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Original question:

TCA9546A: SDA signal is permanently LOW at Power ON Condition

TCA9546A: SDA Voltage level not reaching to 3.3V

Rajat Singh
Intellectual 360 points

Part Number: TCA9546A

Hi Team,

We are using I2C Switch to connect 2 USB HUB (P/N- USB5744).We are facing USB HUB detection issue.At the time of Probe we obsrve that SDA signal level is not raching to 3.3V level.Can you please suggest what can be the issue?

Probe point-

SCL-R57

SDA-R62

Note- Before I2C Switch level SCL and SDA LEVEL ar 3.3V.

SCH

WAVEFORM 

Many Thanks

Rajat

  • 0
    TI__Guru 59108 points

    Hi Rajat,

    Customer is operating the I2C bus at ~100kHz on the output side of the TCA9546A. 

    Is the customer attempting to control the TCA9546A from the output side (SCx/SDx)? This would not work since the channels are disabled upon power up. Customer would need to configure which channels they want ON/OFF via pins 15 and 14. 

    Are there pull-up resistors on the SDA/SCL pin side? 

    What are channels 3 and 4 measuring? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 360 points

    Hi Tyler,

    We are using USB HUB on output side and both the channel are enable at the time of communication same can be visible from waveform which i have shared earlier.

    Pull-up on both the channels as highlighted in attached snap.

    We are not using Channel 3 and 4 design as of now they are Not connected.

    I have attached again schematic snap for you refrence 

    Many Thanks

    Rajat

  • 0 in reply to Rajat Singh
    TI__Guru 59108 points

    Hi Rajat,

    My apologies. I meant channels 3 and 4 of the oscilloscope capture provided. 

    Just to confirm, channels 1 and 2 of the oscilloscope are measuring at pins 4 and 5? 

    And also confirming voltage levels: 

    FPGA = 3.3V

    VCC of TCA9546A  = 3.3V

    USB hub 1/2 = 3.3V

    No level translation occurring? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 360 points

    Hi Tyler,

    Channel 3 and 4 are USB HUB and USB PHY Reset.

    FPGA'SCL and SDA are  working on 1v8 we used one Level Translator to make SCL and SDA at  3v3.I Probed Output of Level Translator- SCL and SDA both are 3v3 Level.

    USB HUB's are on 3v3 level no LT we have used.

    Many Thanks

    Rajat

  • 0 in reply to Rajat Singh
    TI__Guru 59108 points

    Hi Rajat,

    In figure 16 of the datasheet, Vpass is defined as the lowest pull-up voltage in the system. Vpass is used in determining the appropriate VCC for the I2C controlled switch in order for it to act properly as a level translator. 

    Consider that the lowest pullup voltage in the customer system is 1.8V, this is on the FPGA side. 

    Vpass = 1.8V

    Then reference figure 16.

    The supply voltage for VCC for the TCA9546A should be <3V in order for this device to properly level translate. Please change the supply voltage to the device to something like 2.5V or 1.8V and re-check to see if the system level translates properly.

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 360 points

    HI Tyler,

    TCA9546A, we are using as I2C Switch only not as level translator.

    Below is the block level diagram of connection in schematic.I hope this will get clarity

  • 0 in reply to Rajat Singh
    TI__Guru 59108 points

    Hi Rajat,

    May I have a full schematic showing these 5 functional blocks to double check implementation of the level translator? 

    The output of the level translator converting the FPGA signal from 1.8V -> 3.3V is not working correctly. SDA looks to only reach a high-voltage level of about 2.3V. 

    Regards,

    Tyler