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P82B96: Noise and slow rise of SCL and SDA signals

Danilo A.
Mastermind 24705 points
Part Number: P82B96
Other Parts Discussed in Thread: TCA9617B,

Hi Team,

We have a customer using P82B96DG4 in their application. She noticed some noise and slow rise of SCL and SDA signals as shown in the waveform below. The SCL and SDA pins are connected to PB6 and PB7 of STM32F207 and no other devices are connected to these pins. Please refer to the schematic diagram below for reference. What could be the cause of this signal?

Regards,

Danilo

  • 0
    TI__Mastermind 22685 points

    Danilo,

    Thanks so much for including the imagery. It is helpful in seeing what is going on.

    This appears to be an inverted pedestal, which occurs when the buffered-output side of a device like this is driven by a controller. Here is a diagram from the TCA9617B datasheet:

    For buffered-output devices, when the buffer side is released and the signal rises, it will rise to the buffered low and stay there until the non-buffered side rises above the appropriate threshold. This can cause the low level signal to show this "pedestal." Once the voltage on the non-buffered side rises high enough, the buffered side is released as well and continues to rise back up to the rail voltage.

    This occurs because I2C buffers are unable to determine signal directionality. This is the case for the P82B96 when the T and R lines are tied together.

    Best,

    Danny