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PCA9306: UART Application Schematic Review

Jim CHEN
Expert 4795 points
Part Number: PCA9306
Other Parts Discussed in Thread: TXS0102

Dear Team,

My customer used PCA9306 for UART application.

Could you please help to check the below schematic? Especially the pullup resistor and decoupling cap.

Thank you.

Regards,

Jim

  • 0
    Guru 276040 points

    The datasheet suggest 200 kΩ for the VREF2 pullup. Reducing it will not really make much of a difference (it increases the idle current), but why?

    The decoupling capacitor looks OK.

    Please not that using the TXS0102 instead would allow you to get rid of all three resistors.

  • 0
    TI__Guru 57810 points
    Hey Jim,

    Clemen's inputs are correct.

    To add my own:

    I believe you will need to put the pull up resistors on Side1 of RXD and side 2 of TXD otherwise the signal will not get fully pulled up to Vcc on those sides (unless those sides already have pull up resistors else where). Also, the pull up resistors on RXD and TXD are pretty weak, if you are trying to support faster bit rates then those resistors may not be enough.

    As Clemens suggested, TXS0102 will probably work better because it includes rise time accelerators to temporarily supply a fast rising edge and has internal pull ups.

    Thanks
    -Bobby
  • 0 in reply to BOBBY
    TI__Expert 4795 points
    Thanks Clemens.

    Hi Booby,

    Their maximum datarate of UART is 115200 bps.
    So your suggestion is to have more stronger pullup resistor (lower than 100k ohm) on both RXD and TXD, right?
    I will recommend TXS0102 to my customer, but not sure if they can replace PCA9306 at this moment.

    Thank you.
    Regards,
    Jim
  • 0 in reply to Jim CHEN
    TI__Guru 57810 points
    Hey Jim,

    That is correct. The pull up resistor chosen will be dependent on the bus capacitance. I do believe because you are using push-pull the rise time will be helped due to the pull up from the transmitter. I haven't done testing with push pull with this device but You may actually be able to get away with just the 100k resistor now that I think about it with the transceiver helping drive high.

    For now I would say the 100k resistor are probably okay but I would size down a little more to be sure. If the arrows on the lines in the schematic are correct then you need to move the pull up resistors as well. (RX sides need to have pull up resistors not TX sides).

    Thanks,
    -Bobby