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MCU - 100nf cap to GND before and/or after the inductor AVCC pin
Siwastaja:
--- Quote from: thm_w on May 19, 2022, 09:46:04 pm ---No one actually addressed OPs question.
Will the lack of a cap cause the MCU to blow?
--- End quote ---
It's possible. Inductive supply and no bypass capacitor sure is a good way to create voltage peaks. AVDD definitely has some switching going on, and maybe the distributed on-chip capacitance helps bypass it, but maybe not. Now whether these voltage peaks are energetic enough to actually break things... who knows. It's also possible such a spike would somehow cause latch-up and self-destruction.
Maybe not very probable, but completely possible. I tend to just swap parts whenever I make a mistake where I suspect non-zero probability of having a dead chip; I have seen weird things from partial damage and they can be PITA to debug.
T3sl4co1l:
--- Quote from: Siwastaja on May 20, 2022, 12:28:12 pm ---Maybe not very probable, but completely possible. I tend to just swap parts whenever I make a mistake where I suspect non-zero probability of having a dead chip; I have seen weird things from partial damage and they can be PITA to debug.
--- End quote ---
In fact just the other day I seem to have zapped an AVR-DA, with the result that the ADC drawing quite significant current (~0.1mA) from whichever mux channel it's connected to at the time. That is, current into the AINx pin, depending on MUXPOS/NEG selection. Pin functions largely seem to be normal, CPU working 100% as normal. Best guess at cause, ESD on one of my debug wires, in particular one from the analog IO bank, which happened to just slightly fail in this way.
I have an ATMEGA32 laying around I could poke at and measure AVDD current, though I don't really have the time to do so. Or, hmm I think I have one with a program running the ADC even, though it's on a Olimex board I'd have to cut traces to get at AVDD alone I think, dang...
Tim
eugene:
Ugg. "Should the cap go before or after...?" After what? I've seen articles in respected electronics trade magazines that expressed the issue like that.
The answers to all of these questions are nearly obvious if one pays attention to current (and current loops) instead of limiting their thinking to voltages. Any reader that doesn't understand what I mean -- or worse, disagrees -- should start training themselves to think in terms of current. It really is that simple. Learn to think in terms of current flow and questions like these will have obvious answers.
tom66:
I once nuked the AVR8 on my 3D printer by shorting the hotbed sensor to 24V. The result was all the sensors read 1023, but no channel showed any short circuits. A very "transparent" failure, that would only be obvious if you replaced the chip.
Psi:
The failure of the latest chip (3rd) was AVCC pin shorted to GND and pulling down 5V rail through the AVCC inductor.
I was lucky and was powering it off a lab PSU with a 300mA current limit, so I could check with the FLIR and see where the current was going before the chip physically popped.
The series diode and AVCC inductor were getting hot, and there was a tiny area of the MCU package getting hot too.
In the previous 2 chip failures it popped/smoked instantly due to being on battery so I had no idea what happened.
That's what made me think about the AVCC pin and check the schematic to discover no cap to GND on the MCU side.
I assembled 2 more units this time with a bodged in 1uF cap form AVCC to GND and those didn't blow up.
That being said, i have a test board here without any cap bodge and it has been running totally fine for months.
So it maybe silicone lottery as to how sensitive the AVCC pin is to overvoltage/latch-up.
I have been meaning to put my scope on that original board to check whats happening at startup, but have not had time yet.
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