Raspberry Pi HAT identity EEPROMs, a simple guide
I've been working on a RFID scanner than can best be described as an overly large Raspberry Pi HAT recently. One of the things I am grappling with as I get closer to production boards is that I need to be able to identify what version of the HAT is currently installed -- the software can then tweak its behaviour based on the hardware present. I had toyed with using some spare GPIO lines and "hard coded" links on the HAT to identify board versions to the Raspberry Pi, but it turns out others have been here before and there's a much better way. The Raspberry Pi folks have defined something called the "Hardware On Top" (HAT) specification which defines an i2c EEPROM which can be used to identify a HAT to the Raspberry Pi. There are a couple of good resources I've found that help you do this thing -- sparkfun have a tutorial which covers it, and there is an interesting forum post. However, I couldn't find a simple tutorial for HAT designers that just covered exactly what they need to know and nothing else. There were also some gaps in those documents compared with my experiences, and…