I have one EVK board with I2C lines which are pulled up to 1.8V. On the other board again this I2C lines are pulled up to 1.8V and then level translated to 3.3V.

I think the other board I2C lines need not be pulled up.

I also think that providing two pull-ups will violate the KVL.

Please help me to understand what will happen if we provide the pullup in both the boards.

Note: Both the pullup voltage are same and the resistor is also the same.

  • 1
    \$\begingroup\$ Parallel resistors so it behaves as one stronger pull-up, which may or may not be a problem. \$\endgroup\$
    – DKNguyen
    Commented Jun 1, 2021 at 14:37
  • 2
    \$\begingroup\$ At first glance, it's likely to be fine. But what's the resistor value? You just need to be sure that the drivers can sink the combined pull-up current, which they almost certainly will be able to with typical board values. \$\endgroup\$
    – TonyM
    Commented Jun 1, 2021 at 14:38
  • \$\begingroup\$ It is 10k @TonyM \$\endgroup\$ Commented Jun 1, 2021 at 14:39
  • \$\begingroup\$ Am thinking if we apply kvl in the loop considering the open drain mosfet is off, so both the voltage source 1.8V will be acting in parallel and solving the kvl it doest hold good(kvl). Also in theory we learnt that voltage source should not be in prallel.kindly correct me.@DKNguyen @TonyM \$\endgroup\$ Commented Jun 1, 2021 at 14:43
  • \$\begingroup\$ How are the two voltage sources parallel and violate KVL? They are in series in the loop with two pull-up resistors and also negligible internal resistances of the sources. @Ali \$\endgroup\$
    – Mitu Raj
    Commented Jun 1, 2021 at 18:58

1 Answer 1


The 2x 10K parallel resistors on each I2C bus line is the same as having a 5K pull-up on each.

The 5K resistances are pull-ups to 1.8 V. So the I2C drivers will need to be able to sink 360 uA, which IC pin drivers for I2C chips will.

Having separate pull-ups is good if your two boards are ever disconnected from each other. The separate resistors will pull each separated bus idle and keep the mastered bus operational.


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