[SI-LIST] Re: I2C Noise

Steve,

Thanks for your insight.  Yes, I meant to state 100kHz instead of 100mHz.

I think that we might be dealing with item 2 (noise
coupling) as you described below.  We have 3 AC/DC power supplies that each
operate on dual power grids for redundancy purposes.  Each power supply has 2
sets of I2C wires connected to it, one for each power grid.  I believe that each
set consists of SCL, SDA, VDD, VSS.  At one point in the system all 6 sets of 
I2C
wires are bundled together.  

On a system that was failing fairly consistently, we
went in and seperated the cumulative bundle into individual sets by physical 
distance.
We have not seen the failure replicate since.  I don't believe that the orginal 
wire
routing included any special shielding.  I can confirm with a digital scope per 
your
suggestion.

-Chris
 

steve weir wrote:
> 
> Chris,
> 
> I2C is a very high impedance bus, typically 4700 ohms pull-up.  I think you
> mean 100Kbps, the legacy mode.  It is easy to mess I2C up with grounding
> issues.  The states that you are referring to are probably being conveyed
> via IPMB over I2C or similar.  There are several things that can go wrong:
> 
> 1) Incorrect grounding is fouling basic I2C signaling.
> 2) Noise coupling is fouling I2C signaling.
> 3) Voltage translation issues.  The original I2C bus was 5V TTL.  Mixed
> voltages can also cause grief, as can hot swap.
> 
> Check 1 and 2 with any of the many I2C analyzers available, or just put a
> scope on the SCL line in infinite persistance.
> 
> 4) An aberrant device is violating I2C bus negotiation contending with the
> actual master and fouling SCL, SDA or both..
> 
> 5) Your misbehaving peripheral is a poor implementation of I2C and/or
> whatever protocol you may be running on top of it.  Some devices do not
> implement the timing as per the spec.  This can be particularly true of
> microcontrollers that do not include dedicated I2C hardware.  But even a
> number that do violate the specs, or rely on careful programming to comply.
> 
> I would start by looking at the SCL line with a decent digital scope
> first.  If you don't find your problem there it is time to look for mixed
> voltage and/or hot-swap issues.  If you get all that worked out, then I
> would move up to the link layer and see if you are a victim of some bad
> firmware.
> 
> Regards,
> 
> Steve
> 
> At 02:04 PM 5/21/2004 -0700, Christopher Jakubiec wrote:
> >All,
> >
> >Has anyone encountered problems with EMI/noise issues on I2C wires and/or
> >busses?  We use industry standard I2C to interface with our bulk 208V/48V
> >AC/DC power supplies.  We are observing instances where control and status
> >signals (connected via I2C) for the AC/DC power supplies appear to be
> >intermittently
> >in the wrong state.  I believe that we are currently running the I2C clock
> >signal at
> >100MHz, and our wire lengths are significant on the order of 2-3ft.
> >
> >Thanks,
> >
> >Chris Jakubiec
> >Sun Microsystems
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