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TCA9548A: CLK level issue

Tommy Tzeng
Genius 5780 points
Part Number: TCA9548A

Tool/software:

Hi Sir,

We use TCA9548 to connect external I2C devices. One device has power issue so its I2C level drop to …1.8V??? as belo fig. Yellow/Green lines are connected to issued device, and purple SCL_M is connected to host.

1. We see the waveform at SCL_M become unstable, but SCL looks good. Why there’s different voltage level at the two side?

2. At the same condition (power issue at device side), TCA9548’s SDA might be pull down and need RESET to reboot TCA9548. 

  • 0
    Guru 317000 points

    1. The TCA9548A allows voltage translation. Low signals are passed through; for high signals (near or above VCC), the connection is blocked, and the output voltages are determined by the pull-up resistors.

    2. Which device pulls SDA down? What does the TCA9548A have to do with this?

    The SCL_M signal does not look good; it sometimes does not reach VCC. Increase the pull-up current (up to the 3 mA limit) or use a lower frequency.

  • 0 in reply to Clemens Ladisch
    Prodigy 51 points

    Hello Sir:

    our setting is as below

     when the DEVICE is turn off (no power) the signal will become as first picture in this issue

    when the TCA9548 is turn on the channel  the CPU side is direct the Device side ??

    or it by pass by some function?

    do we calculate the pull up resistor need  calculate as the both CPU side  and  DEICE side in parallel or calculate it in cpu side and device side? 

  • 0 in reply to al chang
    Guru 317000 points

    When a channel is enabled, it is connected directly to the master bus (for low-level signals). All pull-up resistors on all enabled channels act in parallel.

  • 0 in reply to Clemens Ladisch
    Genius 5780 points

    Hi Clemens Ladisch,

    sir, thanks for you help and reply. one more question,

    when our channel is connected to a certain device and power off, it will actuall affect the level of the all channel, 

    when this level is affected and is lower than the Vin high(3.3 x 0.7=2.31v) specified by SPEC, it seems that the TCA9548A is still operating, we would like to ask why?

  • 0 in reply to Tommy Tzeng
    Guru 317000 points

    The schematic shows six components that can be connected to some supply. Which ones are powered off?