It was a big OSH Park day today. Among the arrivals were the latest design for the TRR controller+display backpack. This design replaces the ATTiny85 and the MCP23017 with an ATTiny84.
The initial cut at the firmware didn't work perfectly, but it wasn't hard to sort out. Unfortunately, unlike the ATTiny85, the 84 doesn't have a 2.56 volt analog reference built in, so far as I can tell. So instead, it must use Vcc (5 volts), which means that the full 10 bit range covers 1000 °C, which is way, way more than a reflow oven needs. The downside to that is a loss of resolution, but even 1 degree is fine for this application. In principle, this design could also be used to control lower temperature processes, like a sous-vide heater. If you do, then using the 1.1 volt analog reference would yield a maximum temperature of 220 °C (just short of what's necessary for a reflow oven, unfortunately), but with quarter degree resolution.
The alternative part is the MAX31855, but that part is twice as costly and requires a 3.3 volt power supply, as well as level shifting on its input pins. It does, however, offer 14 bit resolution - .25 °C - over an incredibly wide range, which even includes negative Celsius temperatures.
Anyway, I built two of them this evening - and the first built the second. One of them for me, the other for the Hacker Dojo oven. I'll install it sometime tomorrow evening.