Pull-up resistor
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A pull-up resistor is used in the design of electronic logic circuits. Pull-up resistors may be found at inputs to logic systems so that a definite logic level is asserted if an external device is disconnected. Pull-up resistors may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
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.
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 wire-OR function in combinatorial 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, pulldown 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". Pullup 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 inputs change state, 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.
[edit] References
- Paul Horowitz and Winfield Hill, "The Art of Electronics 2nd Edition", Cambridge University Press, Cambridge, Massachusetts USA, 1989 ISBN 0-521-37095-7
