a couple of hours on a train has given me chance to play around a bit. It seems upping the i2c clock frequency helped when the writes were coming from the AVR itself, but not when coming from USB. For now I have a workaround where writes from USB are dealt with one byte at a time - it’s slow but at least it works.

I suspect the problem is that when writing to the EEPROM, after the first write you have 10ms to send another’s write before it commits the data to memory. I suspect a combination of the i2c timing and usb interrupts means that it’s missing that time slot. This would explain why the write protect wasn’t working, but also it probably explains why writes from USB only work one byte at a time. I’ll be able to confirm this and hopefully be more sure of what’s going on once I can check on a scope when I’m home.

The good thing about this is, while I will need to come back to it, it does at least work which allows me to work on the next bit of functionality, which is loading and saving hex files.

Once that’s done, I can work more on the z80 code, where my plan is to build up a set of libraries to make controlling the hardware on the board easier.