Forensics on a dead FRDM-KL25Z

30 Jul 2014

Hi,

I seem to have destroyed a KL25Z tonight, through some stupidity. My design has the KL25Z powered through a 3.3V down converter, but when I was assembling the circuit I realized I only had a 5V converter, no 3.3V in stock. So I simply changed the power lead to use the 5-9V in rather than the 3.3V pin.

Unfortunately I'd forgotten that there was also a nRF24L01+ module powered by the same 5V supply and that has a max of 3.6V. So I fried the nRF24L01+, and that shorted VCC to GND (at least for a while - its open circuit now but it was showing a short when I first tested it shortly after the event). So the microcontroller turned off, and nothing seemed to be working. In order to find out what was going on, I plugged the KL25Z into a USB port, which then destroyed diode D2 (totally fried) and D1 (looks OK but appears to be open circuit).

The thing is, if all I've done is destroy the input protection diodes, the KL25Z should still work when powered from the 3.3V pin. But it doesn't, it seems to be completely dead. Are there any suggestions as to what to do next for (1) determining if the KL25Z really is dead, or of there is something else relatively straightforward that I could fix on it, and (2) is it really plausible that shorting out the VIN pin is the only problem? It is also possible that the fried nRF24L01+ module was at some point putting 5V across an input pin.

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