So after a long hiatus, some of it involving some yak shaving (e.g. #Open-source debugWire debugger yes, I’ll get back to that), I managed to get a version of the firmware together for the power controller that seems to be doing what I ask of it.
The means of overcoming the road block was knocking up a very crude (and slow!) UART driver so I could print data out on the serial port. I avoided doing this previously because I didn’t have an easy way to interface to a TTL serial port. Recently though, I bought some FTDI serial cables, one 5V and one 3.3V, so now I had little excuse.
I feel these will give me some valuable insights into tacking the debugWire project.
I was able though, to bit-bang a UART using avr-libc’s _delay_us, and get a respectable 4800 baud serial stream out. This obviously dropped to 300 baud when I had other tasks running, but still, that’s enough to do what I’m after. (Once upon a time, that was considered fast!)
After figuring out where I was going wrong… perhaps I had been sniffing too much solder smoke that day… I re-wrote my firmware, using this UART library as a means of debugging the code. I set up Timer1 to run at 1.2kHz, which meant I could also use it as a baud rate generator for my software UART and upping the baud rate to 1200bps.
Some further work on a breadboard, and I had more-or-less working firmware.
I’ve thrown the code up on GitHub, it’s very much in a raw state, and I might do a second revision of the PCB, since this prototype seems to be more or less on the money now.