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TCA9617A: I2C over long cable

Part Number: TCA9617A
Other Parts Discussed in Thread: TCA9617B, , P82B715, TCA9517

I am trying to find best solution for I2C (100KHz speed) over a 4 meter cable.

We came across I2C bus repeater (TCA9617A/TCA9617B) and I2C bus extender (P82B71). Which will be the ideal solution for this application. 

  • Justin,

    We also have a design using the P82B71 + a CAN bus that allows for I2C over longer distances. You can find that reference design information here.

    Regards,

    Eric Hackett

  • Eric,

    Thanks for your suggestion. Unfortunately we have limitations with number of wires available to implement this.

    Does P82B71 on both side of cable helps?

    Also could you share is there any limitations for TCA9617A/TCA9617B?

  • Hi Justin,

    Both TCA9617 and P82B715 can help redrive cables.

    4 meters of CAT5 cable is about 200pF of capacitance (around 50pF per meter for cat5 cabling).

    TCA9617 was designed to buffer up to 550pF and P82B715 3000pF. 

    P82B715 is less of a redriver since noise on the cable side can make it to the opposite side so in most cases I generally recommend a redriver.

    I suggest using TCA9617 (or TCA9517) with the A side facing the cabling side and have B side face the I2C interface side. You need to make sure the devices on the I2C bus can drive below the ViLc spec of 9517/9617 of 0.4V (I2C compliant devices can) and accept a VoL of about 520mV as a logic low.

    If you use TCA9617 I would recommend the B revision since it fixes the high-Z issue A version has. TCA9517 should also work as it buffers 400pF of capacitance but made for a max of 400k. 9517 and 9617 have different Vcc rules so you may need to double check those.

    -Bobby

  • Hi Bobby,

    You mentioned TCA9617 was designed to buffer up to 550pF and P82B715 3000pF. Is this for 100KHz?

    When we use TCA9617 on both end of cable, A side has to face B side of other TCA9617, right? So did you mean TCA9617 on the controller side, with the A side facing the cabling side and have B side face the I2C interface side?

    We are using 3.3VCC on both side, is that a problem?

  • All the devices mentioned support 100kHz.

    When we use TCA9617 on both end of cable, A side has to face B side of other TCA9617, right? So did you mean TCA9617 on the controller side, with the A side facing the cabling side and have B side face the I2C interface side?

    Preferably I would suggest both sides of the cable be buffered and both buffers should have the A side face the cables. If you have one B side face the cabling, you may struggle to get the VoL below ViLc on B side across the long cabling. ViL on A side is about 30% of VccA which gives much more margin then a set 0.4V. This is why I suggest the 9617/9517 have B side face the I2C interface (I2C controller or I2C target devices) and the A side face the cable.

    We are using 3.3VCC on both side, is that a problem?

    No, both TCA9617 and 9517 support 3.3V on both sides

    -Bobby