We are using the BQ32002 RTC in our custom board. When board is powered off for one day and powered on again next day the time is coming with 5 seconds delay.
How to correct this timing delay?
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We are not able set FTF bit to 1 in SFR register (0x22). After writing the new value to register , when we are trying to read the register it will revert to default value.
Please suggest me how Configure the IRQ pin to 1 Hz ?
I have configured the IRQ bit to 1Hz. Is it possible to read the frequency of the IRQ and correct the error through software. ? so that it will easy for us to calibrate the device during mass production.
The issue is not consistent and it is varying from board to board. So it will be very helpful if u could suggest some software method to resolve the issue.
If the RTC is 5 seconds slow per day, this translates to an initial frequency error of about -58 ppm ( = -5 sec/day / 86400 sec/day). Based on this rough calculation and the calibration flow described early, try programming S = 1 and CAL = 14 to increase the frequency and calibrate-out the initial error at nominal conditions.
If you know have the crystal's frequency vs. temperature characteristic curve, you can use a nearby temp sensor and look-up the +/- frequency offset from the nominal temperature condition, then compensate the temperature-related offset by adjusting the CAL value from initial CAL setting at nominal condition.
Thank you for your support.
I am successfully able to calibrate and get the proper timing.
I cant use this method in mass production since the drift is not constant in all the board.
It would be very helpful if u could suggest some method to use in mass production.
In Table 6.5 of the datasheet the pre-calibration accuracy of the RTC is given as +/-35 ppm (typical).
+/-35 ppm * 3600 sec/hr * 24 hr = +/-3.03 sec.
Per Note 2 in Table 6.5: Typical accuracy is measured using reference board design and KDS DMX-26S surface-mount 32.768-kHz crystal. Variation in board design and crystal section results in different typical accuracy.
KDS DMX-26S frequency tolerance contributes +/-20 ppm (at 25 degC) toward the +/-35 ppm pre-calibration accuracy spec. So, the remaining +/-15 ppm error could be attributed to variation in load capacitance from the device and board.
If you're seeing a consistent time lag (-5 s --> -58 ppm pre-calibration accuracy) in your boards, it indicates the effective/actual load capacitance seen by the crystal is higher than the C_Load (12 pF?) specified for your crystal model.
Did you take care to minimize PCB stray capacitance on the crystal nets? Things you can try to minimize stray capacitance:
Also, check the crystal frequency tolerance spec is +/-20 ppm or lower; otherwise, this could cause a higher pre-calibration frequency error beyond what we typically specify in our datasheet.
Lastly, if you redesign your crystal layout to reduce stray capacitance, it is possible to overdo it and cause the actual load to be lower than the C_Load spec of your crystal; in this case, you could see a consistent time lead (+ ppm error) instead of time lag. So, you may consider adding the option to try external trim capacitors (DNP initially) to test different crystal trim capacitor values until you can get the pre-calibration accuracy within the datasheet typical spec.