That was a hairy presentation :D

Since this board is a lender from campus, all sorts of things could be wrong with it. I’ve got an Arduino with the ATMEGA328 on it I might fiddle with once this project is behind me.

The reason I dialed back the clock rate was because I noticed that at higher clock speeds (<250KHz PS1 rate), I get jitter on my output LEDs, whereas if I use a lower clock they hold at solid brightness for each button output. Odd I know, but it works.

I'm using the stock 1MHz clock since I need a 1MHz oscillator for the Commodore SID chip I'll be using. Unless there's a clock divider for the output, but that's getting off topic.

I'll invest much more effort when I continue doing this stuff in my spare time, but considering how green I am with MCUs, I'm very happy with my results even if a bit slow.

I had a strange issue with I2C where it only seemed to work at extremely low speeds, and even then not reliably. It turned out to be a failed pull-up resistor on one of the lines. I have never had a resistor fail in a non-obvious way before (i.e. smoke).

For SPI… Capacitance issue maybe? Are you using breadboard? How about the other device, could it be misconfigured or something? SPI should do 10MHz without problems, faster with a bit of effort.

DDRB |= (1 << PB4) | (1 << PB5) | (1 << PB7); // Set SPI out, clock and slave select for output
SPCR = (1 << SPE) | (1 << MSTR); // Enable SPI, set to master
SPCR |= (1 << CPOL) | (1 << CPHA); // Set on fall, sample on rising
SPCR |= (1 << DORD); // LSB first

SPCR |= (1 << SPR1); // SPI clock: 62500 Hz. Best stable speed in practice.

I'm literally just throwing byes at the SPI bus without pauses, so I didn't get stable data until I went to very low speeds. I think I can take the performance hit in my application.

Any tips?

Jochen