On datasheets and in schematics you’ll very often see resistors connected between V+ and devices. They don’t seem to be that important, and they are often neglected or not fully understood. However, like connections to power and ground they can be a fundamental part of how your circuit operates, and merit some consideration.
What is a pull-up resistor?
A pull-up resistor is connected to V+ and the part of your circuit you want to have a positive voltage level on. This can be a pin on a MCU like the ENABLE pin, so that the MCU performs its normal function. Or it could be a RESET pin, and you have a switch in series with the V+ and resistor so that a V+ level can be applied to the pin for a short time to initiate the MCU to reset. The same kind of arrangement is made with a pull-down resistor, but to ground.
Why are pull-up and pull-down resistors used?
Logic input pins on MCU’s should have a definite condition on them, and the common way to do this is to connect to a pull-up or pull-down resistor. The resistor will set the voltage on the pin high in the case of the pull-up resistor or low with a pull-down resistor in your circuit. This is to make sure the pin does not ‘float’ and have a indeterminate or transient voltage applied to the pin which could trigger unexpected behaviors in your circuit.
Why do you need a pull-up resistor instead of connecting to V+ directly?
Connecting a pin directly to V+ or GND might seem like a good idea, however this may result in too much current going through the pin and IC; and if the switch is being used to give an intermittent high or low level, that would mean that the switch is connecting directly from V+ to GND, aka a ‘short’ which will almost certainly cause damage to your project.
Choosing the resistor value.
A good rule of thumb for the pull-up resistor value is 1/10 of the MCU’s input pin impedance. Most applications take 5K-10K or more. Be aware the if your resistor value is too high it might only act as a voltage divider between the V+ (or GND) and the pin, and that would defeat the purpose of having the pull-up/down resistor by setting the voltage somewhere between the high or low levels that might be undefined by the MCU.
Pull-up and pull-down resistors are very useful and are everywhere.
Pull-up and pull-down resistors are so common that many MCU’s incorporate pull-up resistors in their design and can save you board space and hassle (just remember to include the line of code that activates them!). So, keep pull-up resistors in mind and they’ll be your friend in keeping a gate on a FET open for you, keeping your MCU in a stable and chosen state, and other uses where you need to hold a voltage at a high or low level.
