Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance is introduced. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
A pull-up resistor weakly "pulls" the voltage of the wire it is connected to towards its voltage source level when the other components on the line are inactive. When all other connections on the line are inactive, they are high-impedance and act like they are disconnected. Since the other components act as though they are disconnected, the circuit acts as though it is disconnected, and the pull-up resistor brings the wire up to the high logic level. When another component on the line goes active, it will override the high logic level set by the pull-up resistor. The pull-up resistor ensures that the wire is at a defined logic level even if no active devices are connected to it.
A pull-down resistor works in the same way but is connected to ground. It holds the logic signal near zero volts when no other active device is connected.
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.
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. For example, the circuit shown on the right uses 5 V logic level inputs to actuate a relay. If the input is left unconnected, pull-down resistor R1 ensures that the input is pulled down to a logic low. The 7407 TTL device, an open collector buffer, simply outputs whatever it receives as input, but as an open collector device, the output is left effectively unconnected when outputting a "1". Pull-up resistor R2 thus pulls the output all the way up to 12 V when the buffer outputs a "1", providing enough voltage to turn the power MOSFET all the way on and actuate the relay.
Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Many microcontrollers intended for embedded control applications have internal, programmable pull-up resistors for logic inputs so that minimal external components are needed.
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 are susceptible to power 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.
Pull-down resistors can be safely used with CMOS logic gates because the inputs are voltage-controlled. TTL logic inputs that are left un-connected inherently float high, thus they require a much lower valued pull-down resistor to force the input low. This also consumes more current. For that reason, pull-up resistors are preferred in TTL circuits.
In bipolar logic families operating at 5 VDC, a typical pull-up resistor value will be 1000–5000 Ω, based on the requirement to provide the required logic level current over the full operating range of temperature and supply voltage. For CMOS and MOS 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.
Sizes of pull-up resistors in circuits with sensor
In certain applications the switch in the above image may be replaced by a sensor like a bend sensor and hence without the logic gate - the above image describes a voltage divider. So for this case one can use the formulas for the voltage divider to obtain indications about the sizes of the pull-up resistor. Let and denote the resistances of the pullup resistor and the sensor, then:
For getting the highest sensitivity for the sensor one would like to maximize the voltage range of the sensor, i.e. one would want to maximize the range of , i.e. one would like to maximize the difference between the smallest possible sensor voltage (obtained if the sensor resistance value is smallest) and the highest voltage (obtained if the sensor resistance value is largest) in dependence of the variable . I.e. one would like to maximize the function:
One can try to get an extremal value of a differentiable function, like a Maximum, if one takes the derivative of a function, sets it equal to zero and solves the resulting equation (->Solving an equation). That is one would want to solve the equation:
A solution to this equation is
I²C requires pull-up resistors on its clock (SCL) and data line (SDA) because the pins on the chips are of open-collector design. This means that a chip can only pull the lines low, otherwise they float up to VDD. In I²C, pulling the line to ground indicates a logical zero while letting it float to VDD is a logical one. As a channel access method, this allows one node to determine if another is transmitting by asserting an output, letting the external pull-up resistor pull the line to logic 1 level, and monitoring the line state. If a second node pulls the line to zero, then the first node can detect that the other is transmitting.
- Paul Horowitz and Winfield Hill, "The Art of Electronics 2nd Edition", Cambridge University Press, Cambridge, England, 1989 ISBN 0-521-37095-7