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Original question:

BQ32000: BQ32000

BQ32000: Problem of external oscillator after restart

Manu Chang
Intellectual 1050 points
Part Number: BQ32000

Hi,

Customers are evaluating BQ32000 as RTC and facing some problems. Customers first set the correct time to BQ32000 and then restart the BQ32000 by turn off the 3.3V. After the restart some of the BQ32000s are sending wrong real time data. When catching the waveform on OCSO and OSCI, it seems that the external oscillator 

doesn't oscillate or only oscillate for a while. The schematic and waveforms as following, questions from customers are:

1. In the block diagram in datasheet there is a 32k oscillator inside BQ32000, so is an external crystal needed for BQ32000 ?

2. If external crystal is needed, is the schematic looks right ? are C122,C140,R18 needed ? (These elements are added in order to enhance the oscillator's accuracy)

3. Customer cancel the C122, C140 and R18, and the data sent comes to right. However the oscillator stop flag turns to 1, is this normal? (For the sockets that sending wrong data, the oscillator stop flag is 0 but the minutes register data is wrong, which is also very strange.)

4. Besides all above, what problem could cost this problem and cause the external oscillator not work ?

Schematic

OSCI Waveform after reset

OSCO after reset

Thanks a lot, 

Regards, 

Manu

  • 0
    TI__Guru* 85480 points

    Hi Manu,

    If your crystal meets below requirement, you don't need external shunt capacitors.

    The OF flag is always set on initial power-up, and it can be cleared through the serial interface. 

  • 0 in reply to Noel Fung
    TI__Intellectual 1050 points

    Hi Noel,

    Thanks for the answer, I still have some questions:

    1. For the crystal load capacitance, does it means the parasitic capacitor of crystal ? Also, if customers delete this capacitor, it would be 2-3 seconds drift in one day, how could they calibrate/cancel this drift ?

    2. Do you have any comments on the waveform, does that mean that the oscillator is not working ? Or are we measuring them correctly?

    3. About the oscillator stop flag,  is it set to 1 on initial power-up? From datasheet I believe it is initially set to 0. But it turns 1 in this case, so it means that the oscillator stopped once but doesn't mean that the oscillator is stopping right now? After the reset we can direct clear this flag?  Pls correct me if I understand this in a wrong way.

    Thanks a lot!

    Regards,

    Manu

  • 0 in reply to Manu Chang
    TI__Guru* 85480 points

    Hi Manu,

    The crystal being used should has load capacitance between 10.8pF and 13.2pF. We can calibrate the crystal to get better accuracy, details is here: https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/703635/bq32002-bq32002-improper-time#pi320966=1

    The waveforms look like there is an oscillation startup issue, the oscillation eventually die out probably due to too many load capacitance.

    OF is set to 1 at initial.