Teensy 4.0 - shorted 3.3v rail?

[smuggler]

Hi guys,

I'm pretty sure i know the fault but not sure on the cause, just asking for theories. So I soldered wires to all of the GPIO's on the board, no wires to power solder points or programming/usb points. I connected the other end of the wires to a small pcb I have had fabricated but this board was unpowered so no excess voltage came from the board. Also checked all wires and points for short circuits, all ok.

The board worked once, so it allowed me to upload a sketch then died very quickly afterwards. The 3.3V point at the end of the board opposite the usb connector has 2ohm resistance to ground and the 3.3V point next to the usb connector is a dead short so I assume somehow the rail shorted. Now there's no possibility of 5V or higher being applied to any point on the board as there is no external voltage and the board was powered by the usb cable.

I did handle the board with my bare hand at one point trying to press the reset button and this is the only thing I could think of is perhaps shorting the 3.3V and GND at the end of the board with my thumb. Is this likely to be the cause do you think? To be fair, I really should have taken more care and do know better but was just being lazy.
Hope this all makes sense and am I right to assume the board is now useless?

Thanks

 

[MarkT]

What was on the small pcb? An unpowered pcb with active devices is a no-no for connecting to signals due to phantom-powering.

 

[smuggler]

It has a parallel flash rom chip but nothing else was connected to it other than the pins between Teensy and chip. That might actually make more sense. For future reference, how would I design the board so that this doesn't happen, or is it just something that has to be worked with?

 

[MarkT]

If a board has a chip on it, that chip needs to get power and ground before any signals are applied. With CMOS logic all pins
connected to the power and ground through anti-static protection diodes, so if power isn't supplied it gets taken
through these diodes from the signals, possibly overloading the source of the signals and damaging that device, or even
triggering CMOS latch-up (very bad news).

 

[smuggler]

Ok thanks, that makes sense. I just remembered in the main loop of my sketch there was no code to actuate any of the GPIO signals but there was setup code to set the states of the pins to input/output, would this still be enough for the chip to have drawn power from those pins?