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Original question:

TCA9539: tca9539

TCA9539: i2c interface timing requirements

user_CAP
Intellectual 290 points
Part Number: TCA9539
Other Parts Discussed in Thread: TMS570LS1114

Tool/software:

hello:

       I have a question about I2C Fast mode, 400kbps, the specification about of tocf (I2C output fall time- 10-pF to 400-pF Bus), the minimum time is 20*（VCC/5.5V）= 18.18ns@VCC=5V.

Because I had used TMS570LS1114 as the master of I2C, I cannot adjust the I2C sclk output fall time. So the clk output fall time(about 5ns)<18.18ns ,  it can not fullfill the specification but the circuit works well under this condition. I need the reason why this specification about the minimum time of tocf. If the circuit can not fulfill this specification , what bad condition will the circuit take place ?

  • +1
    Guru 294720 points

    That fall time limit is copied from the I²C specification. Its purpose is to prevent ringing on the bus; shorter fall times will work with all devices if there is no ringing that makes the signal go above VIL again.

    If you think that the TMS's outputs are too strong, add a small series resistor.