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Original question:

P82B715: Schematic Review

P82B715: Please help to review Schematics

Ivan Teo
Prodigy 60 points
Part Number: P82B715
Other Parts Discussed in Thread: TCA9517A, P82B96

Hi Team,

Please help to review the schematics whether is ok and whether the pull-up resistors value are ok.

The concept is to allow 3.3V MCU I2C Master to communicate with a 3.3V I2C Slave device through 2.5 meters long 0.64mm pitch IDC Flat Ribbon Cable. The bus speed is 400 KHz.

I2C Bus Master (3.3V) - P82B715 (6V) - Cable (2.5 meters) - P82B715 (6V) - I2C Slave (3.3V)

p82b715.xlsx

Thanks and Best Regards,

Ivan

  • +1
    TI__Genius 15345 points

    Hi Ivan,

    This concept is fine. You can use the P82B715 to level translate between 3.3V and 6V as you have it now. I will test this feature in the lab for you since it is not called out directly in the datasheet.

    When it comes to the pull up resistors I can walk you through how to calculate it, but I can't calculate it for you without know what the capacitance per meter is for your cable. This is something you will have to find out from the manufacturer of your cable.

    To calculate the pull up resistance, you first need to find out the total capacitance on the buffer side (Lx/Ly). This total capacitance is the sum of your device capacitance, trace capacitance, and wiring capacitance.

    You need to set the resistance so that the RC constant is equal to or less than the maximum allowed rise time for I2C fast mode (400 kHz). For fast mode the maximum allowable rise time is 300ns.

    So your total bus resistance = 300ns/(Total Bus capacitance). In your schematic above both of your resistors would add in parallel to give you your total bus resistance. Keep that in mind when you calculate your total bus resistance. You either have to use one pull up resistor or you can have two resistors in parallel, like you have above, as long as they add up to your total bus resistance.

    Best,

    Chris

  • +1 in reply to Chris Ayoub
    TI__Genius 15345 points

    Hi Ivan,

    I finished testing in the lab today and it looks like this device cannot level translate.The next option here would be to either implement a level translator to translate the 3.3V signal up to a 6V signal. The TCA9517A could work for this solution but keep in mind that the static voltage offset if attached to the P82B715 would be duplicated and transmitted through the device. So you would see the offset on the other end and you have to make sure that your device can handle a static voltage offset.

     

    The other option would be to use the P82B96. Keep in mind this device is not p2p with the 715. However, this device can level translate all in one package. It is a buffer so it does have a pretty high VOL of around 0.8 depending on what your pull up voltage is. This should be compatible with most I2C devices that use a VCC of 3.3V. Just like the P82B715 you will need two of these device one on each end of the cable. This device also allows you to connect up to 4000 pF on the transmission side. Also if you only need it to be uni-directional you can just hook up the TX pin of one device to the RX of the other save on cabling costs.

     

    Let me know what you think.

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    Prodigy 60 points

    Hi Chris,

    In view of that P82B715 cannot do the voltage translate and also P82B96 having a high VOL, we've to moved to other solution. 

    Thanks for the help.

    Best Regards,

    Ivan