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How to calculate bus capacitance

Jared Becker
Genius 11495 points
Other Parts Discussed in Thread: TCA9548A

Hi team :)

When I look at the datasheet for an I2C slave, which specs do I need to calculate total bus capacitance?

Thanks in advance.

Jared

  • 0
    TI__Genius 16430 points
    Hello Jared,

    The bus capacitance is determined by many factors, mainly the bus layout and the slaves.

    The general rule is that each slave on the bus will contribute about 15 pF of capacitance.
    Another general rule (albeit, very rough), is that 3 pF/cm of PCB trace.

    To get a more correct rule, you should go to your board vendor and ask them to calculate the capacitance per trace of known trace width at your I2C frequency.
  • 0 in reply to Jonathan Valdez
    TI__Genius 11495 points

    Hmmmm,

    So if you look at the TCA9548A, it shows several specs for capacitance. For this part, would I add the SCL and SDA capacitance for a total capacitance of ~24 pF + ~24 pF = ~48 pF

    Thanks!

    Jared

  • 0 in reply to Jared Becker
    TI__Genius 16430 points
    Jared,

    You are partially correct.

    Because each SDA and SCL line are technically independant (and get their own pull up resistors), you will treat each one as a separate bus. Normally, these two busses will have almost identical capacitance and will allow you to just re use your resistor values.

    In this case, you would assume that the device contributes ~24 (I would assume 28 to give myself headroom)pF to a bus (SDA and SCL are two independent wires/nets).