Excellent! I did have a thought: perhaps you could see if a lower supply voltage would help - perhaps by putting a diode inline. But that wouldn't tell you a great deal, without some instrumentation.

My background is in hardware so I read this thread with good bit of interests but since you already have several experts on the case, I didn't think I can add much value. However, at this point I feel the root problem is being covered up with pull up resistors and ground/VCC modification. Adding ground/VCC to make the problem show up frequently is a good thing. The worst problem to solve is an intermittent problem so anything you can do to make it happen frequently is a good thing. I think signal contention is a strong possibility where better ground/vcc can shift the point of contention to make the problem better or worse. Lower supply voltage will also shift the point of contention. You should probe every signal with a scope and should never see lingering mid value of 1.4 to 2V; signals should transition rapidly from ground to logic high (3V and above).

Your design is solid; you should not see these problems. The problem is likely a mechanical issue (short, open) somewhere in the board. If no signal contentions are observed and intermittent problem persisted, building second board or have another person build a board in parallel may be the best solution forward.
Bill

Good point, Bill, re a possible mechanical issue (short, open) somewhere in the board.

Adrian, one way you can explore that is by applying some vibration. Nothing too violent, mind you. Just use your own judgment re how much is safe. My own approach is to grasp a screwdriver by the wrong end and use it as a little hammer, striking the DUT with the screwdriver handle. If a failure coincides with one of the blows then that's a pretty significant clue...

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

My thoughts exactly.

Just a question for Adrian though .. Are the 6551 ACIAs older NMOS style or CMOS?

Edit: Never mind. I can see in the picture.

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Bill

Hello Everyone and thanks for your replies and greatly appreciated thoughts.

I have been messing a little.

I have a number of ROMs with known working programs (on the breadboard that was used as the prototype for this board). They partially work but usually just stop responding after a few instructions. I even had one program that was responding to interrupts even though I had quite plainly turned them off with SEI. Now, that's just weird.
A little while ago I swapped the 65C02 for another one and a program that wouldn't run properly before ran perfectly - it was a clock program with interrupts updating the time in the background and a foreground task displaying on the LCD. It even had a routine to get the time using I2C at startup that worked. I powered it down and up again and I can no plonger get it working.

This is pointing to some mechanical problem that I can't find - exactly as you seem to be gravitating to. I don't think it was the fact I changed CPU chip, just the fact I had poked about. I am still waiting for the Rockwell 65C02 to arrive and I am going to give that one last try before starting a rebuild. I have spent the last month of (most) eveings debugging this thing. My other half is being very patient I could rebuild it in a few eveings and probably imporve the layout while I am at it.

May I ask for some advice. Should I either use the Rockwell CPU without pull-up resistors in the new build or the WDC 65C02 with resistors?

My list of parts in my parts box are:

WDC 65C02
Rockwell 65C02 (soon!)
2 x Rockwell 65C51 P2
1 x WDC W65C51N
2 x WDC W65C22S
My RAM is Alliance AS6C62256

I could do:
WDC CPU; WDC 65C22S x 2; WDC 65C51N x 1; Rockwell R65C51 + pullup resistors
or
WDC CPU; WDC 65C22S x 2; Rockwell R65C51 x 2 + pullup resistors
or
Rockwell CPU; WDC 65C22S x 2; Rockwell R65C51 x 2 with no pullup resistors (or would I need them for the WDC 65C22S chips? Or should I source some Rockwell 65C22s?

Any other advice?
Thanks folks!

You don't need pull-up resistors at all. You have a full CMOS environment. Those memory chips will perform just fine given the maybe 40-50 uA loading you have.

The Rockwell 65C51 chips have the benefit of not having a transmit bug to code around.

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Bill

Use sockets! And, at least consider including a socket for a SIP resistor array (yes, pullups). Your Rockwell 'C02 surely won't benefit from them but your WDC just might. Or not, but a socket for a 9-pin SIP is inexpensive and small.

I tend to agree, but at the very least I think we can agree there is a degree of compromise involved, albeit perhaps debatably significant.


Check out the datasheet for this CMOS ram.
-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

What a good idea.

Your point is taken Jeff. Two things though..

1) my comments were based on teh chips he is using.
2) That Cypress chip is a fairly atypical chip and seems to have some unusual architecture to enhance speed, reduce supply voltages and reduce die size. I would be willing to bet that the 5V parts are internally regulated down to a lower voltage to allow them to use 3.3V chips. That way they don't have to create a specific 5V part. Probably a good idea to stay away from Cypress 90nm memory technology for our purposes.

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Bill

_________________
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?

It looks like he only has about 1/20 of that.

_________________
Bill

_________________
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?

Why would the current drop off like that above 3V? The spec sheet says a minimum of 1mA @ 2.4V which implies an internal resistance of 2.6K. If that stays constant then current would be .77mA @ Vo=3V and .38mA @ Vo=4V. That's if the internal resistance stays constant. The behavior of small MOSFETS is that their drain resistance decreases as the current through them also decreases.

The WDC stuff only requires Vih > 3.5V, the RAM and ROM are okay with TTL levels.

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Bill

Well, having reviewed the schematic multiple times, I don't see anything that stands out as a problem. It should work... the fact that it doesn't pretty much gets down to either a bad chip, bad socket, possibly some other bad parts or a problem with construction.

If I were to start a build again, I'd go with the WDC 65C02 and 65C22, use the Rockwell R65C51 ACIA, the Alliance RAM and the 74HC logic chips. As for ROM/EEPROM, just stick with a common Atmel AT28C256. Beyond that, multiple 0.1uF bypass caps and a couple decent size electrolytic caps and you shouldn't have a problem.

As for construction, build up the board and run a solid ground between all of the chips first, then add the 0.1 bypass caps to each chip, then connect a +5V power line to each chip. I'd also suggest taking a picture of this and posting it before going any further. Most of my early wire-wrap projects used some proto boards that had bus patterns which could be used for power distribution. I used them to to distribute the ground and +5V to each chip and ensured that I had a solid ground and voltage feed to every component that needed them before moving to the next stage of construction.

On an odd note... are you absolutely certain that your EEPROM/ROM chip is properly programmed and can pass a verify from the programmer? Also make sure you have a solid 5V power supply with ample current, good regulation and low noise. Also make sure you have a solid connection for the power supply that doesn't glitch if you move a wire around.

Good luck...

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

_________________
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?