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GPIO troubles

Posted: Mon Dec 10, 2018 12:08 am
by Peter_Szibo
Hi all,

So I finally opened my new raspberry pi 3 and installed rasbian off the preloaded sd card. I followed some tutorials from this site to flash and LED and I just coulnd't get it to work. After a long troubleshooting session I started checking voltages on the gpio and came to the realization that it is completely different from what it should be. It has only one ground connection that is actually a proper ground, mainly GND 39.
Here are the voltage read outs on some of the pins(Didn't do all):
3.3V = 4.4V 5V= 5V
GPIO2= 3.3V 5V= 5V
GND = 4.4V

Am I missing a driver for it?
Any help would be greatly appreciated.


Re: GPIO troubles

Posted: Mon Dec 10, 2018 6:25 am
by rpdom
Is it possible you are measuring the pins the wrong way?

All the Ground pins on the Pi are connected together. It is impossible for any of them to be anything other then 0V (when compared to any other ground pin).

Check for the proper orientation of the pins.

Getting voltages like 4V, 4.4V etc suggests your voltmeter may have a low battery and is giving false readings. This is quite common.

As a quick test, power up your Pi just the power supply connected and no SD card. Put the negative (black) lead from your meter to the metal outer shell of one of the USB sockets. Put the positive (red) lead to Pin 2 of the GPIO connector. This is the pin nearest the corner of the board. You should get a reading of 3.3V.

Re: GPIO troubles

Posted: Mon Dec 10, 2018 9:49 pm
by Peter_Szibo
Sd card is removed.

Re: GPIO troubles

Posted: Mon Dec 10, 2018 10:19 pm
by rpdom
Peter_Szibo wrote:
Mon Dec 10, 2018 9:49 pm
Sd card is removed.
Ok, I was wrong. Pin 2 is 5V, not 3.3V. Major brain failure.

Your readings look normal.

Battery test shows 9.6V (ish) which is fine for a new 9V battery.

Next test showed about 5V between Pin 2 (+5V - black lead) and pin 39 (ground - red lead). That is normal.

Then you got nearly 5V between Pins 37 and 35, and Pin 2. Probably both of those GPIO pins were low, so a bit under 5V is normal.

Actually, I can't see anything wrong with your readings. The pins that showed 1.4V are at 3.3V level. Comparing them to 5V will show -1.4V.

Just one thing, be careful with that probe. Sometimes shorting pins together isn't a good idea.

Re: GPIO troubles

Posted: Tue Dec 11, 2018 9:45 am
by Burngate
You appear to be putting the black probe on pin 2 - one of the two 5v pins - so I think you're counting from the wrong end of the header.
But I've been wrong in the past, and I'll be wrong in the future.