[chbot] My 2c worth on I2C

[Peter Harris]

We had a problem at work running I2C on a bus between several boxes. In
order to pass EMC requirements it was necessary to put RC filters on the bus
lines which of course screwed up the signal shape and killed the interface.
The answer was to replace the pull-up resistors with constant current
sources set to something like 10mA, these squared up the signals a treat and
put us back in business.

The current source circuits used were similar to the one shown here
http://free-schematic.blogspot.com/2010/02/constant-current-source.html with
the LED replaced by a signal line however we had a PNP/NPN configuration to
reduce the voltage drop across the junctions.

Good luck
  Peter
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