 | This article includes a list ofgeneral references, butit lacks sufficient correspondinginline citations. Please help toimprove this article byintroducing more precise citations.(January 2015) (Learn how and when to remove this message) |
In electroniclogic circuits, apull-up resistor (PU) orpull-down resistor (PD) is aresistor used to ensure a known state for a signal.[1] More specifically, a pull-up resistor or pull-down resistor ensures that a wire will have a highlogic level or low logic level, respectively, in the absence of a driving signal.[2] It is typically used in conjunction with components such asswitches,transistors andconnectors, that physically or electrically interrupt the connection of other components to a lowimpedance logic-level source, such asground,positive supply voltage (VCC), or an actively-driven logic circuit output and thus cause the inputs of those components tofloat (i.e. to have an indeterminatevoltage) — a condition which can lead to unpredictable and potentially damaging circuit behavior.[3]
For example, in the case of a switch which, when closed, connects a circuit to ground orpositive supply voltage, without a PU or PD, when the switch is open, the circuit would be left floating. Implementing pull-up or pull-down resistors ensures stable, reliable, and safe operation of the circuit.
In a circuit with an open switch, nocurrent flows through that branch. Because of this,Kirchhoff's laws do not specify the voltage across the open switch. Thevoltage can vary unpredictably due to ambientelectrical noise,leakage currents, andparasitic capacitance.[4] Without additional sources or boundary conditions, the voltage remains indeterminate and consequently the voltage across connected components are undefined, too.
A pull-up (or pull-down) resistor provides a defined path for current to flow to a voltage source when the switch is open. This ensures the voltage at the connected node is set to a known level, typically logic high (or low). By doing so, it eliminates the indeterminacy caused by the open switch, allowing Kirchhoff's laws to determine the voltage reliably.
For a pull-up resistor to serve only this one purpose and not interfere with the circuit otherwise, a resistor with an appropriate amount ofresistance must be used. For this, it is assumed that the critical components have infinite or sufficiently highimpedance, which is guaranteed, for example, for logic gates made fromFETs. In this case, when the switch is open, the voltage drop across a pull-up resistor (withsufficiently low impedance) practically vanishes, and the circuit looks like a wire directly connected topositive supply voltage. On the other hand, when the switch is closed, the pull-up resistor must havesufficiently high impedance in comparison to the closed switch to not affect the connection to ground. Together, these two conditions can be used to derive an appropriate value for the impedance of the pull-up resistor. However, usually, only a lower bound is derived, assuming that the critical components do indeed have infinite impedance.
A resistor with relatively low resistance (relative to the circuit it is in) is often called a "strong" pull-up or pull-down;[2] when the circuit is open, it will pull the output high or low very quickly (just as the voltage changes in anRC circuit), but will draw more current. A resistor with relatively high resistance is called a "weak" pull-up or pull-down;[2] when the circuit is open, it will pull the output high or low more slowly, but will draw less current. This current, which is essentially wasted energy, only flows when the switch is closed, and technically for a brief period after it is opened until the charge built up in the circuit has been discharged to ground.
A pull-up resistor may be used when interfacing logic gates to inputs. For example, an input signal may be pulled by a resistor, then a switch or jumper strap can be used to connect that input to ground. This can be used for configuration information, to select options or for troubleshooting of a device. For a switch that is used to connect a circuit to ground, a pull-up resistor (connected between the circuit and VCC) ensures a well-definedvoltage (i.e. VCC) when the switch is open. For a switch that is used to connect a circuit to VCC (e.g. if the switch is used to transmit a "high" signal when closed), a pull-down resistor connected between the circuit and ground ensures a well-defined ground voltage (i.e. logical low) across the remainder of the circuit when the switch is open.
| Switch output voltage/signal | Switch opened | Switch closed |
|---|---|---|
| With pull-up resistor | Positive supply voltage Input signal (high or low) | Ground voltage Low signal |
| With pull-down resistor | Ground voltage Low signal | Positive supply voltage Input signal (high or low) |
| Without pull-up or pull-down resistor | Indeterminate voltage | Switch input voltage/signal |
Pull-up resistors may be used at logic outputs where the logic device cannot source current such asopen-collectorTTL logic devices. Such outputs are used for driving external devices, for a wired-OR function incombinational logic, or for a simple way of driving a logic bus with multiple devices connected to it.
Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Manymicrocontrollers intended for embedded control applications have internal, programmable pull-up resistors for logic inputs so that not many external components are needed.
Pull-down resistors can be safely used withCMOS logic gates because the inputs are voltage-controlled.TTL logic inputs that are left unconnected inherently float high, and require a much lower valued pull-down resistor to force the input low. A standard TTL input at logic "1" is normally operated assuming a source current of 40 μA, and a voltage level above 2.4 V, allowing a pull-up resistor of no more than 50 kohms; whereas the TTL input at logic "0" will be expected to sink 1.6 mA at a voltage below 0.8 V, requiring a pull-down resistor less than 500 ohms.[5] Holding unused TTL inputs low consumes more current. For that reason, pull-up resistors are preferred in TTL circuits.
Inbipolar logic families operating at 5 VDC, a typical pull-up resistor value will be 1000–5000Ω, based on the requirement to provide the required logic level current over the full operating range of temperature and supply voltage. ForCMOS andMOS logic, much higher values of resistor can be used, several thousand to a million ohms, since the required leakage current at a logic input is small.
Some disadvantages of pull-up resistors are the extra power consumed when current is drawn through the resistor and the reduced speed of a pull-up compared to an active current source. Certain logic families[which?] are susceptible topower supply transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.