Hi guys

Can I pick your brains a moment? I think I may know the answer to the following, but need some confirmation.
Here goes (it's a bit long winded - but bear with me ) :

I've recently purchased a BBC Micro model B off of ebay with a known fault. It won't boot, but has a blank screen with the only a continuous tone. Anyone who deals with Beebs will know that they normally go "Burrrb Bip" when started. Well, this one keeps emitting just the "Burrrb".

So, I've gone through the socketed ICs, cleaned the legs up with a bit of sandpaper and reseated them. Still the same.
I've fitted known good OS & Basic ICs and removed all other non-essential ROM/EPROMs. Same thing.

Tried the R6502AP CPU in my working beeb and hey presto the same problem. Fitting the original good CPU returns the beeb back to working order.
However, fitting the known good CPU from my working beeb to the other "ebay" beep causes it to do the single tone forever thing along with no display.

Both "good" and "possibly bad" CPUs are Rockwell R6502AP-13 ICs.

I then have a brainwave, rummage through my box of ICs and come up with a WDC 65C02-10 reserved for my homebrew 6502 project.
Now, going by the datasheet this is a 10MHz CPU and completely pin compatible with the R6502 - so should work.

Unfortunately, it causes BOTH beebs to give the above problem.
Why would this be? Surely the 65C02 would run at 2MHz? Or do you think it's faulty? My suspicion is the latter, but I'm not entirely sure.

Does anyone have any thoughts?

Stuck in 'zombie' RESET from faulty hysteresis? I'm speculating at this point, because I'm not sure whether or not the 'Burrrb' needs any active participation from the '02.

Mike

I believe the 'Burrr' is what you get before the sound chip is initialised by the OS, so anything which prevents the OS starting will do this. One thing to try is to swap the two VIAs - the system VIA is needed for the heartbeat interrupt whereas the user VIA is somewhat optional. But it sounds like the CPU is now known bad: it doesn't work in the reference Beeb.

But the good CPU from the reference Beeb doesn't work: I think that means there's more than one fault.

And that means that trying the 65C02 hasn't told us anything new: the second fault will prevent it working.

As it's possible to substitute a 65816 (not quite a pin compatible swap, but see https://sites.google.com/site/beeb816/to-dos), I'm sure it's also possible to substitute in a 65C02, if the machine is good. And yet, it seems this 65C02 doesn't work in the good Beeb. So that result is a little odd. Here's a thing: in our experiment, we added a clock buffer. So we should suppose that the Beeb's clock does not have the fast edges needed for a CMOS part.

Ed

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Thanks for the replies guys.

The puzzling bit is the 65C02 not working and I really need to know if it's dud as this is the chip my very first 6502 home brew is goign to rely on. I might see if I can set up some kind of test. I think I saw one on 6502.org somewhere... NOP test or something. I'll have a look.

In the meantime, good call on the VIAs. I think I ahve some of the old TTL versions somewhere. I'll let you know.

Alas, they're 65C22P2's. I'll check to see if they're pin compatible, but might end up with the same issue as with the 65C02...

[edit] I replaced the system VIA in my working beep with the CMOS version and that seemed ok. Replacing both in the ebay beeb doesn't help. Dammit, I think I bought a lemon

Try the forums at stardot - they have a lot of debugging experience.
http://stardot.org.uk/forums/viewtopic.php?f=3&t=2190

I have a collection of 6502 and 6522 pulls (Rockwell and Synertek). I might have a spare or two.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

That's a good idea. I'll go and post there.
Apologies: I may have given the wrong impression in my earlier post. I meant to ask about the 65C02 and if it should run at a slower speed and work in a BBC Micro at all (the rest was to put it into context - although the advice I did get was great ).

That's extremely generous of you. I'd be happy to pay you for one or two Rockwell ones (if the forum allows that kind of thing). How much would be appropriate?

My thinking is that the C02 will work fine in a (working!) Beeb, but it will need a clock buffer chip to give it a clean clock.
Cheers
Ed

Thanks for the advice BigEd. Can you recommend which buffer to use part-wise? Or will any do?

Hmm, sorry, I'm beginning to doubt myself now. I have a record of a sketch which shows us using two inverters to derive the 6502's two clock outputs, but in that sketch the clock input goes straight from the Beeb to the CMOS CPU. I think it's possible that the C02's clock outputs don't have similar enough timing to drive the Beeb's circuits - the Beeb is a design that's somewhat on the ragged edge.

Edit: or maybe the point was that we were using an '816 and that doesn't have clock outputs, so we had no choice but to derive them. But note that our externally buffered clocks and the original NMOS clock outputs will very likely be slower than a modern C02's clock outputs. Which would be consistent with needing something in addition to a plug-in replacement.

If I can dig up more info about our experiments I'll post here.

Ed

I found some notes on some slightly different experiments, which hint that 74HC14, 74HCT04, 74AC04, 74ACT04 were all OK. Possibly also 74HCT14. These are not necessarily optimal or exhaustive, they are just what we had available. Sorry this is vague!

A lot of designs will ground the NC pins on the 6502, and it goes without saying that that pin 1 is likely grounded. I'm not sure what grounding pins 1, 5 and 36 will do to the W65C02 but it might be an idea to bend those pins out before putting it into the BBC.

_________________
Bill

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