GPIO Expander HAT for ROCKPro64 and Quartz64

[CounterPillow]

Looks cool, man! Now all I need is some application for it... 

Thanks! I'm sort of in the same boat, I don't have a direct application for making these other than wanting to dabble in PCB design and device tree overlays.

The usual applications are either controlling a bunch of LEDs individually, or having a number of external buttons. Linux for example has a device tree binding and driver for something called "gpio-keys" which allows for buttons attached to GPIO pins to act like regular keys on a keyboard. Useful for e.g. industrial control applications, where you need maybe a dozen very reliable buttons to send key commands to some piece of controller software. For LEDS there's gpio-leds, which allows the kernel to control the LEDs with functions like disk activity, or, bizarrely, to notify the user of... mail?. Of course you can also simply have the GPIOs exposed and let user space deal with controlling LEDs, or reading buttons.

I really do like the 2x10 pin stackable header form factor though, I might make boards for other I²C chips in the future, e.g. fan controllers or DACs. I wish I could also use SPI to make something like a CAN interface board, but the SPI broken out on the RP64 is already used by the SPI flash chip.