Reversed ROM bytes

A project log for GTXL Gigatron clone computer with keyboard

Microcomputer that runs without a CPU

justin-davisJustin Davis 07/27/2019 at 14:170 Comments

I got the program stepping through one instruction at a time so I could probe all my lines.  First thing I found out is that my bytes are swapped coming out of my ROM.  I didn't realize that the instruction was the lower byte and the data was the upper byte.  Of course it's obvious in the schematic, but I didn't put that together with the software.  I reprogrammed by flash ROM pretty quickly, but it still didn't run as expected.  For some reason after reprogramming, the ROM started to refuse to put out anything.  I put it back into my Arduino programmer, and it could read the values, so I'm not sure what changed.  I ran out of time, so that all for today.  I'll need to put more time into it.  But at least I'm knocking out problems one by one.