Re: [Balloon] I2C troubles on Balloon 3

On Aug 17 2011, Wookey wrote:

>+++ Chris Jones [2011-08-17 13:31 +0100]:
>> Hello all,
>>
>> I know some people in the Balloon community have done a lot more
>> work with I2C than I have. I'm now using I2C more intensively and
>> having some problems. It works most of the time but will more or
>> less randomly give up and say, 'i2c-pxa: timeout waiting for bus
>> free'. Adding some debug to the i2c code shows that the PXA's I2C
>> controller is claiming to be busy at these times. Has anyone else
>> seen this?
>
>No, or at least not that I've noticed. My solar controller is using
>I2C down about 60cm of unshielded wire and has generally been
>reliable. It does sometimes jam up but I think that's at the 1-wire or
>OWFS level, not I2C, although it's hard to be sure.
>
>This is all with a very old kernel now (2.6.26 or so and OABI) so may
>not be directly relevant to current experience. PaulF is the real I2C
>expert.

I wouldn't call myself an expert...

I have seen transient I2C errors, probably caused by spikes on the
bus, when using I2C to talk to a PIC that was driving a 12V DC
motor (via an L293). It was usually best to avoid turning the motor
from full on to full reverse as the next I2C message was then usually
missed.

I don't actually recall whether I had that specific error ('i2c-pxa:
timeout waiting for bus free') although I may well have done. I have
seen a few 'timeout' style messages over the years, most of them
entirely due to dodgy soldering (usually my own). These errors were
generally due to shorts with either SDA or SCL to either high, low, or
each other. On one occasion the 'short' was actually the presence on the
bus of a PCF8574 which had been previously been fried by a student.
Removing it made everything work again...

Best wishes,

Paul.


Paul Fidler
--
Cambridge University Engineering Department | Tel: +44 1223 332816
Trumpington Street, Cambridge, CB2 1PZ, UK | Fax: +44 1223 332662