Can't wait to get my hands on an Astro-Pi HAT! I'm aware it’s not RPF's intention for the ATtiny88 to be reprogrammable by the end-user, but let's not have that stop us from doing it anyway. Thanks for making the code available!
At what frequency is the ATtiny88 running, and what are the fuse settings? Is the AVR RESET pin connected to GPIO pin 25?
The factory default is that it runs off the internal 8MHz clock with CKDIV8 fuse bit enabled and therefore at 1MHz, and RESET to pin 25 considering the reference to @kcuzner's linuxspi avrdude in the README, but that's just guessing.
There are references to the individual components here: https://www.raspberrypi.org/astro-pi-tech-specs/
which is great, but will there be schematics for the full HAT?