I spent some time poking and prodding now that I've got another functional board to compare against. Observations:
1) double checked NRST is high (2.85v) on both boards as expected
2) double checked BOOT0 is low on both boards as expected
3) Voltage across C1 is 2.85v on both boards as expected
4) Voltage across C2 and C3 is 1.25V on the good board but 140mA and 100mA on the bad board
5) Continuity test shows the SWD connections are good from the debug connector to the STM
6) I've visually inspected both boards for any obvious problems and I discovered the power switch is completely gone on the bad board. It's been busted for a while now but had previously been mostly intact. Based on the schematic I don't see any reason for this to be related, right?
Observation #4 is possibly interesting but probably not: C2 and C3 are coming off the voltage regulator in the STM. The datasheet says the output of the voltage regulator should be 1.2v, and is enabled by default on this particular package (larger packages expose a pin to bypass the functionality). I guess all this means, though, is that the STM isn't booting properly.
Communication to the STM32 fails -- something's broken
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Re: Communication to the STM32 fails -- something's broken
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Re: Communication to the STM32 fails -- something's broken
I've returned to trying to fix this board after a year
What's the pinout of the X1 crystal? I'm not seeing any voltage on any pin on the broken board and am wondering if the crystal is broken, or if it's still an STM32 problem (i.e. does the STM32 power the crystal from the PH0/PH1 pins?). I'm thinking it's still a problem with the STM32 since I can't get the chip to boot into DFU mode even by manually shorting out the BOOT0 pin at power-on.
What's the pinout of the X1 crystal? I'm not seeing any voltage on any pin on the broken board and am wondering if the crystal is broken, or if it's still an STM32 problem (i.e. does the STM32 power the crystal from the PH0/PH1 pins?). I'm thinking it's still a problem with the STM32 since I can't get the chip to boot into DFU mode even by manually shorting out the BOOT0 pin at power-on.
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Re: Communication to the STM32 fails -- something's broken
I think it is hard to measure the crystal pins and as the STM32 boots from the internal crystal I wouldn't expect the behavior you are seeing but I'm not sure. Can you see any damage to the crystal? Can you program the STM32 through the SWD?
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Re: Communication to the STM32 fails -- something's broken
Makes sense. As you say, I guess DFU and SWD modes should work even if the external crystal wasn't working. Oh well, I guess the mystery will continue for now. Not a big deal because I have other boards, but I like the challenge of trying to diagnose the hardware problem
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