This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ32000: RTC time registers gets reset after power cycling

Part Number: BQ32000

hi,

we use BQ32000 and super cap as backup supply for the rtc.

On power cycling we observe the date and time getting reset in some cases.

we could also observe a drift in the rtc time, which varies from 1 or 2 minutes upto 50 to 60 minutes.


we found the time drift after a few hours of setting the RTC time properly and also we observed this time drift (sudden change in time upto a few minutes) immediately after a power cycle in some cases.

what can be the reason for this behavior of rtc time drift and the rtc getting reset.

  • Hello Nikhil,

    Sorry for your troubles.

    Is the drift always in the same direction?  That is always reporting accelerated time or always reporting delayed time?

    Can you provide the specifications of the crystal you are using?

    73,
    Timothy

  • hi Timothy,

    we always have drift backwards. today after 21 hours, we observed about 40 mins drift backwards in 1 device and a few seconds (4-5 sec) drift backwards in other devices.

    we use CM7V-T1A-32.768kHz-12.5pF-100ppm-TA-QC

  • Hi Nikhil,

    Check out this post which discusses calibrating the BQ32000 to achieve better accuracy: https://e2e.ti.com/support/clock-and-timing/f/48/t/703635

    In Table 6.5 of the datasheet the pre-calibration accuracy of the RTC is given as +/-35 ppm (typical), which corresponds to ~+/-3 seconds per day. You must calibrate the device to the XTAL for better accuracy. Note the accuracy may vary with several factors including board design and XTAL selection.

    +/-35 ppm * 3600 sec/hr * 24 hr = +/-3.03 sec

    In regards to the date and time being reset in some cases, the date and time will reset after a total power loss scenario. In other words, if VCC is removed and then reapplied, then the RTC would not lose the date and time if the supercapacitor is still providing Vback. Note that the timekeeping registers are updated one second after VCC is reapplied.

    Also, be sure to initialize the backup supply as detailed in section 8.2.2.3.

    Kind regards,
    Lane