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How to eliminate DEVICE_XREADY bit

SERGIO DURAN
Prodigy 40 points
Other Parts Discussed in Thread: BQ76940

Hello.

We are developing a new Project with the BQ76940 IC and sometimes, when the current flows and we are measuring CC current, the BQ76940 stops the FETs and give the DEVICE_XREADY fault condition.

The I2C commands are still working and we send a status reset (0x3F) and  a go to ship command (normally it is working) and the device give us the ACK but it does not go to ship mode a does not erase the fault, so we can not delete de DEVICE_XREADY bit and the unit can't work propertly.

I can delete it by disconnecting the AFE from the battery, but it is not posible ir real application (and it is not recommended in the datasheet), so, is there any way to reset the DEVICE_XREADY bit via I2C commands ?. Why the AFE replay with ACK but it does not do anything ? We have a microcontroller sending commands, but after the bit is reset, we can not close the charge or discharge FET, so the unit stop working at all.

Best regards,

Sergio.

  • 0
    TI__Guru 54715 points

    If the upper cell group power is above VSHUT and the TSx pins are pulled down, setting the XREADY bit status to "1" should clear the XREADY condition.  The 0x3F sent to the status register should do that.

    The part measures cell voltages in 3 groups and communicates the voltages to the bottom group with a propriteary internal interface.  XREADY may be thought of as a watchdog on the interface. If the interface is sufficiently disrupted the part will think it does not know the voltages of the upper cell groups and will turn off FETs.  It thinks the MCU also does not know the cell voltages and does not allow the MCU to override this condition.

    Check that the group power pins stay above VSHUT durig operation. (BAT - VC10X, VC10X - VC5X, VC5X - VSS).  If the groups do not have power they will not respond and the IC will show XREADY which can not be cleared.

    Be sure the TSx pins are pulled down to the the appropriate reference voltage.  The TSx have a wake up function which can prevent shutdown or start up in some conditions if the pins are allowed to float or held up.

    The part ACKs communications because it is on and is reporting a fault.  The pin conditions must allow normal operation for the fault to be cleared. 

     

  • 0 in reply to WM5295
    Prodigy 20 points

    Aha! Thank you for this. 

    DEVICE_XREADY would get set a few ms after power up on my board. After reading this I realized that I had TS1, TS2, and TS3 pulled to "ground" with a 10k resistor to prevent it from floating. This is incorrect and pulls TS2 and TS3 beyond their relative ground - should be VC5x and VC10x. After removing the 10k pull downs my board started performing great. Thanks for pointing out that the thermistor inputs can also cause a DEVICE_XREADY error bit to be set.