H743 flash corruption with brownout.

[geofrancis]

From my testing the H743 chip can corrupt its flash memory if its power supply doesnt come up fast enough. I can make my matek boards corrupt the memory by changing the voltage between 0 and 5v slowly. my understanding is that the cube carriers have an extra power management chip so it doesn't start up until the voltage is adequate.

When this happens the board appears dead since reflashing in DFU mode doesnt fix it, you need to do a full flash erase with STM32CUBE programmer and reflash the board to get it to start up again.

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