Got me an Atom

[cmjones01]

I'd suspect the reset circuit too. It's got the classic simple reset arrangement also popular on arcade games - an electrolytic (C11) and a resistor (R38) feeding into two LS04 gates (IC9). For some reason I find that the capacitor has frequently gone short circuit.

Chris

[jay_oldstuff]

Hmm what should be a 10uf electrolytic for C11 has been replaced by a nice 0.01uf 400v mullard mustard. wonder if that could be it. got to go out now but will investigate on my return.

Jay

[julie_m]

The reset pulse will be about 1000 times too short .....

[jay_oldstuff]

Right replaced C11 with 10uf and replaced R38 with the correct 1k not the 4.7k fitted. still nothing on screen but some patterning. the only other thing I can find so far is that x2 should be 3.58mhz and the one fitted is 3.57mhz the component fitted doesn’t look right somehow and I think is a later replacement. x1 the 4mhz crystal looks right and fits the board properly. any suggestions on what to try next folks.

Jay

[dave cox]

As the CPU comes out of reset (goes high) there will we 6 clock cycles then 2 read operations (from $FFFC / $FFFD). The RDY line can hold this process up for slow memory / IO ...

dc

[SiriusHardware]

Originally Posted by jay_oldstuff

Right replaced C11 with 10uf and replaced R38 with the correct 1k not the 4.7k fitted. still nothing on screen but some patterning.

On a 6502, which I presume it what it is: Pin 40 is the reset pin. In normal running mode this pin should be high (= ~ 5V). Look at the reset pin with a logic probe, a scope, or if it's all you have, a meter. You should see it go momentarily low only during the reset phase, then return high. If it is permanently low, something is holding it in reset and you need to follow that through.

The component discrepancies you have found (if they were not an official factory modification) suggest that someone else has already been hunting for a fault in the area of the reset circuit. I would definitely keep looking in that area.

[jay_oldstuff]

Yes it is 6502 based and pressing the brake key pulls pin 40 of the 6502 low. (I do have a logic probe.) releasing the brake key lets it go high again. but holding it in, it stays low.

Jay

[SiriusHardware]

Well - that's the behaviour we would expect on the Reset pin. If you've got the circuit diagram (from what you've said, I think you have) then follow the reset signal from the reset circuit to any other place it goes to (ie, reset pins on other chips) and make sure it is getting to them all.

Edit: Just found the diagram. Check the low-going reset signal is also getting to the 6522 pin 34. An inverted version of it (normally low, but high during reset) should also be reaching the 8755 pin 35.

If it is, then, as Dave Cox mentioned, another pin which can be used by external devices to halt the processor is the RDY pin, pin 2. What's that doing? For normal operation, it should be steady high or mostly high. It shouldn't be steady low. Looking at the main diagram there doesn't seem to be anything using it, it is normally pulled high by R15 so it would be suspicious if it was low.

Also check pin 4 (IRQ) and pin 6 (NMI) - these would normally be steady high, or mostly high with occasional low or periodic low pulses depending upon how the Atom hardware uses them.

[jay_oldstuff]

Ok now checked the reset lines through and all appear to do as intended,
my atom isn’t fitted with the 6522 VIA though I have ordered one, but the reset signal is present at pin 34 of IC1 the 6522's socket. I can also see the inverted version at pin 35 of the 8755.
the RDY signal at pin2 of the 6502 is a steady high.
have also checked pins 4 and 6 and both seen to be at a steady high too.

Jay

Maybe the system PROM has got a bit or two corrupted? They only had a design life of 10 years or so.

[jay_oldstuff]

That’s pretty much the way my thought process has been heading, and would explain the lack of activity on the data lines. ill pull the ROM and try reading it. fortunately if it does prove to be bad the ROM images are fairly easy to acquire. finding a suitable eprom to burn it to could be the hard bit. ive plenty of 27c128 and larger but think this is only about 8K

Jay

[SiriusHardware]

Actually, even if you take the ROMS out completely you should still see a blizzard of activity on the address lines as the processor hunts through the whole address range trying to execute the 'code' it is reading from the nonexistent ROMS and probably the random 'code' in the RAM as well.

