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Re : I2C Routing SCL,SDA as 100ohm differential pair

kannan sai

Hi Folks,

I have done many designs with I2C and routed SCL,SDA as 50ohm SE lines.Recently I have done this as 100ohm diff pair for one of our customer. Whether it is ok in-terms of functionality?

  • RFinley

    Most of the time, 100 ohm diff will still work.   But, when it doesn't work, it will be intermittent and cause blood to shoot out of the ears of your software nerds.

    100 diff pair advantages don't exist without a differential transmitter and receiver on both ends.  Without that, you are just coupling noise from your clock line to your data line. 

    May not notice at short lengths.  But, after a certain length, you will have random intermittent things happening.   You don't want that in production.

    Usually what causes me to grip the desk tightly is engineers running I2C through multiple boards and none of the 100-mil headers have ground pins adjacent to these edge-sensitive lines.

    I know software is going to blame me for that.

  • excellon1

    I2C can be routed as a diff pair, but with I2C you need to take into account "CrossTalk" between the clock and data line. You would want them separated so that crosstalk between the two lines is minimal. The actual buss speed for i2c is very slow. Something like an 8mil trace with a spacing of 15 mil between SDA & SCL would be good. From the perspective of crosstalk you would be looking at an attenuation level of about -40dB for a risetime of 300ns in either I2C fast or Highspeed mode. One note, when using i2c you would want those SDA and SCL lines running over a ground plane or reference plane.

    Don't forget your pullup resistors :)

    All the best.

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