HAT Compatability RPI A,B and B+

Question

I'm working through the HAT specification.

The raspberry PI A and B have a 26 and GPIO headers, while B+ has 40 pin GPIO. https://github.com/raspberrypi/hats

Am I correct to reason that if I design a HAT using only the first 26 pins that my HAT will be future proof?

The documentation states "There are obviously a lot of add-on boards designed for the original model A and B boards (which interface to the original 26 way GPIO header). The first 26 pins of the 40W GPIO header are identical to those of the original models, so most existing boards will still work."

The concerning word is "most". Can any further elaboration be provided for how to ensure compatibility?

Answer 1

You can’t make an official HAT because it would miss the configuration pins.

The pin mapping on 26 pin headers differs. Only you can decide if this is important. I think only 3 pins differ. See Which GPIO pins on previous modela are equivalent to those on the B+ model?

The mechanical position also differs.