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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

Mangostain
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  • 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 08 '19 at 02:23
  • 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. https://howchoo.com/g/mwnlytk3zmm/how-to-add-a-power-button-to-your-raspberry-pi – tlfong01 Oct 08 '19 at 02:27
  • @tlfong01 The poster has a Pi 3, not Pi 4. – Dmitry Grigoryev Oct 08 '19 at 06:44
  • Try touching an oscilloscope probe with a stray wire that you hold in your hands. You will be surprised. – Dmitry Grigoryev Oct 08 '19 at 06:53
  • @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 08 '19 at 07:03
  • @Dmitry Grigoryev, my second comment is related to using GPIO to turn on/off Rpi. I read about 10+ discussions and tutorials, and this is belongs to top 3 of all I read so far. The last one I read is some weeks ago, when a guy said about similar things, but added a long instruction to how to update the boot loader. Now this morning before I recommend the tutorial, I read it again to make sure it is a good tutorial (over selling or over recommending a product/service is against my engineering societies's code of conduct.) , to continue, ... – tlfong01 Oct 08 '19 at 07:11
  • Anyway, when I read it again, I was glad that the author has an update that only applies to buster release 2019sep26. I think this is good for anyone who read this before, no harm if the reader is using Rpi3 or Rpi4. Actually the article was written for Rpi3, and updated for Rpi4. This is one of the reasons that I respect the author and recommend the article. You might have read my other very long "tutorial" on GPS, LIRC, DS18B20, Watchdog timer ( the list on other forums is lont!etc. As soon as possible, I updated my with a comment that I have verified that it is also good for Rpi4. – tlfong01 Oct 08 '19 at 07:15
  • You might like to comment on my explanation, before I think of other things related. No hurry though. – tlfong01 Oct 08 '19 at 07:16
  • @tlfong01 Sorry, please see the update on my post, one extreme of the wire was touching the SCL pin.... Now im at work, I will read carefully all your reply when @ home, thank you! – Mangostain Oct 08 '19 at 12:35
  • @Mangostain, Sorry if I have misunderstood you situation. My long comments are mainly to clarify >Dmitry Grigoryev's possible confusion of me pointing out something about Rpi4, but your question has not mentioned Rpi4. Perhaps I should have said that my answer is applicable to both Rpi3B and Rpi4B. – tlfong01 Oct 08 '19 at 12:42
  • I tried to explain the human body and the dangling wire from an I2C pin (actually also other "floating" wires) acts as an antenna picking up noise. You might like to read my other answer trying to explain the noise picked up in SPI wiring: Raspberry Pi 3B+ interface with Heart Monitor AD8232 using MCP3008 and SPI pins: https://raspberrypi.stackexchange.com/questions/96482/raspberry-pi-3b-interface-with-heart-monitor-ad8232-using-mcp3008-and-spi-pins. The human body picking up noise confused even ECG engineers who invented the clever "Enhoven Triangle" circuit to eliminate the noise, ... – tlfong01 Oct 08 '19 at 12:55
  • And after I explained why floating wire can as antenna picking up noise and wrongly turn on a sleeping Rpi. At this point I noticed that you are trying to find a way to turn on Rpi. I remember I saw another related article using a button, and I thought you might be interested to read it. The confusion actually starts at this point. I found the article mentions that the trick worked for Rpi4B only for buster release 2019sep26. I was very happy to read this, so even your question has nothing to do with Rpi4B, but you or others might do it later. So I mentioned this "off topic" good news. – tlfong01 Oct 08 '19 at 13:09

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