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TCA9517: Unexpected bump

Antony Lin
Genius 13480 points
Part Number: TCA9517
Other Parts Discussed in Thread: TCA9548A

Hi,

Customer is using TCA9517 between one I2C master components and six I2C slave components as described below.

  • There's one set of 4.7kohm pull-up resistor @ clock generator.
  • There's another set of 4.7kohm pull-up resistor @ TCA9517 SDAA/SCLA

Now they observe some unexpected bump like below, and need our comments for the root cause.  Can you share your inputs?  Is it normal there're totally two sets of 4.7kohm external pull-up resistor at A-side(slave side)?

THanks!

Antony

  • 0
    TI__Guru 62920 points

    Hi Antony,

    This is the expected waveform on the B side of the device.  The signal at SDAB (for example) will reach a "low" level close to 0 V when another device pulls down on the bus.  This causes SDAA to be pulled low by TCA9517.  When the device on SDAB releases, the voltage will start to rise.  Since SDAA is still low, though, the TCA9517 will drive SDAB to its "VOL" level (of about 0.5 V).  This only lasts a moment since this "VOL" level is set higher than the "VILC" level of the B side (0.4 V), meaning the buffer considers the "B" bus high and releases SDAA.  When SDAA goes high, SDAB releases and is pulled up to VCCB.

    You can see an illustration of this effect in Figure 6 of the datasheet.

    For more information on how I2C buffers operate, you may want to reference this application note:

    Alternatively, you could watch this short training video:

    https://training.ti.com/ti-precision-labs-i2c-buffers-overview

    If you have any further questions please just let us know.

    Regards,
    Max