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PCA9545A Multiple Slave Interface

Pradeep Das81
Prodigy 20 points
Other Parts Discussed in Thread: PCA9545A, TCA9545A

Hello,

The I2C signal from micro-controller(master) is connected to PCA9545A, and 4 slaves are connected to 4 individual channels in our design. We are able to communicate with individual slave by selecting respective channel(by writing to control register of PCA9545A). We are also able to communicate with multiple slave by selective multiple channels.

But we are facing issue when switching on all 4 channels and any one of slave is not powered on. In this scenario we are neither able to communicate any of slaves nor  with control register of PCA9545A. We are getting NACK after Slave ID.

We are working at 3.3 V logic. We have probed the signal and found that signal level in is coming down to 1.5 V from 3.3 V in case all channel activated.

Please help us to find out the issue.


Regards

Pradeep

  • 0
    TI__Mastermind 30260 points

    First of all, when the package option is available, we recommend using the TCA9545A device as a replacement for the PCA9545A. The TCA version resolves the POR power supply constraints and extends the voltage range down to 1.65V, in addition to having a newer datasheet with more complete application information.

    To address your issue: You essentially have 5 pull-up resistors in parallel.

    As can be seen in the TCA9545A datasheet, "If multiple slave channels will be activated simultaneously in the application, then the total IOL from SCL/SDA to GND on the master side will be the sum of the currents through all pull-up resistors, Rp"

    Not only will it be very difficult to achieve a valid low signal, VOL, the current consumption can potentially increase to dangerous levels for the TCA9545A device or even an I2C driver which needs to be externally limited.

    Please refrain from enabling all channels simultaneously by modifying your processor's firmware appropriately.

  • 0 in reply to Brian Berner
    Prodigy 20 points

    Thank you  for your prompt and useful reply. 

    We will revert back for further help if required.

    Thanks & Regards

    Pradeep