Michael Rigby-Jones wrote:

> What does this resistor protect against?  Surely it just reduces noise
> immunity by raising the logic zero level when the slave is sending data to
> the master?


Hi Michael,

The I2C specification has many references to these series resistors that
will answer your question:

"Optional series resistors Rs protect the I/O stages of the I2C-bus devices
from high-voltage spikes on the bus lines and minimize ringing and
interference."

My philosophy on this is why take a chance when resistors are just a few
pennies each? Like I said before, it's very cheap insurance whether one
thinks they need them or not, and in spite of the fact that the
specification calls them "optional" (although Philips goes to great length
to help the designer choose the correct value of Rs for the designer's
particular application).

It's defensive design -- anything that a designer can do to make a more
robust design is a good thing -- even if the I2C device (like an EEPROM) is
not in a computer monitor application (see picture tube flash-over reference
below).

A few more excerpts from Philips' I2C Bus Specification (Version 2.1,
January 2000)

"series protection resistors"

"As shown in Fig.36, series resistors (RS) of e.g. 300 ohms can be used for
protection against high-voltage spikes on the SDA and SCL lines (resulting
from the flash-over of a TV picture tube, for example)."

"Fig.36 Series resistors (Rs) for protection against high-voltage spikes."

"Series resistors Rs are optional. They protect the I/O stages of the
I2C-bus devices from high-voltage spikes on the bus lines, and minimize
crosstalk and undershoot of the bus line signals. The maximum value of Rs is
determined by the maximum permitted voltage drop across this resistor when
the bus line is switched to the LOW level in order to switch off Rp2."



Michael, I hope this helps you out and answers your question.

Best regards,

Ken Pergola


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