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Old 23rd Sep 2017, 11:16 am   #33
PETERg0rsq
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Join Date: Jun 2009
Location: St Helens, Lancashire, UK.
Posts: 641
Default Re: Help with 6802 Microcontroller

Thanks

Yes absolutely correct on the pulses I saw. I expected to see a "sharp" rising and falling edge. (the picture was taken when no other components connected to the BUS except the pull up resistors, but the immage was identical when all other components installed.

I was puzzled about what was triggering the IRQ, as it was being sent by the PIA (a 6821), but there was NO activity on its CA1, CA2, CB1, CB2 or its PA BUS.

The IRQ was effectively "waking up" the 6802 (no activity on the ADDRESS BUS) untill the IRQ, then both BUS's went active, then it completely stopped for about half a second, then another IRQ would appear and off it would go again performing exactly the same instructions. Nothing was triggering the IRQ externally that I could see!

Anyway a late night on it found the fault, and all is fully working again

I was correct, and I had created this second fault on the board.

After finding the high resistance CA track I had checked continuity between all the DATA pins between the 6802 and all the other chips (ROM, RAM, PIA etc) using a bench meter. All looked good.

I then went through it again using a "ToneOhm" and noticed a different "pitch" when checking between the 6802 and the first EPROM on D7.

Further investigation showed a resistance of 150 ohms. Re-soldering all the vias and pins made no difference, to I linked out the bad part (via to EPROM) and after that everything is now working exactly as it should.

I had previously run a soldering iron down every pin on the EPROM sockets looking for dry joints, after finding the last bad high resistance track (I spotted this with a scope, as the CE pin was "floating") and must have disturbed a dodgy joint.

SO a big thanks to all the help and support, and corrections, received here without it, and also the opportunity to write, and think, about the problems I observed, I doubt I would have got it sorted.

I have learnt a great deal and now understand the basics of how these processor circuits work, but also know there is a great deal more i need to learn.

Thanks

Peter
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