Yesterday I just researched the HAT spec to understand what all is required for compatibility other than just using the pins. I then started sketching out some solutions in spreadsheets to get a feel for how the available pins might work with the Atomo Controller Module interfaces. The plan is to use something for giving the ADC capability over SPI, since the Pi doesn't have ADC.

After looking at the ADC ICs and available MCUs, the MCU direction makes a lot more sense. It is significantly cheaper and is more flexible. There are two options that make sense:

1. Two module design with the Pi providing all GPIO and bus connections, only provide the ADC for the two modules via a MKE04Z8VTG4
2. Four module design using a MKE02Z16VLC4 to support all ADC plus two modules of GPIO and PWM.

Either way, need to sort out the easiest way to communicate using what pins are available and not interfere with the current backplane design for multiplexed SPI selects.