Pull-up & Pull-down Resistors

Resistors are used in one of the single-most important and common utility configurations in circuit design; as pull-up and pull-down resistors. Pull-up and pull-down resistors provide a default value (HIGH or LOW) in a circuit where otherwise, the circuit might have an indeterminate value part of the time.

Floating Circuits

To understand the usefulness of this, consider the following button circuit:

Illustration of a floating button circuit: one button leg is connected to 3.3 volts and the other is connected to an input pin.

Note that the symbol in the center of the circuit represents a common pushbutton:

Pushbutton symbol with two outer connection dots and a line across those points with a solid button rectangle protruding above it.

In this case, when the button is in its default state; not pressed, the value at the input port, 1, is in a floating state, in which it's neither connected to ground or high, and can actually be indeterminate. In fact, a floating wire (or wire trace on a PCB), acts like an antenna and can pick up interference, giving it a fluctuating voltage signal.

There may not be much power/current in the floating termination, but many modern circuit components react to voltage changes, as opposed to current.

Pull-Down Resistors

However, a pull-down resistor can be used, which connects the floating wire to GND:

Illustration of a button wired with a pull-down resistor: the left leg connected to 3.3 volts and the right leg connected to both an input pin and a resistor connected to ground.

This provides a known, default value of LOW when the button isn't pressed.

Logic

When then button is pressed, the input at 1 will see a HIGH signal, even though some current will flow to ground.

Pull-Up Resistors

A pull-up resistor is much like a pull-down resistor, but provides a default HIGH value, and can be used when the logic is inverted; as in the following circuit, where the button connects to ground when pressed:

Illustration of a button wired with a pull-up resistor: the left leg connected to ground and the right leg connected to both an input pin and a resistor connected to 3.3 volts.

Just as with a pull-down resistor, when the button is pressed, it shorts to ground, so the input at 1 will see a LOW signal, because any voltage at 1 will sink to ground.

Strong vs. Weak Pulling Resistors

Typically, in modern, power-efficient circuits where the switching components react to voltage, rather than current, the resistance used in pull ups and pull downs is very high, 10kΩ or more, up to millions of Ohms, which allows only a small amount of current to leak.

A lower amount of resistance will exert a stronger "pull" one way or the other, and a higher amount of resistance will exert a weaker pull. For this reason, high value pulling resistors are known as weak pull-downs or pull-ups, and lower value resistors are known as strong pull-ups or pull-downs.

Because of the physical nature of electricity, stronger pull-ups and pull-downs will react also faster than weaker ones.

We'll examine these considerations more deeply later.

Internal Pull-Ups and Pull-Downs

This is such a common design that many microcontrollers have configurable pull-up and/or pull-down resistors available on inputs. In fact, all the digital inputs on the F7 Meadow board have both pull-ups and pull-downs.

Bias Resistors

As we'll examine later, many times a design actually requires a pull-down or pull-up resistor in order to bias, or set a default (AKA preferential) controlling input level in a circuit. This is extremely common when using transistors (electrical switches), to make sure they are either fully ON or OFF.