@fzabkar - That's a
very interesting idea
As you say, some assumptions (hopes) in there, but IMHO certainly worthy of deeper consideration.
@Nick_CT - before you give up on this idea due to the worries about soldering, here are a couple of comments. Obviously this is assuming that no-one gives you any other way of doing what you want to do "officially" (without needing to use the "patient" MCU), and also assuming you don't want to / can't move that MCU. I don't have one of these boards in front of me, so please take my comments with a pinch of salt
Nick_CT wrote:
So removing the smooth chip alone won't give me access to the rom assuming there are no other faulty components on the board?
Obviously it's fzabkar's idea, so he may have a different view, but IMHO the answer to your question is
no. The SMOOTH chip drives 2 external NPN pass transistors that power the MCU - so a dead (or removed) SMOOTH chip == dead MCU. We just have to hope that the (unknown) exact mode of failure didn't also fry the MCU...
To be 100% sure that the SMOOTH chip is no longer providing MCU power, the emitter pins of the external transistors on the problem board, are the place to be measuring and you can then compare those voltages to the donor board (obviously being very careful to avoid your test probes slipping
) Another thing that faulty SMOOTH chip could be doing, is providing power, but also (wrongly) keeping the MCU held in reset - and the MCU would then also not be accessible via the PC3K.
Nick_CT wrote:
Your idea in previous post is good, however I don't think my skills are up to connecting up the MCU's Vcore and Vio on such a small scale.
It might not be quite as bad as you think, if you were thinking that you'd have to solder to the pads where the SMOOTH chip is/was. IMHO given the catastrophic failure of that chip, its effect (e.g. loading) on all its external connections is highly suspect, so removing the whole chip is the best option - it's doing no good as it is now, and lifting individual pins on such a chip is possible but non-trivial.
The connections from a donor board's two MCU power supplies which fzabkar suggested, onto the faulty board, would likely be possible onto the emitter pins of the NPN pass transistors (probably Q2 & Q3 - but you'd need to check that). As you can see these are larger (and more robust) than the pcb pads where the SMOOTH chip is located, and so easier for you to connect to.
In order to check that the donor board's MCU power supplies could also power the "patient" board, I would actually go a bit deeper and measure the current drain on the donor board of those supplies (though that would need more soldering) and guesstimate the power dissipation in the pass transistors. Using external power supplies for the MCU "patient" (i.e. not from a donor board) would remove that worry about the current capabilities of those parts of the donor board.
The MCU reset signal (driven by SMOOTH chip pin 18) is more of a concern for accessibility, if you don't have steady hands. I use a microscope for this type of work, though when I was younger, my eyesight was better and I could probably have managed without - but not now
You'd need to either use that SMOOTH PCB pad, or trace the signal from there towards the MCU, to see if you have any easier places to get to that signal (e.g. a possible TP pad) and inject a low->high pulse (and keep it high, if there is no onboard pull-up, to avoid that signal floating).
Anyway, those are just a few comments from me, as an electronics engineer but without such a board in front of me. If I was in your situation, and needed to gain access to the MCU via SATA, and had run out of other options, then fzabkar's idea is one that I would definitely be considering, perhaps with externally generated power inputs to the "patient" board.
Good luck...
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