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BQ40Z50-R2: SMBUS Read Errors

Christopher Kellar
Prodigy 170 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ40Z50, BQ24770

Hi, 

Is there a mode of the BQ40z50 where the SMBUS is unresponsive? We are having an issue where at some point the SMBUS communication seems to have continuous read errors. We are unable to recover with a reboot, but it does seem to recover when the battery pack is removed and replaced. In addition, we also see a flood of SMBUS read errors when the battery packs are removed. Is this normal?

We have a 2 cell configuration and use a BQ24770 as the charger.

Thank you,

Chris

  • 0
    TI__Guru** 105800 points

    Hello Chris,

    Can you share the logic analyzer captures of the bus? What is the cause of the read errors (communication lines held low too long? bad packet formation? capacitance on lines causing miscommunication?, etc)

    If you have the FLASH_BUSY_WAIT bit set the gauge should clock stretch longer in order to still complete the packet, if the host cannot do this then it can lead to errors.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 170 points

    Hi Wyatt, 

    Sorry, I do not have any captures of the issue. We are observing the issue at random in the field. We normally do not have issues with the communication, and I believe the communication protocol (with clock stretching) is all correct.

    It seems like the gauge gets into a state where the SMBUS has read issues. Does a shutdown state prevent SMBUS communication? Or is there another state at which the gauge prevents communication?

    Thank you for your help,

    Chris

  • 0 in reply to Christopher Kellar
    TI__Guru** 105800 points

    Hello Chris,

    If the gauge is in shutdown everything is turned off, it will not respond to any communication. In this case you must apply a voltage on the PACK pin higher than Vstartup to wakeup the gauge.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 170 points

    Hi Wyatt, 

    What is Vstartup typically?

    In our design we read from the gauge and then send the charge/current levels to the charger. If the batteries are depleted and the gauge is in shutdown, the batteries seem unable to charge due to lack of communication. How can I ever get the PACK pin to be higher than Vstartup? Should I just send the charge a constant value to apply a charge to PACK? 

    Thank you again for your help,

    Chris

  • +1 in reply to Christopher Kellar
    TI__Guru** 105800 points

    Hello Chris,

    Vstartup is defined in the datasheet, it is nominal at 2.25V. Yes if the gauge goes to shutdown based on low cell voltages it will not communicate until the charger voltage is applied to the PACK pin and then it will become active again. If you require the communication with the gauge before applying voltage to the battery the gauge may never wakeup as you are seeing.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 170 points

    Thank you Wyatt,

    I did find the VStartup after I wrote the message. I have been so focused on the Reference Manual and did not look on the datasheet.

    I certainly appreciate the insight. 

    Thank you,

    Chris

  • 0 in reply to Wyatt Keller
    Prodigy 170 points

    Hi Wyatt, 

    Another related question. Would a permanent fail cause SMBUS to stop communication?

    Thank you,

    Chris

  • 0 in reply to Christopher Kellar
    TI__Guru** 105800 points

    Hello Chris,

    No the gauge should still allow communication after a permanent failure.

    Sincerely,

    Wyatt Keller