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I2C Total Bus Capacitance Calculation

Luis Cordova
Intellectual 1930 points

Hi,

I wanted to double check some Pull-Up resistor values I calculated for my I2C bus.

This board will only have 1 master 2 slaves, each contributing 5-10pF of load capacitance.

I am unsure on exactly how the total bus capacitance is calculated, but I performed two calculations.

The first was done if total bus capacitance is calculated by adding both SDA and SCL pins as if they were in parallel, and the value I got was 7kOhms. Very reasonable.

If the total was calculated per lane (one for SDA and SCL), then I got 14.1kOhms of course. This seemed reasonable, but larger than I've seen so far.

My guess is the lower one is safer either way, but I wanted to double-check before I finalized anything and make sure I am correctly calculating total bus capacitance.

Thanks!

Luis Cordova

  • 0
    TI__Genius 16430 points
    Hello,

    Your assumptions are generally true.

    The general rule of thumb for capacitance is 10-15 pF per IC. Your PCB adds capacitance to the bus as well, but the actual value can vary significantly depending on the trace width and the board material, so it's best to get this information from your board vendor.

    However, when calculating, you do not add the SDA and SCL lines together. Each line will require its own pull-up value, therefor they should be kept separate.

    14k ohms is not unusually high for a small bus load, but you are correct that 7K is 'safer'. Remember, that the calculation you use is only a maximum resistance value to achieve a speed for a given bus capacitance, so you should give yourself some head room.
  • 0 in reply to Jonathan Valdez
    TI__Intellectual 1930 points
    Thanks for this extremely fast and excellent response Jonathan, your support is appreciated!