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Original question:

TCA9617A: TCA9617A 上升沿时间

TCA9617A: IIC repeater

Chambin Zhang
Prodigy 165 points
Part Number: TCA9617A
Other Parts Discussed in Thread: PCA9306,

Hello,

I found a formula to calculate IIC pullup resistor value in the document SLVA689 about "I2C Bus Pullup Resistor Calculation"

When I use a IIC repeater, there are two couples of pullup resistors both side of repeater as follow:

My first question is these two couples of pullup resistors use same calculate formula above? that mean they have same maximum pullup resistor limit?

My second question is why master side(red side) rise time no obviously change when I change slave side(blue side) pullup resistor?  

My third question is rise time of master side can bigger than slave side? 

My forth question is master side pullup resistor value must less than slave side?

Thanks.

  • 0
    Guru 319150 points

    With a buffer like the TCA9617A, the two sides are isolated, and you have to compute the pull-up resistors separately. (With passive switches like the PCA9306, the resistors on both sides act in parallel.) Please note that the A and B sides could have different VOL values.

    The allowed rise time is limited by the I²C specification, so it is usually the same.

    There can be multiple masters. The formula results in different values because VCC is different.

  • +1 in reply to Clemens Ladisch
    Prodigy 165 points

    Hello,

    Very thanks your answer.

    The isolation you said mean the rise time master side and slave side can't impact each other?

    Is it right that master side and slave side have different capacitive load (Cb)? 

    Thanks.

  • 0 in reply to Chambin Zhang
    Guru 319150 points

    Yes, isolation means that there are separate input and output buffers on both sides. Each side is a separate bus with up to 550 pF.