With a tweaked RC oscillator calibration setting, the UART is behaving and I can format a disk that can be completely copied by an Apple ][. Read performance seems to be around 1 soft error in 1000 sectors read. This is about 1 soft error in 2.7*10^6 bits. I’m not sure what is expected with one of these old drives using old media, but I’m declaring success (at least for now).

I’m running out of things to work on with this first proto board.

I need to start working on proto ][ with more memory, USB interface and an improved low drop out voltage regulator. I’m likely to switch to a crystal based oscillator since the built in RC oscillator calibration setting seems too course for this application. Seems like there is only 1 calibration value that works reliably. and I’d like a little more precise clock.