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The baudrate for the RPI can definitely be set in the range of 100-400kHz by editing the /boot/config.txt:

dtparam=i2c_arm=on
dtparam=i2c=on
dtparam=i2c_baudrate=10000

Following this tutorial.

But I haven't yet found any reference to what was the minimum baudrate successfully used.

Further: what is the minimum pullup-resistor that can be used for the RPI4? On page 9 in this datasheet it reads the pullup resistance is tbd but I think it is quite known to be around 1.8k, as reported here.

My questions are:

  1. What is the lowest I2C baudrate, how do you set it, does it work stable?
  2. What is the standard I2C pullup resistance?
  3. What is the limit I can use for adding an external I2C pullup resistance?
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  • (1) A practical lower limit is 9k6. It is meaningless to try 4k8 or lower (2) Rpi4B I2C pin has built default 1k8. The limiting factor is 400pF of the bus, implying wiring cannot be too long. (3) I usually remove the pullups of all the externally connected I2C modules, because the more modules with their own pullups you connected, the more likely that the resultant pullup is too strong. (4) You might like to read my answer to the following SO Q&A: stackoverflow.com/questions/62757687/…. Happy I2Cing. Cheers.
    – tlfong01
    Dec 7, 2021 at 14:09
  • These questions are mainly asking for opinions and have little specific to do with the Raspberry Pi.
    – joan
    Dec 7, 2021 at 14:19

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