Hi all,
In a homebrew computer I'm making I'm currently having this curious issue of a single bit consistently going missing over the USB to TTL serial connection between my PC and my single board computer.
The expected start-up message should be: Cold [C] or Warm [W] start?
The message I receive is as follows: /,$ /2 7!2- 34!24?
The binary for the first message is:
01000011 01101111 01101100 01100100 00100000 01011011 01000011 01011101 00100000 01101111 01110010 00100000 01010111 01100001 01110010 01101101 00100000 01011011 01010111 01011101 00100000 01110011 01110100 01100001 01110010 01110100 00111111
The binary for the second is:
00101111 00101100 00100100 00100000 00101111 00110010 00100000 00110111 00100001 00110010 00101101 00100000 00110011 00110100 00100001 00110010 00110100 00111111
Comparing the first and second set of data from the last byte received backwards, you can see that from the second last byte the data is identical except for b6 (from right to left starting with b0).
b6 is consistently low in the second set of data even though it is expected to be high for some bytes as seen in the first set of data.
The MC68B50 is running at 7.3728MHz which is well above it's given spec but I've seen the likes of Grant Searle using it at these frequencies in his z80 computer:
http://searle.hostei.com/grant/z80/SimpleZ80.htmlTo further this, on rare occasions the the data will come trough correctly and the computer will function as desired.
Any suggestions or ideas as to what is causing this issue? And even perhaps how to fix it?
I've done a good bit of digging myself but haven't been able to find much about it online.