Atmega32u4 mini - button with alternative ground

Using Arduino General Electronics
1

A while ago I made some mouse jigglers using a leonardo, proximity sensor for auto operation, button to switch between off, auto and on and a multi colour LED to show the mode.

Works great whilst working at the desk but I want a miniaturised version to take on the road. I've sourced a tiny atmega32u4 on a usb plug from ebay. In the interest of space and to avoid using links or resistors, I want to use two digital pins for the momentary push button I'm using and have found this code:

const byte groundPin = 9;
const byte switchPin = 11;
pinMode(switchPin, INPUT_PULLUP);
pinMode(groundPin, OUTPUT);
digitalWrite(groundPin, LOW);

tl;dr is this a safe configuration or do I risk damaging the board? I think the INPUT_PULLUP negates the need for a resistor. The tutorials I found always include ground.

Thanks

2

Why would you want a ground pin that you can take HIGH?
Is that to save energy during some sleep mode?

3

You'll have to excuse my ignorance - I'm very much a novice. Saving energy is not important. I was trying to avoid using a resistor and also having to solder a link to the ground pin. I came across the INPUT_PULLUP which I think means the input pin will be HIGH when the switch is open.

4

Yes, that's exactly how it works. If you use INPUT_PULLUP, you don't need an external pullup resistor.

It is OK to use the output pin to connect to an input pin, either through a button or directly. The danger you could have is configuring both pins for OUTPUT pins and then having one HIGH and one LOW somehow. If that is a possibility, you could protect the chip from the short with an external resistor to limit the current flowing between the pins.

5

Many thanks for the explanation.

6

better you don't use the code without understanding its purpose.
do you need button? switch pin in INPUT_PULLUP and when it's connected to GND you'll know it's state is LOW now and it doesn't matter how you made that connection, via a button, a switch, a straight wire to GND, or a screwdriver.

7

Is there a reason why you can't use the normal GND?