Martin's Acorn Atom (poorly)

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martinw
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Martin's Acorn Atom (poorly)

Postby martinw » Thu Aug 03, 2017 7:28 pm

My Atom has not been well for a while now, I'm embarking on open heart surgery currently.

IMG_5182.JPG

IMG_5190.JPG


Any advice welcome.

I'm currently on the address bus, triggering on channel 1 of my scope, connected to reset.

I'm guessing someone will say I need a minimal configuration, but am not sure what that is and if I do I might forget which chip goes where if I take them out.

Martin


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Re: Martin's Acorn Atom (poorly)

Postby 1024MAK » Thu Aug 03, 2017 7:55 pm

Use the force Luke, or rather, use your camera to take photos of your board before you fiddle... :wink: :wink: :wink:

Yes, a minimal configuration helps, because it removes potentially suspect chips that may be the cause, allowing the minimal system to work (hopefully).
I'll scarper now, as there are more experienced Atom members that will know off the top of their heads (I'd have to go and find out...) [edit - see one's popped up before I could even finish this...! :P ]

But before I go, confirm that the reset circuit (key) works :wink:

Mark
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Re: Martin's Acorn Atom (poorly)

Postby martinw » Thu Aug 03, 2017 8:25 pm

Cheers guys, I'll work through all that and report back :)

Martin

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Thu Aug 03, 2017 9:21 pm

Hi Martin,

Can you summarise the main symptoms of yours sickly Atom? i.e. what do you see on the screen?

That's a Rigol 1102D you have there, correct?

The specs for the logic analyzer say:
- Channels: 16
- Sample Rate: 200MSa/s (each channel)
- Record Length: 512kpts (each channel)
- Trigger Modes: Pattern, Duration
- Threshold Selections: TTL = 1.4V, CMOS = 2.5V, ECL = -1.3V, USER = -8V to 8V

It's got a pretty decent capture depth.

If you set the sample rate to 10MSa/s and trigger off the rising edge of reset, it should the capture about 50ms of samples at sufficient resolution that we can see exactly what the address bus is doing.

But we need the actual data!

I have some tools that will help post process it into something meaningful.

Once you have this, if you save it to a USB stick, zip, and upload it.

From that, it should be possible to tell how far through the reset sequence it gets.

Dave

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Re: Martin's Acorn Atom (poorly)

Postby Elminster » Thu Aug 03, 2017 9:35 pm

Not much help from me,just general interest. But I think it took me about 4-6 hours to walk through the Atom boot sequence with a logic analyser till I found the stucky bit and Dave put me on the right path. Was quite interesting really, I learnt lots about 6502 assembly/hex and boot sequences.

I just used the old F000 atom Disassembly on our old friend acornatom.nl, starting at FFFC and worked my way through.

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 8:21 am

Thanks guys, yes it's a Rigol 1102D, I'll report back with more info. shortly.

Martin

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Re: Martin's Acorn Atom (poorly)

Postby oss003 » Fri Aug 04, 2017 9:02 am

Hi Martin,


It would indeed help if you describe the symptoms or make a video of it.

Greetings
Kees

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 10:48 am

Will do Kees, out of interest (anybody) I'm drawing about 1.7 amps on my 5 volt bench supply, does this sound about right?

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Fri Aug 04, 2017 10:52 am

martinw wrote:Will do Kees, out of interest (anybody) I'm drawing about 1.7 amps on my 5 volt bench supply, does this sound about right?

Yes, for a fully loaded Atom that is what I would expect.

Dave

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 11:07 am

hoglet wrote:
martinw wrote:Will do Kees, out of interest (anybody) I'm drawing about 1.7 amps on my 5 volt bench supply, does this sound about right?

Yes, for a fully loaded Atom that is what I would expect.

Dave

Excellent, at least I'm not cooking anything ... nothing was warm ... but I thought I'd check.

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Re: Martin's Acorn Atom (poorly)

Postby oss003 » Fri Aug 04, 2017 11:30 am

Yes, those were the days that you could keep your cup of coffee warm just by putting in on the power supply....... :lol:

Greetings
Kees

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Fri Aug 04, 2017 11:34 am

Please don't keep us in suspense... :D

What are the symptoms?

A screenshot of the screen after power up and pressing reset would be handy.

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 11:40 am

Hi Dave,

I'm at work at the moment, will have something when I get home.

I was just running RF into the old CRT TV I have and it just looked like a mess, wiggly lines.

I'll setup a composite video display when I get back :D

Martin

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Re: Martin's Acorn Atom (poorly)

Postby trixster » Fri Aug 04, 2017 12:23 pm

Martin, send it to Bas for a looksee, that's my advice!
A3020 | A3000 | BBC B + 128K RAM/ROM + 20K Shadow + Pi0 + VideoNuLA
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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 12:47 pm

trixster wrote:Martin, send it to Bas for a looksee, that's my advice!


I was contemplating something along those lines, I'll see if it's an easy fix first, might learn something too, quite impressive with all this mental know how being brought to bear :)

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Re: Martin's Acorn Atom (poorly)

Postby 1024MAK » Fri Aug 04, 2017 2:08 pm

Not all TVs like the Atom, at least not via RF :(

Try using a composite input (often a yellow phono socket). The Atom has composite video available on pin 9 and GND on pin 10 of PL4, which is the connector between where the regulators/heatsink used to be and the cassette DIN socket.

Here is a good Atom schematic:
Image

Mark
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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 2:14 pm

Hi Mark,

Thank you for that.

I've had it on composite before, from memory when I switched it on I just got lots of different shaded blocks with characters interspersed between then, early finish today so we'll see what happens later.

