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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Forgive me if I am totally misunderstanding you but are you suggesting that I could try a lower power supply voltage? As I mentioned somewhere above my power +ve is a little high. I think I said it was 5.15V - ish.
I shall go and try it anyway!

Interesting.

I don't have a variable "bench" power supply. I found a wall-wart (I think thats what you guys over The Pond call them) that outputs 4.97V and although its not perfect, my test program gets a lot further (before crashing). But that is a REAL change. It crashed before, as far as I can tell, on the first or second level of a subroutine - i.e. a few bytes of stack used and probably corrupted. Now it calls display soubroutines several times before crashing.

So, I think we really are converging on TTL/CMOS levels. Nice one Dr Jefyll, back on page 3.

Now, pullup resistors didn't seem to make a lot of difference. I still have them in circuit - 2K2. I wonder if stronger pullups would help. 1K5 or 1K even.

If you have a rectifier diode in your parts box (1N4001 or similar) you could try sticking that in series with the power supply - that should drop it down to somewhere near 4V. Then see if that makes it any more reliable.

Yes, and I would suggest you do this to the W65C02 only. If it begins working then, it's almost 100% an I/O threshold problem due to the RAM.

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Bill

Hang in there BillO - I'm typing another post.

I should have thought of that! Thanks...

...and BINGO!

At last, AT LAST. IT WORKS! W65C02 WORKS!

Its still a little unstable - I think it would benefit with just a little lower voltage - it runs at 5.5V but occasionally crashes but before it only got one character to the screen with this program and died. Now it runs for minutes at a time. Unfortunaltely my display won't run at any lower voltage so I can't go lower - at least not tonight.

So, I am assuming it is indeed TTL/CMOS levels and stronger data line pullup may indeed work.

I will do a proper thank you to everyone tomorrow.

To give you an idea of how low you can drop the voltage, I ran memory diagnostic while lowering voltage. It stopped at 4.5V, but that's due to the voltage supervisor resetting. Removing the voltage supervisor and manually reset with push button, I can lower the voltage down to 2.6V before it stopped. It never reported memory error, just stopped so I infer memory diagnostic is still passing just something else quitting. While I'm confident it will operate at 3.6V, 2.6V is quite unexpected simply because 7430 is a TTL logic that shouldn't operate much lower than 3.3V.

When an intermittent problem is "cured" by lower supply voltage, system noise is a likely candidate. This is because lower voltage slows down the devices and reduces rail-to-rail voltage swing thus reducing system noise. Contention can also be affected by supply voltage because the drive output weaken and caused less power spike.

The part list for this experiment are W65C02, KM62256, AT28C256, W65C51, 2x1.8432MHz can oscillators, 7430, 74ACT138, and 74AHC00.

I can test whether TTL/CMOS of RAM is a problem by replacing KM62256 with AS6C1008 which is quite similar to AS6C62256. I'll let you know.
Bill

Replacing KM62256 with AS6C1008 (pull up A15 and A16 inputs) and verify it still passes memory diagnostic at 2.6V (it should, AS6C1008 is rated 2.7V-5.5V, same as AS6C62256); then raise voltage slowly while running memory diagnostic to 5.5V. Memory test passes from 2.6V to 5.5V. Attached is the memory diagnostic program which resides in AT28C256 EPROM.
Bill

Edit, Since a fast 55nS AS6C1008 is now installed, I decided to do a speed test. I don't expect the board to run much faster than 2MHz because AT28C256 is a 250nS part, so I replace it with Winbond W27C512 which is a 45nS device. The existing wiring are compatible with W27C512 as long as the code is from $C000-$FFFF. So with W27C512 and AS6C1008 installed, the board will run 12MHz at 5V. It will not run at 14.7MHz @5V, but will run 14.7MHz @5.5V.

Well, this is an oddity no doubt! So, another test you could try to confirm it's a voltage threshold problem with the W65C02 would be to use the diode and ONLY insert it in the +5V to the W65C02. This would lower the CPU's voltage, but maintain the normal +5V (or whatever it is) for the rest of the logic chips... and the memory. In theory, the CPU's threshold voltage would be a bit lower and the memory output levels would remain the same.

I'm traveling this week and next, so I don't have access to my normal test lab... in any case, I did bring one of my C02 Pocket SBC setups and my usual toolbag, which includes a Fluke 115. Running a sequential block read test of a Compact Flash, the voltage at the Alliance SRAM is hovering around 5.045 volts. Again, this is one of several boards I've built up with the Alliance SRAM and none are experiencing any issues. I've also run a RAM test on the boards in the past with zero failures.

Sadly, your puzzle continues.... ugh!

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Regards, KM
https://github.com/floobydust

Thanks Folks for your thoughts and physical work. I continue to be humbled by your kindness and help.

Indeed, on sleeping on it I agree that things are not "fixed" - the underlying cause is still there. Levels, noise, whatever. I will spend more time this evening thinking about your posts. I do think though that I can rule out simple wiring faults.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

BDD, can you explain what you are saying there. Are you agreeing the 1mA is feeble or saying it's okay?

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x86?  We ain't got no x86.  We don't NEED no stinking x86!