If the address lines are static / inactive (as you seem to have been saying) the ROMs are not your problem.

But if you look through the ROMs anyway you should be able to see plenty of plain text words if your eprom programmer's software has an ASCII window at the right hand side as usual - if they are present and correct and readable there is a reasonable chance the whole PROM is OK, although there's always the possibility of a failed data bit or two somewhere in the middle of the code area. However, such an error would not stop the processor dead. It would just keep on running randomly through whatever code it found, trying to execute it.

Do you have another 6502 to try?

[KeithsTV]

If you need any 2764 EPROMs let me know and I'll send you a couple or you could pick them up at Golborne.

Keith

[SiriusHardware]

Some further pins for investigation:

When the machine is not in reset (ie, should be running normally) what do you have on:

6502 pin 37 (Clock in ) - what is its actual frequency (can you measure it?)
6502 pin 39 (Clock out 2) or ('Phase 2) - what do you see there?
6502 pin 9 (A0) - is it active, steady high, or steady low?
6502 pin 34 (R/W) - active, steady high or steady low?

If the old brain is still working (always an insiders view) the 00 opcode for a 6502 (the Atoms chip) is BRK and with 00 on the bus it will go back to 00, so no data in the PROM, no bus activity.

[SiriusHardware]

That's a good thought, but the default 'empty' state of an EPROM is all FF, so when an EPROM is starting to lose its data all the bits will gradually return high. That's assuming that the ROM is an EPROM, and not a masked PROM.

With the EPROM out, I would expect the data lines to float high rather than stay low so I suppose the processor would then read in FF from each address - if so, I have no idea what 6502 instruction that would be. (Z80 man here).

Anyway, it seems we can't absolutely rely on address bus activity to tell us whether the processor is running or held, but there should still be:-

-Activity on the R/W pin if the processor is running
-No activity on the R/W pin if the processor is being held somehow.

I'd still also like to know if the clock-in signal to the processor on pin 37 is 1Mhz (That's what I think it should be, after passing through the 74393 divider) and if it is, whether the clock is emerging on the clock2 output on pin 39.

[KeithsTV]

I suspect the ROM is either an OTP EPROM or masked ROM as the ROMs in my Atom do not have windows. I'll have to dig it out and check it and check the various waveforms.

Check the activity on the /CE input to the ROM as this would show if it was being accessed.

Keith

[jay_oldstuff]

Just had a few minuets to play. I have attached a quick pic of my scope screen first pic is pin 37 of the 6502 which the scope reckons is 253hz the second pic is the waveform from pin 39 it looks the same but is constantly moving on screen where as the waveform from pin 37 is solid. I don't trust the accuracy if this scope it's cheep tat but shows there is at least something there. I really need a proper scope.
Pin 9 looks active and responds to the brake key.
Pin 34 looks active and also responds to the brake key.

Keith the rom (IC20) is a masked rom but should be easy to swap out for an eprom if it becomes necessary.

[KeithsTV]

I don't have immediate access to the Atom circuit at the moment but pin 37 is the clock input which should be 1MHz. Check the connection to the divide by 4 divider. I'll dig out the circuit tonight and if I can extract my Atom from its hiding place I'll check the waveforms.

Found a circuit so check pin 10 of IC44 (74LS393) there should be a 1MHz square wave which is the clock for the processor (pin 37). There should also be a 250kHz square wave on pin 8 which is fed to IC47 and the other half of IC44 for the tape output.

Keith

[jay_oldstuff]

Ive dug out a proper oscilloscope. please disregard the last images that pocket scope is useless. the first pic shown below is from pin 10 of ic44 the second is from pin 8, they are a bit misshapen but don't look too bad.

Jay

Edit ive just tried scopeing pin 39 and 37 of the 6502 and third and fourth pic are the results (sorry for poor image)