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P82B715: I2C pullup resistor values

Mark Ramsey
Prodigy 10 points
Part Number: P82B715
Other Parts Discussed in Thread: LM1117

Hello,

There seems to be conflicting information on the TI website regarding I2C pullup resistors.

I2C Bus Pullup Resistor Calculation (2015)
https://www.ti.com/lit/an/slva689/slva689.pdf

P82B715 I2C Bus Extender (2016)
https://www.ti.com/lit/ds/symlink/p82b715.pdf

The example in the first link on page 4 suggests values from 1 to 2 K.

However page 12 in the datasheet shows values much higher. Is that specific to the P82B715 or have opinions changed on the recommendations?

The circuit is 5V powered. Mcu is ESP8266 (ESP-01), supplied by a LM1117 to drop to 3.3V. The local I2C devices are powered by 5V with the SDA and SCL lines pulled up to 3.3V. Lx and Ly are pulled up to 5V, as well as the remote SDA and SCL.

Thanks all for your insight.

Mark.

  • +1
    TI__Guru 50795 points

    Hi Mark,

    The maximum value for an I2C pull-up resistor is limited by the line capacitance of the system to ensure the rise time of the signal can support the desired data rate. The first document here calculates the worst-case bus load of 400pF (max allowed by I2C spec), and thus limits the maximum pull-up value to a fairly low resistance (~1k - 2k). The example in the P82B715 datasheet does not use worst-case values and instead does it's calculations based off the estimated bus capacitance between 30pF and 50pF. Because this capacitance is much smaller, the pull-up resistor does not have to be as strong (small resistance value) in order to achieve the required rise time. This allows for the pull-up value to be in the ~20k-ohm range. This relationship is depicted in Figure 3 of the app note (shown below). The app note assumes max capacitance so the far right of the graph. The datasheet uses an example with values on the far left of the graph. 


    Let me know if anything here is still unclear and if you have any more questions.

    Regards,
    Eric Schott