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PCA9545A: How to use PCA9545A

Andy Liu3
Intellectual 2500 points
Part Number: PCA9545A
Other Parts Discussed in Thread: TCA9543A, SN74LVC2G66

Hi team,

This is Andy, FAE of Inspur Group.

Inspur is using our PCA9545A at Server, and I have some question on how use PCA9545A, please help me check.

1. PCA9545A have four channels, if we just need 2 channels, how can I deal with the other unused channels?

2. PCA9545A also have voltage translation function, so I use 3V3 standby power rail powered PCA9545A VCC, Pull up Master and Slave channels SD0~SD2, but use another power rail 3V3 main pull up Slave SD3.   The problem is 3V3 standby power on quickly, but 3V3 main will power on later, determined power-on time based on system requirements;  During this time, the SD3 channel can be considered to be floating, so does PCA9545A support this application?, If we power PCA9545A like this, will the BMC appear abnormal when scanning PCA9545A?
If can't use like this, do you have some suggestions for this application to give our the customer, customer needs to use 3V3 main to pull up the SD3 channel, the purpose is to maintain the same power supply rail as the downstream device to avoid leakage current. 

Thanks for your help and look forward your reply.

 

Best regards,

Andy Liu

  • 0
    Guru 316400 points

    1. I²C is a very slow protocol, so you can leave the I/O pins of unused channels open. (The TCA9543A is a two-channel switch.)

    2. The PCA9545A itself is not affected by floating signals on slave channels. But when SD3 is enabled while still powered down, the pull-up resistors on the master side will pull up SD3 signals, so the slave device(s) will see a voltage of 3.3 V at their I²C pins. This is typcially forbidden because a current can flow through the ESD clamping diodes into the main 3V3 supply, and partially power up anything connected to it. You have to tell the software on the master to not scan SD3 when the power is not yet up, or you have to disconnect SD3 with some other mechanism (e.g., a SN74LVC2G66 controlled with a separate GPIO).

  • 0
    TI__Guru 53515 points

    Thanks Clemens!

    Hi Andy,

    Just to add a bit to the answers above:

    1. You may also tie the unused channels to Vcc through a pull-up resistor. This will protect the system in the event that the unused channels are inadvertently enabled. Perhaps the 2-channel switch Clemens recommended would be a better fit here though. 

    2. As long as the unpowered channels are not enabled (default state of all channels), this is not a issue. The local I2C bus can even communicate in this state. These channels can be enabled once power is available to them so the can communicate with the I2C network. If the connection is made before the channel's pull-up resistors are available, this may impact communication, as Clemens points out, and thus should be avoided. 

    Let us know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Intellectual 2500 points

    Hi Ladisch, Schott,

    Got it, very clear, thank you!

    Just a small case, 

    "This is typcially forbidden because a current can flow through the ESD clamping diodes into the main 3V3 supply, and partially power up anything connected to it. "

    How understand through ESD clamping diodes?  which direction of this ESD clamping diodes? 

    Thanks,

    Best regards,

    Andy Liu

  • +1 in reply to Andy Liu3
    TI__Guru 53515 points

    Hi Andy,

    This diode would be facing from the supply to the bus pin and typically blocks voltage > Vcc from impacting the supply. However, this may still act as a small leakage path to the supply and back-bias an otherwise off system and thus should be avoided. 

    Regards,
    Eric Schott