If you've followed my other project, you'll find that this machine lacks some very simple interface features. Also mentioned is the oven's use of the Cypress/Fujitsu CY96F613, an obsolete, 16bit microcontroller with an unobtainable build chain.
To overcome this, I'm going to completely replace this microcontroller with a new one. The existing microcontroller has a TQFP footprint in a pretty tight space. I did look at creating an adapter to replace the existing one with a new SAMC21 (5V capable) VQFN variant:
But, this still leaves us with the poor thermocouple interface to contend with, could be quite tricky to solder in place and would be difficult to initially program the bootloader/reprogram if something goes wrong.
So instead, we look underneath:
All of the (low voltage) components we need to connect to are through hole devices. So we can easily create a mod board that connects up to all these pins, completely populated with the SAMC21 (and ICSP interface) and a couple of MAX31855's to replace the existing thermocouple circuit. In addition to this, we'll be piggybacking on the existing optocoupled serial interface, which can be broken out into a USB interface for programming and debug.
This all removes the need to create a completely new, full sized board as all the other components are already there.