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Parameters of I2C bus capacitive load

mehmet emin okudan
Prodigy 40 points

Part Number: PCF8575
Other Parts Discussed in Thread: PCA9548A

Hi,

I use PCF8575 to create new IO lines for microprocessor. I want to calculate total bus capacitance on the I2C bus in order to choose right pull up resistors. So I want to learn that capacitance of P ports (Cio) are adding to total bus capacitance or not. For example Cio of SDA is 7 pF and I use 2 ports which have 10pF each of them. At the end the total bus capacitance of the SDA line between PCF8575 and microprocessor is 27pF?

Also the datasheet said that Cb is the total bus capacitance of one bus line in pF and it is 400pF. So can I understand that SCL and SDA line has 400pF capacitive load separately.

Thanks in advance.

  • 0
    Guru 326490 points

    A very rough estimate is 1 pF per cm of trace, and 10 pF per device.

    In practice, to determine whether you have appropriate pull-up resistors, check the waveforms with an oscilloscope.

  • 0
    TI__Guru 64288 points

    Hi Mehmet,

    The I2C SDA/SCL lines input cap is not effected by the number of p-port I/O's used. These are two separate entities of the device. The main capacitance on the I2C bus would come from the input cap present on SDA and SCL + PCB trace capacitance, which Clemens mentioned "1pF / cm" of PCB trace is a good estimate when design at a PCB level. 

    mehmet emin okudan said:
    Also the datasheet said that Cb is the total bus capacitance of one bus line in pF and it is 400pF. So can I understand that SCL and SDA line has 400pF capacitive load separately.

    I believe your thinking is correct. I2C standard states 400pF limit for SDA and 400pF limit for SCL.

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 40 points

    Thank you for your answers. I'll try them as soon as possible.

    Besides  I have another question related to this topic. I also used I2C switch PCA9548A. My question is that does the total bus capacitance change according to active channel (increase or dicrease) or does the I2C switch act as a buffer for different channels?

    Regards.

  • 0 in reply to mehmet emin okudan
    Guru 326490 points

    The PCA9548A is a passive switch. All enabled bug segments are directly connected together, and their capacitances add up.