HP97 The journey begins

[teenix]

Hi all,

More progress.

A bug appeared shortly after last post which was a bit weird where the display would start flickering with a jumbled mix of numbers for no reason that I could see. This is one of those moments where the shoulders slump while a myriad of possible problems float through the brain.

I thought the processor was resetting for some reason and tried to nail down if it was my code causing the problem, but all seemed ok.

I enabled some special test code, that is in all of the CPU modules, which sends the HP microcode program counter value and any internal registers that I need back to the PC so I can see actual HP program flow. This enabled me to see that the HP microcode thought the RUN/PRGM switch was being switched back and forth really quickly, which mechanically is impossible.

While playing around, I realized that just moving my hand near the switch caused the problem but only in RUN mode. In PRGM mode, the input pin is grounded, so the only sensible reason for this is a floating input to the processor causing it to oscillate, probably at mains frequency while my hand was nearby. I traced the circuit to a resistor that holds this pin in a Logic 1 state until the PRGM switch is selected an lo and behold, I forgot to solder one of the pins. Lots of small parts on this board, so easy to do I suppose.

So far the calculator seems to be working ok. I have tested the program storage and the Continuous Memory. I entered a small program the other day and is still in memory when switched on. I added the clock chip and that seems to be working as well.

I had to make a calculation for a software delay I had to implement in my code, so for its own internals, the 97 did its first job

cheers

Tony