[SI-LIST] Re: I2C Noise
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Christopher.Jakubiec@xxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 21 May 2004 14:59:59 -0700
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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