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PCA9557: communication time out

Kazuki Kuramochi
Genius 10525 points
Part Number: PCA9557

Hello expert,

I have question for PCA9557's behavior regarding SMBus communication time out.

My customer observed that PCA9557 hasn't happened time out even if adding more than 25ms wait between Reg addr and write data.
I think SMBus specification has limitation regarding communication time out(Max 35ms).
So We would like to confirm about whether this observed behavior is expected behavior or error.
Also if this is expected, could you tell me about how long does it take to time out?

Best regards,
Kazuki Kuramochi

  • 0
    TI__Guru 73321 points

    Hey Kazuki-san,

    Are you saying that our device is getting stuck (stuck bus) and not releasing the SDA line after 35ms?

    Our device does not have a timer built in that detects a stuck bus and releases the SDA line (time out) as it was spec'd for I2C. I believe only SMbus has a spec for timeout.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 10525 points

    Hi Bobby-san,

    I meant that our device didn't suspended communication even if adding more than 35ms wait. After this wait, our device received data packet and work correctly following packet.

    I think this device could support electric specification of SMBus. However timing specification(including time out) is comply with I2C.
    So this device could support long wait time between Reg addr and write data.
    Is this correct?

    Best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Genius 10525 points

    Hi Bobby-san,

    Would you provide answer for my question?

    I'm waiting your reply.

    Best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Guru 73321 points

    Hey Kazuki-san,

    Sorry, I did not see your follow up question.

    Our device does not have a timeout feature. It was designed with the I2C spec in mind which has a frequency range support of 0 Hz to 100kHz and 0Hz to 400kHz, at very low speeds we would easily see the timeout interfere with the I2C spec.

    "So this device could support long wait time between Reg addr and write data."

    Yes, as we do not have a time out.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 10525 points

    Hi Bobby-san,

    I understood about this device support SMBus regarding electric specification but timing specification compliant with I2C.
    So this device support clock stretch and there isn't time out.|

    Best regards,
    Kazuki Kuramochi