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BQ76942: The PF flag will not reset

richard wilson
Intellectual 555 points
Part Number: BQ76942

Hi,

 I am using the BQ76942. During testing, the 8 cells under test drifted apart 3.1 -3.75V triggering the permanent fail. The cells have been brought back into balance and after multiple power disconnects from the battery, PF Resets and Resets that have been issued to the board, the PF stubbornly refuses to Reset. As you can see there are no errors that would cause a PF enable following a reset. has anyone else seen this problem? 

Thanks for your help in advance,

Richard

  • 0
    Intellectual 555 points

    I've just noticed that the Alert pin is also high which is a little hard to understand as it is correctly configured as a thermistor input. 

  • 0
    Intellectual 555 points

    I returned to the evaluation board and I see the same behavior

  • 0 in reply to richard wilson
    TI__Genius 11315 points

    Hey Richard, that's odd behavior. Do you mind exporting your config file and sharing it here? 

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Intellectual 555 points

    calibration_2021-5-12_j.gg.csv

    Here is the file for my board, it is set up for 8S, thanks for taking a look. This behavior also affects all three evaluation kits that I have and it is occurring with the default settings.

  • 0 in reply to richard wilson
    TI__Genius 11315 points

    Hey Richard, 

    Are you referring to the PF flag at Bit 5 of the Alarm Enable register? I don't see any other PF registers that are high in your screenshot.

    Also, my EVM alert pin is not high. Try resetting your alarm status by writing 0x001 to the Alarm Status Register to see if that helps.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Intellectual 555 points

    Thanks Caleb

     I can't write to the alarm status register using BQ software as the command sequence only allows for a 2 digit address. Can I send you one of the evaluation boards to take a look at?

  • 0 in reply to richard wilson
    Intellectual 555 points

    Yes I am referring to the PF flag at bit 5

  • 0 in reply to richard wilson
    Intellectual 555 points

    I wrote to register 0x62 and 0x64 , no change. 

  • 0 in reply to richard wilson
    TI__Genius 11315 points

    Hey Richard, 

    After looking at the Technical Reference Manual, it looks like that PF Flag in bit 5 of the Alarm Enable Register is to enable the alarm when the PF flag is set high through the Alarm Raw register. Meaning that it is supposed to be user configured to be high or low, and it will not be reset using the PF_Clear command or any other command that clears permanent fails.

    I noticed that while hovering over the the Alarm Enable PF bit in BQ studio it shows that "the bit is set high when an enabled permanent fail fault triggers" but this seems to be an error. This description is for the PF bit in the Alarm Raw Status Register. Sorry for the confusion.

    Thanks,
    Caleb

  • +1 in reply to Caleb Perez
    Intellectual 555 points

    Thanks for the explanation and help Caleb, 

    So, if I understand correctly PF (RAW) indicates that a permanent fail condition has occurred if the PF safety bits are set appropriately. PF Alarm Enable is a mask bit. The previous behavior stopped once I disabled PF Bit 5 in the Default alarm mask. After re-enabling it , the previous problems went away and I can charge and discharge. the act of disabling and reenabling the mask bit cleared the problems I was seeing which doesn't make sense to me, I suspect there may be an error with this functionality.

    If you want to replicate the problem I was seeing, select the PF mask bit, select manufacturing Status PF enable. Then reduce the cell voltage below the SUV. The PF flag trips and the discharge FET turns off.  Then increase the cell voltage above SUV, send a reset and the charge FET still doesn't (or didn't) turn back on. 

    I have just replicated the 'fix' on a second board - 

    The (second) board came up with no alarm flags on and the raw PF flag off, however the charge FET was off and the All Fets On command did not cure the problem. I set the Default Alarm Mask PF bit to 0 , programmed the device with just that setting altered, then set the PF alarm mask back to 1 and the board works as expected. I hope that helps you get to the bottom of the issue.