bit();}i2c_write_bit(nack);if(send_stop){i2c_stop_cond();}returnbyte;}voidI2C_delay(void){volatileintv;inti;for(i=0;i<I2CSPEED/2;++i){v;}}
GENERIC kernels of OpenBSD.GENERIC i386/amd64 builds.When developing or troubleshooting systems using I2C, visibility at the level of hardware signals can be important.
There are a number of I2C host adapter hardware solutions for making a I2C controller or target connection to host computers, runningLinux,Mac orWindows. Most options areUSB-to-I2C adapters. Not all of them require proprietary drivers orAPIs.
I2C protocol analyzers are tools that sample an I2C bus and decode the electrical signals to provide a higher-level view of the data being transmitted on the bus.
When developing and/or troubleshooting the I2C bus, examination of hardware signals can be very important.Logic analyzers are tools that collect, analyze, decode, and store signals, so people can view the high-speed waveforms at their leisure. Logic analyzers display time stamps of each signal level change, which can help find protocol problems. Most logic analyzers have the capability to decode bus signals into high-level protocol data and show ASCII data.
On various off the shelf modules, there are some main connectors and pinouts:[29]
The assignment of target addresses is a weakness of I2C. Seven bits is too few to prevent address collisions between the many thousands of available devices. What alleviates the issue of address collisions between different vendors and also allows to connect to several identical devices is that manufacturers dedicate pins that can be used to set the target address to one of a few address options per device. Two or three pins is typical, and with many devices, there are three or more wiring options per address pin.[35][36][37]
10-bit I2C addresses are not yet widely used, and many host operating systems do not support them.[38] Neither is the complex SMBus "ARP" scheme for dynamically assigning addresses (other than for PCI cards with SMBus presence, for which it is required).
Automatic bus configuration is a related issue. A given address may be used by a number of different protocol-incompatible devices in various systems, and hardly any device types can be detected at runtime. For example,0x51 may be used by a 24LC02 or 24C32EEPROM, with incompatible addressing; or by a PCF8563RTC, which cannot reliably be distinguished from either (without changing device state, which might not be allowed). The only reliable configuration mechanisms available to hosts involve out-of-band mechanisms such as tables provided by system firmware, which list the available devices. Again, this issue can partially be addressed by ARP in SMBus systems, especially when vendor and product identifiers are used; but that has not really caught on. The Rev. 3 version of the I2C specification adds a device ID mechanism.
I2C supports a limited range of speeds. Hosts supporting the multi-megabit speeds are rare. Support for the Fm+1 Mbit/s speed is more widespread, since its electronics are simple variants of what is used at lower speeds. Many devices do not support the400 kbit/s speed (in part because SMBus does not yet support it). I2C nodes implemented in software (instead of dedicated hardware) may not even support the100 kbit/s speed; so the whole range defined in the specification is rarely usable. All devices must at least partially support the highest speed used or they may spuriously detect their device address.
Devices are allowed to stretch clock cycles to suit their particular needs, which can starve bandwidth needed by faster devices and increase latencies when talking to other device addresses. Bus capacitance also places a limit on the transfer speed, especially when current sources are not used to decrease signal rise times.
Because I2C is a shared bus, there is the potential for any device to have a fault and hang the entire bus. For example, if any device holds the SDA or SCL line low, it prevents the controller from sending START or STOP commands to reset the bus. Thus it is common for designs to include a reset signal that provides an external method of resetting the bus devices. However many devices do not have a dedicated reset pin, forcing the designer to put in circuitry to allow devices to be power-cycled if they need to be reset.
Because of these limits (address management, bus configuration, potential faults, speed), few I2C bus segments have even a dozen devices. Instead, it is common for systems to have several smaller segments. One might be dedicated to use with high-speed devices, for low-latency power management. Another might be used to control a few devices where latency and throughput are not important issues; yet another segment might be used only to read EEPROM chips describing add-on cards (such as theSPD standard used with DRAM sticks).
On very low-power systems, the pull-up resistors can use more power than the entire rest of the design combined. On these, the resistors are often powered by a switchable voltage source, such as a DIO from a microcontroller. The pull-ups also limit the speed of the bus and have a small additional cost. Therefore, some designers are turning to other serial buses that do not need pull-ups, such asI3C orSPI.
I2C is the basis for theACCESS.bus, theVESADisplay Data Channel (DDC) interface, theSystem Management Bus (SMBus),Power Management Bus (PMBus) and the Intelligent Platform Management Bus (IPMB, one of the protocols ofIPMI). These variants have differences in voltage and clock frequency ranges, and may haveinterrupt lines.
High-availability systems (AdvancedTCA,MicroTCA) use 2-way redundant I2C for shelf management. Multi-controller I2C capability is a requirement in these systems.
TWI (Two-Wire Interface) or TWSI (Two-Wire Serial Interface) is essentially the same bus implemented on various system-on-chip processors fromAtmel and other vendors.[39] Vendors use the name TWI, even though I2C is not a registered trademark as of 2014-11-07.[40] Trademark protection only exists for the respective logo (see upper right corner), and patents on I2C have now lapsed.[citation needed] According toMicrochip Technology, TWI and I2C have a few differences. One of them is that TWI does not support START byte.[41]
In some cases, use of the term "two-wire interface" indicates incomplete implementation of the I2C specification. Not supporting arbitration or clock stretching is one common limitation, which is still useful for a single controller communicating with simple targets that never stretch the clock.
MIPI I3C sensor interface standard (I3C) is a development of I2C, under development in 2017.[42]
| Year | Version | Notes | Refs |
|---|---|---|---|
| 1981 | Patent | U.S. Patent 4,689,740 filed on November 2, 1981 by U.S. Philips Corporation. | [43][44] |
| 1982 | Original | The100 kbit/s I2C system was created as a simple internal bus system for building control electronics with various Philips chips. | — |
| 1992 | 1 | Added400 kbit/sFast-mode (Fm) and a 10-bit addressing mode to increase capacity to 1008 nodes. This was the first standardized version. | — |
| 1998 | 2 | Added3.4 Mbit/sHigh-speed mode (Hs) with power-saving requirements for electric voltage and current. | [45] |
| 2000 | 2.1 | Clarified version 2, without significant functional changes. | [46] |
| 2007 | 3 | Added1 Mbit/sFast-mode plus (Fm+) (using 20 mA drivers), and a device ID mechanism. | [47] |
| 2012 | 4 | Added5 Mbit/sUltra Fast-mode (UFm) for new USDA (data) and USCL (clock) lines usingpush-pull logic withoutpull-up resistors, and added an assigned manufacturer ID table. It is only aunidirectional bus. | [48] |
| 2012 | 5 | Corrected mistakes. | [49] |
| 2014 | 6 | Corrected two graphs. | [50] |
| 2021 | 7 | Changed terms "master/slave" to "controller/target" to align withI3C bus specification. Updated Table 5 assigned manufacturer IDs. Added Section 9 overview of I3C bus. This is the current standard (login required). | [2] |
The 7-bit portion of the slave address for the BMC is 0010_000b
static u_int8_t probe_val[256];
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Interfaces are listed by their speed in the (roughly) ascending order, so the interface at the end of each section should be the fastest.  Category | |
