I have programmed a CPLD to insert additional waitstates to the Z80 to slow down I/O access. The additional waitstates can be set from 0 (disabled) to 255.
As a test I just let the Z80 run on 10MHz and play with number of needed extra waitstates to get a properly working screen.
Some findings I can use some help on explaining:
- With a certain amount of extra IO waitstates I get a normal basic screen but the area where the function keys are printed is always garbled (ie. list => iist').
- When I do a listing of a short basic program the list show up normal a few times on the screen until a get near the bottom lines then it appears garbled but then also the rest of the screen gets garbled.
with a CTRL-L to clear the screen and everything back to normal (except the function keys).
What could cause this issue?
