Issues with interfacing two boards via I2C through cable

[stanislavb]

Hi,
The system include two boards connected each other with 1.5 - 2meter cable. The interface is I2C. Do I have to implement any impedance matching or standard pull resistor enough to get good signal shape?
Thank you

 

[betwixt]

i2c cable length

The simple answer is yes. But, it depends on the I2C speed you are using and to some degree on the number of loads you are placing on the bus.

I2C does not have a minimum speed so you might be able to go slower if the cable length becomes a problem, that depends on how quickly you need to pass information around the system.

All I2C lines are open-collector (or open-drain) so you absolutely must use pull-up resistors, preferably at the slave end of the cable where they can help to dampen reflections. The value again depends on the bus speed. Lower value resistors will give sharper rising edges on the waveform, allowing higher speed to be used, but also need more current to be sunk by the driver to pull the signal lines down again. You should refer to the manufacturers data sheet for optimal values.

Try to use low capacitance screened cables if possible and remember that the ground potential between the two boards will appear as noise to the slave so keep it as low as possible.

Brian.

 

[treitmey]

i2c cable

If you need to go farther, or are having problems, take a look at this
http://www.ccsinfo.com/forum/viewtopic.php?t=39938
He is a great poster. He knows what he is talking about.
Follow what he says and use that chip P82B96L.
**broken link removed**

Also see figure 1.
http://www.standardics.nxp.com/support/documents/i2c/pdf/an255.pdf