[SI-LIST] Re: I2C Noise
- From: Christopher Jakubiec <Christopher.Jakubiec@xxxxxxx>
- To: steve weir <weirsp@xxxxxxxxxx>
- Date: Fri, 21 May 2004 15:24:42 -0700
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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