Arduino misconceptions 1: need to use external pull-up resistors

One of the first things you need to learn when interfacing switches to microcontrollers is the use of pull-up resistors. These ensure that the inputs to the microcontroller settle in either logic high or low when the switch is not made. They are fundamental, so fundamental that Atmel decided to build-in weak pull-up resistors into the ATmega328P used in the Arduino!

Simple pull-up resistor

Yet a lot of tutorials show external pull-up resistors being used with switches. There’s not really a problem with this – just that it is not required 99% of the time. It might be good to teach the concept, but I regularly see posts on forums where people have connected pull-ups incorrectly, causing them problems. I’ve even seen people confident enough to design a PCB, but have multiple external pull-ups for interfacing to a keypad!

InternalPullUp

So how do you use the internal pull-ups? It is very simple – when the data direction register is set to input, write a high to the output port:

pinMode(pin, INPUT);
digitalWrite(pin, HIGH);

Or, if you have moved away from the Arduino libraries:

DDRx &= ~(1 << pin);
PORTx |= (1 << pin);

What’s the other 1% where these internal pull-ups won’t do? There are two situations:

  1. The switch requires a pull-down rather than pull-up – though this can generally be avoided
  2. You need a strong rather than weak pull-up – sometimes devices draw power from the pull-up, and the 20kOhm internal pull-up is too large.

So, simplify  your circuits and remember this feature!

2 thoughts on “Arduino misconceptions 1: need to use external pull-up resistors

  1. Permalink  ⋅ Reply

    VSz

    July 26, 2016 at 12:36pm

    There is a third situation where an external pull-up might make sense: If the power supply is extremely weak and too much current is flowing through the pull-ups, you can use a larger external pull-up.
    Makes sense if you are in a low-current environment, getting your supply from a capacitor which is in turn powered by piezoelectricity or some other low-current source through a booster circuit. In such a case the internal pull-ups could contribute to a much larger consumption than the rest of the device!

  2. Permalink  ⋅ Reply

    Noel

    April 20, 2017 at 3:46am

    pinMode(pin, INPUT_PULLUP);

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