Martin

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Re: Martin's Acorn Atom (poorly)

Postby 1024MAK » Fri Aug 04, 2017 2:20 pm

martinw wrote:I've had it on composite before, from memory when I switched it on I just got lots of different shaded blocks with characters interspersed between then, early finish today so we'll see what happens later.
That's exactly what mine does if it does not reset properly.

So test and confirm if the CPU gets a reset pulse on it's reset pin.
Other possible problems are:
  • The CPU can't write to screen RAM due to a buffer chip problem (or control line to the buffer chips)
  • One of the screen RAM chips is faulty (try swapping RAM chips)
  • A fault is preventing the CPU from executing the ROM code, so the CPU never clears the screen memory

Mark
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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 7:24 pm

Sorry for not replying earlier but I've been making progress.

The video output was intermittent/non-existent to say the least.

A while back I was where I am now (which I will explain later) and thought I would swap out the big chip sockets for turned sockets and it made it worse, so I thought f**k it and left it.

So when I came back to it I remembered. I have since (tonight) reflowed all the turned pins very meticulously.

IMG_5213.JPG


I am now back where I started if you see what I mean :)

Which doesn't seem like much but I am consistently getting to the same point (which is reassuring).

When I apply power I get this.

IMG_5211.JPG


Forgot to mention I've gone minimal too.

IMG_5210.JPG

IMG_5212.JPG


But when I reset/press break (sometimes I have to do it a couple/few times) I now get this.

IMG_5209.JPG


Where it sticks, no typing, no ctrl-g beep.

Next !!!! :)

Martin

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Re: Martin's Acorn Atom (poorly)

Postby oss003 » Fri Aug 04, 2017 7:29 pm

Could mean that the keyboard is not working.
You can try to swap the 7445 and if that doesn't solve the problem, try switching the 8255.

Maybe reseating the chips is enough ...

Greetings
Kees
Last edited by oss003 on Fri Aug 04, 2017 7:31 pm, edited 1 time in total.

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Fri Aug 04, 2017 7:30 pm

This is progress!

As you have a spare 8255, try swapping them over.

If that doesn't do anything, try capturing (and uploading) a logic analyzer trace of the address bus, as we discussed yesterday.

Dave

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Fri Aug 04, 2017 7:33 pm

oss003 wrote:Could mean that the keyboard is not working. You can try to swap the 7445 or the 8255.

You should also easily be able to trace the keyboard scanning with the scope:
- is the 8255 CS input pulsing?
- are the 8255 PA0..3 outputs counting?
- are the 7445 outputs each pulsing?

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Fri Aug 04, 2017 7:43 pm

Cheers, I'll try all this at the weekend, time for a single malt now :D

Martin

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Re: Martin's Acorn Atom (poorly)

Postby martinw » Sat Aug 05, 2017 10:43 am

It's looking like the 8255 chip select is not working properly.

This is the RD pin 5

IMG_5218.JPG


This is the CS pin 6

IMG_5219.JPG


I've swapped out the 8255 the 139 and the 138 with the same results.

I've measured continuity from the 6502 to the address pins on the 139 and 138, all OK.

Any ideas?

Martin

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Re: Martin's Acorn Atom (poorly)

Postby hoglet » Sat Aug 05, 2017 10:54 am

martinw wrote:It's looking like the 8255 chip select is not working properly.

This is the RD pin 5

IMG_5218.JPG

This is the CS pin 6

IMG_5219.JPG

I've swapped out the 8255 the 139 and the 138 with the same results.

I've measured continuity from the 6502 to the address pins on the 139 and 138, all OK.

Any ideas?

When you were scoping the CS pin, your trigger level was 4.32V. Is this because if set lower (e.g. 2v) it never triggers after reset is released?

This is strange, because to get to the point where ACORN ATOM is printed, there will have been several writes to the 8255. Any your have verified continuity, and replaced the ICs involved (were the replacements known to be working?)

Can you double check the CS signal again, but this time measure it on pin 4 of IC 39 (the LS139).

While you are there, measure the outputs of IC23 (the LS138) and see which are active at some point after reset. Have the trigger set to about 1.5V, falling edge. On each pin just release reset and see if the scope triggers.

The only thing I can think of at this point is a short on either the 8255 CS signal, or on the nBxxx signal out of the LS138. The short could be to an adjacent output.

Dave
Last edited by hoglet on Sat Aug 05, 2017 11:01 am, edited 3 times in total.

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Re: Martin's Acorn Atom (poorly)

Postby oss003 » Sat Aug 05, 2017 10:55 am

Can you check pin 1,2,3,4 of the 8255 if there is any activity?

Greetings
Kees

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Re: Martin's Acorn Atom (poorly)

Postby 1024MAK » Sat Aug 05, 2017 10:57 am

Have you checked the connections between chip pins? Just in case one of the sockets has a poor connection?

Also check the connection between the 74LS138 and the 74LS139.

And lastly, if you hook up to the phase2 clock, the relevant CPU address lines and the relevant 74LS138 and the 74LS139 outputs to your 'scope, it should be possible to see if the CPU is actually attempting to read or write to the 8255. And to see if the 74LS138 and the 74LS139 are responding to their inputs.

Mark
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Re: Martin's Acorn Atom (poorly)

Postby 1024MAK » Sat Aug 05, 2017 11:00 am

martinw wrote:Any ideas?

The one inch drop test :lol:
























Only joking :P
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Re: Martin's Acorn Atom (poorly)

Postby Elminster » Sat Aug 05, 2017 1:28 pm

Bang .... joking !!!!??? .... send bill to Mark.


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