[time-nuts] Net4501's cheap...

lists at lazygranch.com lists at lazygranch.com
Fri May 24 14:22:53 EDT 2013

My solution is not to use the R Pi. An extra $20 gets you any number of superior arm SBCs. Go cortex-A7 type cpu. 

The Beagleboard XM I'm using has a bug in the built-in usb hub (patchable), but it has no issues with hot plugging.  

The R Pi is designed to be cheap, but you spend a lot of time doing work-arounds, hardware and software. At some point, an extra $20 looks like a bargain.

-----Original Message-----
From: Dan Kemppainen <dan at irtelemetrics.com>
Sender: time-nuts-bounces at febo.com
Date: Fri, 24 May 2013 09:44:46 
To: <time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: [time-nuts] Net4501's cheap...

This may be way off topic, but the inrush current problem is probably 
with devices not meeting the USB specification. As it turns out, a lot 
of low end devices do not meet this spec.

I spent some time redesigning the power supply system of one such device 
I was using. This time was covered by the company that built the device, 
and eventually it went into production. High speed clamp on probe showed 
2Amps inrush, where it was supposed to be much smaller than that. That 
caused all sorts of havoc. It worked fine on a PC USB, where the 5V line 
is very stiff. USB hubs were a different story.

My bet is a workaround would be a stiff cap (ceramic, not 'lytic) at the 
USB on the Pi board to counteract empty filters on the hot plugged 
device may help. Maybe a small impedance between the 5V supply of the Pi 
board and the cap would help.


On 5/23/2013 8:37 PM, time-nuts-request at febo.com wrote:
> "by mahjongg ? Mon Nov 19, 2012 12:09 am
> Don't hot plug USB devices directly into the PI, if you must hot plug
> plug into a hub instead!
> Its called "the rush in current problem", and it makes that the
> current PI itself is non "hot pluggable", note that the revision 1 PI
> was hot pluggable, as the two polyfuses prevented any rush in current
> because the fuses had a non zero resistance. But in the current board
> there is literally zero resistance between the PI's 5V supply, and any
> USB device you plug in, that means that if you plug in any USB device
> with empty power decouplers, (which act as a complete short for a very
> small time) then you are actually simply shorting the 5V when you plug
> de USB device in.

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