Although the firmware was fairly simple (about 4 Kbytes) it had to get FDA approval as a class II medical device. Lots of records, module-level testing and system testing. It actually made me think about the process of software development like never before. The firmware was a big state machine so I could avoid using interrupts but still keep up with the real time activities using a very slow processor clock (32 kHz / 8192 instructions/second). The device would run months or years on a set of batteries. I enjoyed working with my wife and we did really well together. It was a pretty big load doing this project as well as my full-time work at BDL.
The wire was added to force the PIC into
reset as the unit was switched off to prevent errant writes
to an EEPROM.