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TCA9517: I2C Bus Repeater - Is connecting B side to A side ok?

Mark Sample
Prodigy 110 points
Part Number: TCA9517
Other Parts Discussed in Thread: TCA9803

Tool/software:

Hello, The design consist of two circuit boards connected by ribbon cable. On the master side, the MCU, is connected to the A side of the TCA9517 with VccA = 3.3v and VccB = 5v. B side to ribbon cable.

On the other side of the ribbon cable, on the other board, the connection is to the A side of another TCA9517DGKR with VccA = 5v and VccB=3.3v. The B side connects to some sensors.

The idea with operating the ribbon cable at 5 volts was for better noise immunity.

My question is, is it ok to interface the B side to the A side?  Or should the higher voltage ribbon cable be connected to the B side on both boards?

Thank you!

Mark

  • 0
    Guru 317280 points

    Yes, you can connect A to B. But VCCA > VCCB is not allowed.

    Just use 2× TCA9803 buffers with 3.3 V everywhere.

  • 0 in reply to Clemens Ladisch
    Prodigy 110 points

    Thank you for the fast response! We do have VCCA >VCCB. We will fix that. What issues would that cause?

    Also, why do you suggest 2× TCA9803 buffers with 3.3 V everywhere over what we have now (with VCCA >VCCB issue fixed)?

    Thank you!

    Mark

  • 0 in reply to Mark Sample
    Guru 317280 points

    You must not connect two B sides together.

    I suggest the TCA9803 because it will work.

  • 0 in reply to Mark Sample
    TI__Mastermind 48108 points

    Hi Mark,

    THank you for your question. 

    The static voltage offset of two buffers must not connected to each other. See the available connection schemes for connecting to SVO buffers: 

    TCA9517 has SVO on B-side. If the first TCA9517 buffer has A-side face 3.3V and 5V side face long ribbon cable, then the A-side of the second buffer must connect to the cable side. If VCCA = 5V, then VCCB must = 5 V in order to stay within datasheet specifications. 

    If you use VCCA = VCCB = 3.3V, this is also okay as well. You might not need to shift to 5V for greater noise immunity. How long is your cable and do you suspect significant noise in your system? 

    Two TCA9517 buffers in series with VCCA = VCCB = 3.3V for both buffers should be fine, even if the cable is long. 

    Regard,s

    Tyler