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This is a model 3B+, due to some under-power warnings on the screen I was testing the actual voltage between the 5V and ground pins on the header, but managed to touch pin 40 (SCLK) to pin 39 (GND) with the multimeter probe.

This caused the Pi to reset and seemingly be dead - the polyfuse is fine (5v present at test point P7), the red power LED is lit but I don't get anything from the activity LED at all. I've tried the SD card in an identical 3B+ and it boots fine, also tried a fresh SD card with a new install of Raspbian, no help.

Have I just killed to the board, plain and simple, or is there anything else worth trying to resurrect it?

(And if anyone can explain why shorting pin 40 to ground would have such a dire effect, I'm also curious...)

  • "why shorting pin 40 to ground would have such a dire effect" -> Because it's a short, which means there was no resistance from a voltage source to ground, which means the amount of current involved is only limited by the how much it takes to cause enough damage that resistance is introduced (e.g., if the wires burn through). Realistically the pin needs to be configured as an output set to high; the default state is an input, unless you had SPI enabled, since pin 40 aka GPIO 11 is used there as the clock (SCLK) line -- which would be an output. – goldilocks Jun 29 '19 at 14:00
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Even with 50 years experience as an Electrical Engineer I wouldn't poke at a 0.1" header with a multimeter probe (although the 50 years experience probably taught me the hard way).

Attach Du-pont leads to the pins, and measure the flying leads.

Test the voltage on Pin 1 (3.3V) - if this is absent the Pi is dead.

Connecting Pin 39 and 40 will not cause any damage - I do this regularly, and have a push button connected there to shut the Pi down. Even if Pin 40 (GPIO 21) is configured as an output this extremely unlikely to cause damage.

I suspect you may have shorted Pin 1 & Pin 2 - which is invariably fatal.

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  • I guess this is my learning the hard way :) I shall invest in some DuPont leads for the future. – James Turner Jun 30 '19 at 9:47
  • Tested Pin 1 and indeed it's absent (well 0.7v or something), so I agree with the guesses, likely I shorted pin 1 to pin 2. I am a muppet. – James Turner Jun 30 '19 at 12:24
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I don't see that connecting pin 40 (GPIO21, aux SPI SCLK) to ground would have any bad effect.

Most GPIO are set as inputs. Connecting an input mode GPIO to ground or 3V3 is normal operation and has no bad effect.

If the GPIO was set in output mode and set high then it might damage the GPIO if connected to ground for an extended period. I don't think a brief (few seconds) touch would do any harm.

You are more likely to have damaged the Pi/GPIO by connecting 5V to a 3V3 pin or connecting 5V to a GPIO.

The most likely cause of damage is you inadvertently connected 5V to a 3V3 pin.

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