I was searching for ways to power on my rpi from shutdown (o halt) mode, then I noticed somewhere else that shorting SCL and GND will turn on...

Well in fact, I tried this and my RPI turned on ONLY with touching the wire (the other extreme was not touching GND or anything else)

I dont know why this occurs....

Can anyone explain me? (BTW I have I2C mode turned on)

UPDATE: Sorry, I meant that RPI turned on ONLY with touching the SCL pin with a short wire, (the other extreme of the wire was not touching GND or anything else)

Sorry for my english

  • Hi @Mangostain, Ah, let me see. If Rpi SCL/SDA in I2C mode, they are each connected with a built in pull down 1k7 resistor to Vcc, and any signal to these pins can do something. Now you are using connecting wires from these pins to your I2C devices. These connecting wires, if a "little bit" long, say > 30 cm, acts as a "radio" antenna (perhaps weak) , pickup up nearby noises, say 50/60 Hz mains noise. Now if you are holding the "antenna" by hand, you big body joins the antenna to form a big antenna. So in your story, you are feeding mains signal to Rpi GPIO to switch on/off power.
    – tlfong01
    Oct 8, 2019 at 2:23
  • And the following tutorial might help you understand more. The tutorial mentions that Rpi4B bootloader need to be modified. But I heard that buster 2019sep26 release will take care of that. howchoo.com/g/mwnlytk3zmm/…
    – tlfong01
    Oct 8, 2019 at 2:27
  • @tlfong01 The poster has a Pi 3, not Pi 4. Oct 8, 2019 at 6:44
  • Try touching an oscilloscope probe with a stray wire that you hold in your hands. You will be surprised. Oct 8, 2019 at 6:53
  • @Dmitry Grigoryev, many thanks for point out that. This time I can explain. (1) The OP asks about I2C wire picking up noise. Actually this is a generic thing which applies to all computers, from Adruino to Microbit, Rpi, ... you name it. I answered with this in mind. So you see my answer is general, not specific to Rpi3B. Actually I thought about adding that my answer applies to floating wires in general. So this comment should not mislead any users. Counter explanations welcome.
    – tlfong01
    Oct 8, 2019 at 7:03


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