Hello Pandurang,
1. Can we know the relation of Load capacitance with RTC running behavior? I mean if the value of load capacitance is not matching with what should be required, what will be the effect on RTC run? If the load capacitance has higher value than required, will RTC run faster or slower?
[A] If the load capacitance value is higher, than required the RTC should run slower, because it takes longer time to charge and discharge the capacitor, resulting in lower oscillator frequency. But in your case the crystal requires 6pF and you had the load capacitance set to default 7pF, which should result in negative deviation, but your deviation is positive. Could you please provide the crystal datasheet?
2. For the Offset register, if the RTC runs faster, we should write a positive value, correct?
[A] Yes, that is correct.
3. Offset calculation.
One pulse time observed is 30.60 uS. So, the calculation should be as below:
Dmeas = ((1/32768)*1000000 - 30.60 uS = −0.082421875 uS
Eppm = 10^6 * −0.082421875/30.60 = −2693.525326797.
Now, as we know this value is out of range for Offset register, so we cannot use this value, right?
[A] I have checked your calculation and they are correct. The deviation is too large and cannot be corrected with the Offset register. Have you set the load capacitance to the 6pF as required per crystal? How did you measure the CLKOUT frequency?
The datasheet of the oscillator indicates that the ppm value is +/-20ppm.
According to this, the calculation results in 9.830425166.
the offset register should have 9 or 10 value in fast mode (OFFM = 1). Kindly confirm.
[A] You can test it, but I don't think it will correct the large deviation. A crystal with +/- 100ppm or better is recommended. Yours crystal has +/-20ppm, which is excellent. So there shouldn't be such large deviation. Could you please test another crystal? Another piece in case this one is faulty or from an other manufacturer.
With Best Regards,
Jozef
