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TCA9509: pull-ups on the A SIDE

Israel Rosner
Prodigy 20 points
Part Number: TCA9509


Tool/software:

Hi,

i'm using the part in order to buffer the I2C line going to a sensor out from board. but i use it also to on-board device which i connected directly to the MCU without buffering. 
i read in the datasheet that pullups in the A side are not allowed? i didn't understand if not allowed or not needed? 
the problem is that I thought to disable the buffer whenever the external sensor is not in use. and then i won't have the 1mA current source active, is that true?
so can i connect pullups to the A side or not?
all sides uses the same 3.3V voltage.

pls see the diagrem:


my question is about the circled resistors.

Thanks,
Israel

  • 0
    Guru 317730 points

    Section 8.2.1 says that "Pullup resistors should not be placed on the A-side ports".
    Section 8.2.2.2 says that "The pull-up resistance on the A-side must be carefully selected to ensure that the logic levels will be transferred correctly to the B-side."

    If you have pull-ups, they must be large enough so that all devices on that bus segment are able to pull the lines below 110 mV.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    ok, 
    and this is because of the current source on the A side? if i'll take 10k reisitors, that can work? i have only my MCU, an I/O expender, and the buffer. what do you think? 

  • 0 in reply to Israel Rosner
    Guru 317730 points

    I do not know the drive strength of the MCU or the I/O expander.

  • 0
    TI__Guru 50108 points

    Hi Israel,

    This description of the TCA9509 helps to add clarity on why PU resistors are not recommended on A-side.

    Israel Rosner said:
    ok, 
    and this is because of the current source on the A side? if i'll take 10k reisitors, that can work? i have only my MCU, an I/O expender, and the buffer. what do you think? 

    Adding 10k PU resistors on A-side increasing the VOL seen on A-side when the MCU pulls LOW. Removing these 10k PU resistors decreases the VOL. 

    The VOL from the MCU increases because now it would need to sink the current from the 1 mA current source of the TCA9509 + the additional current from the 10k PU resistors. 

    As Clemens has already suggested, every device on A-side would need to be able to pull the SDA/SCL line below 110 mV in order to pass a LOW signal from A-side to B-side. 

    This is the PU resistor limitation. 

    If your MCU is strong enough to sink the current of the 10k PU + 1 mA current source, then you are probably okay. 

    Regards,

    Tyler