Author: Kaz Kylheku Date: To: ada-mp1 Subject: New file: fixing startup problem on old ADAs.
I had the MP1 open, so I took a shot of a fix that I did to make it
start up reliably.
I spent weeks debugging the issue a couple of years ago, and then I
it down to the flaky NMI-signal generating circuit whose purpose is to
cause the Z80
CPU to jump to some safe infinite loop when the power is cut.
Unfortunately, NMI is edge-triggered and the circuit can flake out at
generating a spike to that input, causing the CPU to enter that same
like when power is cut.
I found this myself, and then I learned that they fixed it in later
(mine is from 1987).
Here is my fix: cut the trace to the NMI pin (17), and jumper
pin 17 to pin 11 (pulling NMI to +5V).