Shutting down the RPi gracefully

One of the things thats worth considering when using an embedded linux system, is the integrity of the file system when it comes to pulling the plug. In the past, I've had issues (especially with poor quality SD cards) with file corruption. The RPi should not be simply disconnected from power when you're done, and if you're running an application without console access (either direct or via ssh), its a good idea to implement the means to shut down the system gracefully.

There is a large number of documentation on how this can be done via a physical switch connected to the GPIO pins. For my implementation, I aimed for the following:

- as few components as possible

- as straight forward header connections as possible

To achieve this, I ended up using pin 05 (GPIO03), which is conveniently located next to a GND (pin 6):

img source: https://www.raspberrypi.org/forums/viewtopic.php?f=78&t=82397

This pin has internal pull-up resistors enabled by default, which means you can use an active-low switch without having to add a resistor. To hook this up simply requires a 1x2 header connection attached to a switch (or bare wires, if you're into that sort of thing) on the other side.

Then, I employed the interrupt method (because polling constantly sucks, right? :P) as described here. The interrupt needs to be modified to trigger on the falling edge instead of rising, due to the active-low logic we're dealing with.

The unintentional bonus of using these two pins is that once the Pi is in the halted state, triggering this pin again will boot it up again! (This is likely a built-in feature which I luckily stumbled upon...)

The one weird thing I discovered, was that the interrupts do not fire when the script is running in the background, which might not be suitable for some usage scenarios since it requires load the python script last and keeping it in the foreground. I'm still investigating this...