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Pull-up resistor
In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. More specifically, a pull-up resistor or pull-down resistor ensures that a wire will have a high logic level or low logic level, respectively, in the absence of a driving signal. It is typically used in conjunction with components such as switches, transistors and connectors, that physically or electrically interrupt the connection of other components to a low impedance logic-level source, such as ground, positive supply voltage (VCC), or an actively-driven logic circuit output and thus cause the inputs of those components to float (i.e. to have an indeterminate voltage) — a condition which can lead to unpredictable and potentially damaging circuit behavior.
For example, in the case of a switch which, when closed, connects a circuit to ground or positive 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, no current flows through that branch. Because of this, Kirchhoff's laws do not specify the voltage across the open switch. The voltage can vary unpredictably due to ambient electrical noise, leakage currents, and parasitic capacitance. 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 of resistance must be used. For this, it is assumed that the critical components have infinite or sufficiently high impedance, which is guaranteed, for example, for logic gates made from FETs. In this case, when the switch is open, the voltage drop across a pull-up resistor (with sufficiently low impedance) practically vanishes, and the circuit looks like a wire directly connected to positive supply voltage. On the other hand, when the switch is closed, the pull-up resistor must have sufficiently 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; when the circuit is open, it will pull the output high or low very quickly (just as the voltage changes in an RC circuit), but will draw more current. A resistor with relatively high resistance is called a "weak" pull-up or pull-down; 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-defined voltage (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.
Pull-up resistors may be used at logic outputs where the logic device cannot source current such as open-collector TTL logic devices. Such outputs are used for driving external devices, for a wired-OR function in combinational logic, or for a simple way of driving a logic bus with multiple devices connected to it.
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Pull-up resistor AI simulator
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Pull-up resistor
In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. More specifically, a pull-up resistor or pull-down resistor ensures that a wire will have a high logic level or low logic level, respectively, in the absence of a driving signal. It is typically used in conjunction with components such as switches, transistors and connectors, that physically or electrically interrupt the connection of other components to a low impedance logic-level source, such as ground, positive supply voltage (VCC), or an actively-driven logic circuit output and thus cause the inputs of those components to float (i.e. to have an indeterminate voltage) — a condition which can lead to unpredictable and potentially damaging circuit behavior.
For example, in the case of a switch which, when closed, connects a circuit to ground or positive 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, no current flows through that branch. Because of this, Kirchhoff's laws do not specify the voltage across the open switch. The voltage can vary unpredictably due to ambient electrical noise, leakage currents, and parasitic capacitance. 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 of resistance must be used. For this, it is assumed that the critical components have infinite or sufficiently high impedance, which is guaranteed, for example, for logic gates made from FETs. In this case, when the switch is open, the voltage drop across a pull-up resistor (with sufficiently low impedance) practically vanishes, and the circuit looks like a wire directly connected to positive supply voltage. On the other hand, when the switch is closed, the pull-up resistor must have sufficiently 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; when the circuit is open, it will pull the output high or low very quickly (just as the voltage changes in an RC circuit), but will draw more current. A resistor with relatively high resistance is called a "weak" pull-up or pull-down; 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-defined voltage (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.
Pull-up resistors may be used at logic outputs where the logic device cannot source current such as open-collector TTL logic devices. Such outputs are used for driving external devices, for a wired-OR function in combinational logic, or for a simple way of driving a logic bus with multiple devices connected to it.