Electronics > Beginners
Troubleshooting Microprocessors
Shock:
Here is a video showing testing from an almost dead state. Minimizing the board into a basic running state with the dead test cart seems reasonable to me, especially if you have nothing on the screen. If you have a working board to compare the cart on even easier, with the right triggering you should be able to determine exactly what is working and how clean the signals should be between your two boards.
You should have no problem probing the signals with your oscilloscope it won't load it significantly. If you are concerned about the quality of signals compared to the other board you should look at how you are grounding the probe and use the closest ground. Unexpected noise on the power or specific data line requires attention. As you notice in the video a change in voltage level can indicate a faulty component. In fact not measuring all voltage levels/ripple and ground continuity to all ICs to start with is punishable by death.
In your case don't make any assumptions in your testing or having introduced a marginal fault by accident with the power supply, video and cables. What worked before might suddenly be the culprit until you double check (even if it works on another system). Pays to use two testing methods followed up with measurement to confirm your findings.
For example, the display flickers for you therefore it must be able to work on this system would be an assumption. So if you haven't thoroughly tested the circuitry involved or double checked your changes to the circuit or changes in your testing environment since the last time it was working correctly you need to revisit it.
He mentions at the beginning his ram looks fine but he based that off seeing just activity alone with the oscilloscope. I think there is a large chance however this video covers your problem. Looks like he has the same board revision as well.
bostonman:
I haven't had a chance to watch the video or continue addressing the problem, but I have a question.
Why can't the microprocessor be removed, and a socket be installed that has all the pins either grounded or pulled to Vcc (ignoring the ones that are either a clock in, Vcc in, etc...)? This would allow the entire address and data bus to remain in one state and allow for measuring all the chips to see where issues are.
ogden:
--- Quote from: bostonman on January 07, 2020, 08:04:15 pm ---Why can't the microprocessor be removed, and a socket be installed that has all the pins either grounded or pulled to Vcc (ignoring the ones that are either a clock in, Vcc in, etc...)?
--- End quote ---
No need. If everybody else can find faults w/o replacing CPU with switchers, you can do it as well.
Shock:
--- Quote from: bostonman on January 07, 2020, 08:04:15 pm ---I haven't had a chance to watch the video or continue addressing the problem, but I have a question.
Why can't the microprocessor be removed, and a socket be installed that has all the pins either grounded or pulled to Vcc (ignoring the ones that are either a clock in, Vcc in, etc...)? This would allow the entire address and data bus to remain in one state and allow for measuring all the chips to see where issues are.
--- End quote ---
If the microprocessor is functional it is the main diagnosis tool (think of it as a multi channel signal generator), but lifting pins and isolating is a troubleshooting technique you can use if required.
Watch that last video and this one as well, both useful as they give ideas on how to sniff out the problem from a similar state. Ignore that bit about the ram not being used by the dead test cart and soldering through hole plated joints (aside from verifying they are still connected). But otherwise the only place he went wrong (shown in the video at least) was a few assumptions based on previous work which weren't that fruitful, checking previous work though is a good idea.
NivagSwerdna:
--- Quote from: bostonman on January 07, 2020, 08:04:15 pm ---Why can't the microprocessor be removed, and a socket be installed that has all the pins either grounded or pulled to Vcc (ignoring the ones that are either a clock in, Vcc in, etc...)? This would allow the entire address and data bus to remain in one state and allow for measuring all the chips to see where issues are.
--- End quote ---
Simplistically it can... and people have built gadgets using 5V compatible technology to tweak the lines and simulate the uP however life gets rapidly more complex.... to reduce costs DRAM was used rather than SRAM and DRAM requires refresh so there will at least be bus contention and in some cases (e.g. Z80) the actual DRAM refresh is done by the uP itself... further video architectures differ but some work asynchronously with respect to the uP and use some bus logic to arbitrate between the uP and the video... your gadget would need to respect that arbitration. On very simple architectures you could achieve what you are suggesting... in fact it might be fun to try... an Arduino Mega 2560 would be an obvious starting point as it is 5V and has lots of GPIO but you might find yourself tearing your hair out with the number of wires required and then you would get into the land of a custom PCB for an adapter... and... the rabbit hole goes deep.
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