zenbike
Posts: 8
Joined: Tue Jul 10, 2012 5:19 pm

Pi reboots when 3.3v jumper is placed?

Sun Oct 21, 2012 2:23 am

Is it normal for the procedure for testing the 3.3v power on J7 to reboot the Pi? It's very consistently causing it to power cycle or at least to reboot from the color test screen, as if just powered on, when I touch the J7 pin and a ground, or when I place the jumper on J7 to power the board.

I do get 3.31v power on the multimeter.

I assume this is not normal. So really, the question is why this might happen, and what to do try to find the problem...

User avatar
Gert van Loo
Posts: 2487
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Pi reboots when 3.3v jumper is placed?

Sun Oct 21, 2012 11:00 am

When you say you are measuring 3.31V, I assume you measure that on the J7 connector.
That means your board is connected and your regulator is working.
It does not say anything about the rest of the circuits.

Before you place the jumper, measure the resistance between a 3v3 point and a GND point
(There are several of those) On a typical board the resistance should be about 200K..800K.
If it is very low (smaller then 10 Ohms) you have a short. All you can do is look at each solder connection
very carefully and attentive. Using a good magnifier will help there.

ejfield
Posts: 17
Joined: Fri Oct 19, 2012 10:21 am

Re: Pi reboots when 3.3v jumper is placed?

Mon Oct 22, 2012 8:33 am

When you say it reboots, does it reboot with the jumper still on, or only when you remove the jumper? If it's the former, it's unlikely to be a short, since this would take down the 3v3 permanently. In this case, it could be that the inrush current to the gertboard is causing a brown-out on the 3v3 rail, in which case trying a PSU with a bit more current capability might be the answer.

zenbikr
Posts: 3
Joined: Fri Oct 12, 2012 9:21 am

Re: Pi reboots when 3.3v jumper is placed?

Mon Oct 22, 2012 11:58 am

Originally, it was rebooting when the DMM probes connected ground to the J7 pin, still reading 3.31v there. Afterward, when I placed the jumper, it still rebooted. Once it boots, it works fine. (Although I'm not done testing the various components.)

Removing the jumper, and trying again in the morning, it appears to no longer have the issue, although I apparently did nothing to fix it.

HumblePi
Posts: 7
Joined: Thu Jul 19, 2012 8:54 pm

Re: Pi reboots when 3.3v jumper is placed?

Tue Oct 23, 2012 5:08 pm

This is precisely what happened on my board. I checked with a DMM on the 3V3 and 0V pads getting 3.31V. As soon as the jumper touched the two pins the Pi re-booted so I pulled the jumper fearing a direct short onto the Pi itself.

I spent a whole evening checking for bad joints or perhaps a short under one of the surface mounted components. Oh how my eyesight has suffered these last years! I couldn't find anything so put the jumper back on the board and then plugged the power (5V, 1A Samsung switching) into the Pi and it ran fine.
This could just be a dip in the Pi's voltage causing a brown-out but to my intrained eye, the Gertboard doesn't look like it is full of caps and stuff that soak up amps on start-up?

All seems to work from the limited amount of doodling I have done but I would like to know what the cause is?

G

zenbike
Posts: 8
Joined: Tue Jul 10, 2012 5:19 pm

Re: Pi reboots when 3.3v jumper is placed?

Wed Oct 24, 2012 2:52 am

HumblePi wrote:This could just be a dip in the Pi's voltage causing a brown-out but to my untrained eye, the Gertboard doesn't look like it is full of caps and stuff that soak up amps on start-up? All seems to work from the limited amount of doodling I have done but I would like to know what the cause is?
That's my thing, too. I got the Pi and the Gertboard as educational tools. When things like this happen, it's no problem, but I want to understand what is happening. There has to be a reason...

plugwash
Forum Moderator
Forum Moderator
Posts: 3621
Joined: Wed Dec 28, 2011 11:45 pm

Re: Pi reboots when 3.3v jumper is placed?

Thu Oct 25, 2012 1:52 am

I don't think this was related to the capacitance on the rails. most of the capacitance seems to be before the 3V3 jumper and the gertboard uses a seperate 3.3V reg from the main board anyway. The most likely explanation I can think of is current surges caused by either static on your body or by the fact that floating mains powered devices have a nasty habbit of floating to half the mains voltage (with a very high impedance so it shouldn't be hazardous to users but it can still cause problems for electronics)

BTW if you do have a short there is a trick to finding them. Get a current limited bench supply, set the current limit to about 1A and use it to apply power to the short. Then set your multimeter on it's most sensitive voltage ranges and you can trace the current flow by measuring the volt drop in the tracks.

Bernie.H
Posts: 14
Joined: Wed Oct 03, 2012 3:16 pm

Re: Pi reboots when 3.3v jumper is placed?

Thu Oct 25, 2012 12:33 pm

I too suffered the 3v3 jumper reboot, and really thought I hade messed the board up, as I did mess the c17 capacitor pad up, but seems I got away with that one.
However, I digress. I've fainally got my board working and am a very happy bunny.
I removed all the chips, and connected the Pi and placed jumper 7. Pi powered up ok, no reboot.
I then proceeded to fit the chips one by one onto the board, testing each one for reboots. This all worked and I was faced with the last one, the ATmega which I purposely left till last (c17 pad problem).
I examined the board for the upteenth time, and decided to resolder all the pins incase of dry joint. This I did, refitted the chip and fired the Pi up. It worked, and no frying with the regulator. Job done.
Ran the leds program and admired the dancing leds.
Now I need to run all the tests, but suffice to say, no more reboots and the board is working.

Return to “HATs and other add-ons”