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TCA9517: Pull-up resistor selection on both port

Hideki Hayashi1
Genius 9230 points

Part Number: TCA9517

HI team,

Customer think using pull-up resistor as below with Vcca = Vccb = 5V. Does it work? I read I2C Bus Pullup Resistor Calculation from ti.com and think port B needs 7.3kohm (5-0.6 V / 0.6mA) at least, is my understanding correct?

SDAA, SCLA: 47kohm

SDAB, SCLB: 2.2kohm

Another question, Is there any problem to place pull-up resistor on EN pin of 4.7kohm? I know there is an internal pull-up.

Best regards,

Hayashi

  • 0
    TI__Genius 15345 points

    Hayashi-san,

    47K is most definitely going to be too large of a pull up resistor. What speed are you trying to communicate at?

    For I2C spec if you are trying to achieve 100 khz (standard mode) the maximum rise time is 1000ns. With a maximum bus capacitance of 400 pF that gives you a maximum pull up resistor of 2.9K:

    This will change based on what your bus capacitance is. Most likely you are not using the maximum bus capacitance allotted for standard mode. So it is not impossible to use a 47K ohm resistor just make sure that you are using the equation above to find your maximum pull up resistance.

    I highly recommend reading through this app note on how to size pull up resistors. 

    You can use an external pull up on EN it will make your pin more robust to noise.

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Genius 9230 points

    Hi Chris,

    I'm not aware of I2C speed right now, need to check. I know the app note so am wondering the minimum value of pull-up resistor on B side. Can you please comment to it?

    Best regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Genius 15345 points

    Hayashi-san,

    The value of the resistor on the B side looks fine. What you are controlling with this pull up resistor value is the VOLs of the devices that are connected to the TCA9517 not the TCA9517 it self. The TCA9517 has a static voltage offset so it will always output the same VOL. You have to make sure that all the devices on the B side are below the VILC of the TCA9517. That is where the pull up resistor comes into play. If you make the pull up resistor too small then the VOL of those devices will be above the VILC of the TCA9517 and communication will not work. Calcualting the minimum value of the pull up resistor is also shown in the app note I highlighted above.

    Best,

    Chris