Karen Shaeffer shaeffer at neuralscape.com
Tue Jun 17 07:32:31 CEST 2014

On Tue, Jun 17, 2014 at 06:42:52AM +0200, Wolfgang Denk wrote:
> Dear Larry,
> In message <5070A944-CC27-4388-91E5-78B0792E590C at usgs.gov> you wrote:
> > 
> > A recent situation has awakened me to a a behavior of U-Boot that maybe should be changed.
> U-Boot related questions should better be discussed on the U-Boot
> (rather than the ELDK) mailing list.  I'm adding the U-Boot list on
> Cc.
> > I am experimenting with a BeagleBone Black (BBB) single-board
> > computer.  Occasionally when I perform a reboot, the BBB never
> > finishes booting Linux.  When I happen to have a console cable
> > connected, I see the U-Boot prompt.  This explains why the BBB has not
> > booted Linux.  But it is a mystery why U-Boot is waiting for a command
> > when I did not request interrupting the autoboot sequence.
> > 
> > I found many others reporting the same experience -- and no solution.
> > Until I found Andrew Glen's 10/28/2013 post at
> > https://groups.google.com/forum/#!topic/beagleboard/aXv6An1xfqI about
> > U-Boot hangs likely due to noise on the UART0 port.  The solution he
> > applied was to rebuild U-Boot to require specific text -- "uboot" --
> > to interrupt the autoboot sequence.  (BBB have no flash, so there is
> > no opportunity to permanently alter any U-Boot environment variables,
> > if this is settable.)
> You should be able to store the environment on SDCard.
> > It occurs to me that this might be a more common occurrence on any
> > number of circuit boards.
> No, this is in no way a common issue.  If you have any such line noise
> on a serial port, you should start looking for hardware (design)
> problems.  This is _not_ normal.  IF several boards of a specific
> brand show such a problem, then I'm willing to bet that it's caused by
> broken  (or simply too cheap) hardware design.
> No industrial grade board I've ever seen has any such issues.
I agree line noise on a serial port is a hardware design problem. But I
have seen such problems on servers in data centers, where there was a lot
of electrical noise coupled into the serial port cable, acting as an
antenna coupling that noise into the serial port terminal. This only
happened on a small percentage of that hardware model's instances in that
lab, but it was fully investigated and was easily repeatable on the
defective hardware. We patched the software to close a related security
issue and documented the potential hardware issue. It wasn't an issue
we cared to engage the hardware vendor about.

Karen Shaeffer                 Be aware: If you see an obstacle in your path,
Neuralscape Services           that obstacle is your path.        Zen proverb

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