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TCA9548A: TCA9548A SDx and SCx output question

Dane Zhang
Genius 9605 points

Part Number: TCA9548A

Hi team,

Are SDx and SCx of TCA9548A in OpenDrain mode when outputting signals?

If so, how does this device judge the input and output directions? The function diagram in the manual is relatively simple, is there a more detailed diagram?

  • 0
    Guru 320660 points

    The I²C signals always are open drain.

    The device dos not judge directions. The pass transistors are symmetrical, and switch on when any side goes low. In multi-master or clock stretching applications, both sides transmit at the same time.

  • 0 in reply to Clemens Ladisch
    TI__Guru 53535 points

    Hi Dane,

    Clemens comment is correct. The I2C switch does not re-drive any signal to propagate to different channels and instead only connects active channels, allowing the original signal driver to pull the connected lines low. This is important to understand because the driving responsibility still lies with the original signal driver and bus capacitance is not isolated between channels.

    Let me know if you have any other questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    TI__Genius 9605 points 
    Hi Eric,
    Is the symmetrical transfer transistor you are talking about similar to the structure shown in the figure below?
     
    During the test, we found that in a special case, SDA was pulled high, SDx was pulled low, SDx appeared half-high, and a voltage drop of about 1V appeared before and after I2C_Swtich;
     If it is the output of a pure OpenDrain structure, there should be no half-high level but SDx is pulled down, right? But if it is through, why is there a 1V voltage drop before and after? 
    So I hope to learn more about the internal structure of I2C_Swtich to facilitate analysis, thank you~ 
    The following are the SDA test waveforms before I2C_Swtich and SDx after I2C_Swtich:
  • 0 in reply to Dane Zhang
    TI__Guru 53535 points

    Hi Dane,

    The structure you show is indeed similar to the pass-FET architecture used by TCA9548A. 

    Some voltage drop is expected across the switch when passing current (perhaps half a volt when passing several mA) due to the resistance of the switch. It is strange to see such a large drop however when a half-high level is driven to one side. I believe this may be some improper interaction between the undetermined input voltage and the internal reference used for the Vcontrol equivalent. It appears that the low-signal is not being driven properly by the original signal drive. Because TCA9548A does not have any way of pulling the SDA line low itself, I don't expect this is caused explicitly by the switch. 

    Would you be able to share a schematic of the test setup showing what devices are driving these signals and how each line is being pulled-up to Vcc? If you would like to share off-line of the public forum, please find my email my clicking my E2E name.

    Regards,
    Eric Schott