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PCA9548A: Master and Slave side I2C line total pull-up resistor calculation when channel x is selected

Johnny Guo
Expert 4910 points
Part Number: PCA9548A

Hi, team,

I have one question about PCA9548A.

SDA/SCL and SDx/SCx are both pulled up to 3.3V separately, how to calculate the I2C DATA and CLK line total pull-up resistor value? Is it just calculate as resistor in parallel?

For example, when channel 0 is selected, and no other device is connected to I2C bus, is the whole DATA line pull-up resistor 4.7k//4.7k=2.35k? Or still calculate separately?

And for internal switch, is it just a CMOS switch? What is the resistor topology for both side if one switch is ON?

Thanks.

Johnny

  • 0
    TI__Expert 4910 points
    Anyone can help on this?
  • 0 in reply to Johnny Guo
    TI__Guru 62920 points
    Hi Johnny,

    You are right that the resistances will be considered in parallel when the switch is on. It is a FET-based switch with low on-state resistance.

    Max
  • 0 in reply to Miguel Robertson
    TI__Expert 4910 points
    Max,

    1. Customer found if SDA/SCL is pulled up, and SD0/SC0 is not pulled up through resistor, output voltage will be abnormal when channel 0 is selected. If internal switch is with low on-state resistance, i think SD0/SC0 voltage should be equal to SDA/SCL. Could you help explain this?

    2. If SDA/SCL and SD0/SC0 are all pulled up, once SD0/SC0 is shorted to GND, will there be a current flowing through internal switch without any protection?

    Thanks.
    Johnny
  • 0 in reply to Miguel Robertson
    TI__Expert 4910 points

    Please help on questions raised. Thanks.

  • 0 in reply to Johnny Guo
    TI__Guru 62920 points
    Hi Johnny,

    In order to facilitate voltage level translation, the active switches only conduct current when one side of the device is pulled low (e.g., by an open-drain I2C driver). This means that you need pull-up resistors on both sides (SDA/SCL and SD0/SC0). Otherwise, there will be no way for the high-level signals to be driven.

    Max
  • 0 in reply to Miguel Robertson
    TI__Expert 4910 points
    Max,

    Thanks for reply. How about another question below?

    1. Customer found if SDA/SCL is pulled up, and SD0/SC0 is not pulled up through resistor, output voltage will be abnormal when channel 0 is selected. If internal switch is with low on-state resistance, i think SD0/SC0 voltage should be equal to SDA/SCL. Could you help explain this?

    Thanks.
    Johnny
  • 0 in reply to Johnny Guo
    TI__Guru 62920 points
    Johnny,

    That's what I was trying to answer. The switch only has low resistance when driving low levels. High levels need to be pulled up by resistors. If pull-up resistors are not used then you won't get the right output voltages/waveforms.

    Max
  • 0 in reply to Miguel Robertson
    TI__Expert 4910 points

    Max,

    I am confused about this "The switch only has low resistance when driving low levels.".

    You mentioned that "the resistances will be considered in parallel when the switch is on. It is a FET-based switch with low on-state resistance."

    So how about the resistance when driving high and switch is on? Not in parallel?

    Thanks.

    Johnny

  • 0 in reply to Johnny Guo
    TI__Guru 62920 points
    When the bus state is high, each side is pulled up individually by pull-up resistors to their own respective power supplies. This allow for different pull-up voltages to be used on either side of the switch.

    Max