[ETS/PARC List] Raspberry Pi power

[CHRISTOPHER FINE]

Len and fellow list members - 

First, Len, thanks so much to you and the team for all the repair and maintenance work on the repeaters, and for keeping us so well informed.

Couple of comments on your Pi USB power observations…

I’ve found exactly the same thing: Power is key to the proper operation of the Pi.  And USB power supplies can be less than reliable (both the power source and the cable).

First comment is that the Pi often shows misleading symptoms when there’s a power problem.  The symptoms you were seeing are typical, where part of the Pi works and part doesn’t.  You were able to boot the OS, but not Asterisk, for example.  I suspect, without real proof, that this is because the various chips have different requirements and tolerances for voltage and current.  That explains why, if power isn’t accurate, some of them work and some of them don’t. Since most of the chips are fully-contained systems, if that one chip works, then a whole portion of the system works.  Same is true for add-on boards. I had a case where the power wasn’t correct, and the same sort of thing happened - the Pi passed power-on-self-test and booted, but the software wouldn’t run properly.

Second comment, for any fellow Pi (or other USB-powered-board) fans, is that it’s worth the time and possibly money to get a USB power adaptor and cable that’s high quality and has more than enough current capacity.  Usually there are a few models either sold with the device or listed as compatible.  Your average phone charger isn’t powerful enough.  I learned that the hard way.  Also, these power supplies don’t always just fail to zero.  Sometimes they start declining.  So one day your Pi stops working, but some of the lights are on and it’s a situation where the voltage just dipped below the operating threshold.  

USB is an interesting story all round - a classic case of something designed for use cases A and B, and ending up covering use cases A through Z.  In theory, USB-C is better and more consistent.

73 all,
Chris KC2PI

> On Aug 1, 2019, at 8:55 AM, Len Griffin <lengriff at optimum.net> wrote:
> 
> We've had some intermittent problems with Asterisk shutting down at our
> K2ETS remote site, and I've discovered something important. The Raspberry Pi
> MINUMUM operating voltage is 4.8v. The USB cable you use to power it is
> CRITICAL. To check,  I used a USB cable which I cut and stripped, and
> measured across the red and black conductors. I plugged this into a USB port
> on the Pi. I checked 4 cables on a Pi running with one URI connected, and
> got 4 DIFFERENT Pi voltages. The lowest reading was 4.5 volts on 2 cables!
> One read 4.7 volts. Finally, the fourth one read 4.92 volts. These cables
> are not rated for current, so you must check them!
> 
> 
> 
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