[chbot] My 2c worth on I2C

[Charles Manning]

Of course I2C was never intended to go between boxes.

Perhaps try CAN next time :-).


On Tue, May 31, 2011 at 2:24 PM, Peter Harris <petes.username at gmail.com> wrote:
> Having looked at the actual design again here are some updates:
> The circuit uses 2 PNP devices and may in fact be temperature compensated.
> The current is ~3mA (220R/VEB)
> There is a potential gotcha with an active pull-up:
> When the line is high the current source has no load and so presents a very
> low impedance between the line it is pulling up and Vcc. In this situation a
> device attempting to pull the line low may experience a large transient
> current as the current source stabilises. I know for a fact that this can
> hang the I2C interface on an ST7. The cure for this was to add 47R between
> the current source output and the line.
>
>   Peter
>
> On 31 May 2011 09:48, Peter Harris <petes.username at gmail.com> wrote:
>>
>> 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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