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PCA9515 abnormal signal

Thomas Chang15933
Expert 4770 points
Other Parts Discussed in Thread: PCA9515

Dear Team:

Our customer  used PCA9515, but the datapin sometimes add extra one high signal. Could you help the case?

The circuit and wave form as attached.

  • 0
    Guru 243120 points

    The PCA requires that the input low voltage level of the connected I²C devices is lower than its own output low level (see VIL, VILc, and VOL − VILc in sections 6.3 and 6.4 of the datasheet, and section 9.1.2).

    On the 5V SDA waveform (2), one can clearly see the difference. But there is no such step on the 3.3 V SDA waveform (3). What is the output low voltage level of the master?

    If that is indeed the problem, increasing the pullup resistors on the 3.3 V side (and reducing the frequency) should help.

  • 0 in reply to Clemens Ladisch
    TI__Genius 17205 points

    Hello Clemens,

    Thanks for the helping with the forum!  

    Thomas,

    I agree that it might be the pull up resistors.  I didn't see the pull up resistor values for the 3.3V side, are they on another page?  The signals looks like they have week pull up resistors and you need to increase the strength (lower resistance).  Is this a large board with long traces or does it have a harness or back plane connections?  All this needs to be considered when selecting the correct pull up resistor.  

    link to pullup resistor calculation for I2C bus:

    http://www.ti.com/lit/an/slva689/slva689.pdf

    Francis Houde

  • 0 in reply to fhoude
    TI__Expert 4770 points

    Dear Team:

    I got the wavefrom detail.

  • 0 in reply to Thomas Chang15933
    TI__Genius 17205 points
    Hello Thomas,
    Did they try strengthening the pull ups (reduce resistance)? Did that work?

    Can we also see all the information before and after the device; SDA0, SCL0, SDA1, and SCL1?
    -Francis Houde