Problem with RTC time- Calibration

[Sam Cristtina]

I'm using with RTC. I have some problem with it.
my clock is slowing with 1.768sec/Day. I'm using with 32768Hz crystal with -20 to +20 PPM.
I Have solved it using calibration register. but it is difficult to calibrate all RTC (I have 5 RTC)using Calibration register.
So I like to ask u that what is better that I have to use tight tolerance crystal or calibration the RTC manually?

Thanks..

 

[betwixt]

It depends on the RTC device.
If it has the feature of a clock output as most do, divided down from the main crystal, the easiest solution is to compare it with an accurate frequency source and work out the calibration value based on the difference. For example, if it has a clock/4 output at 8192Hz, read that frequency and derive the correction factor from it. I would not advise connecting to the crystal directly as your probe will shift the frequency slightly. I did it once using a microphone and accurate frequency counter, the microphone could hear the vibration inside the crystal so no electrical connection was necessary!

Brian.

 

[Sam Cristtina]

Yes,u r right. my RTC has a feature of a clock output and using that i have calibrated the RTC and get perfect time.
But i think another way to get perfect time using tight tolerance crystal. so no need to calibration RTC.
Which is better way that calibrate the RTC or use tight tolerance crystal?

 

[betwixt]

I would go for calibration and a cheaper crystal. Although inexpensive crystals may not be absolutely correct in frequency, they are just as stable as tight tolerance ones. What you pay for is a standard crystal that has been selected for best accuracy, not one with superior quality. Other factors will also influence the frequency, such as loading capacitance and possibly a bias resistor, these can also be corrected by calibration at the same time.

Brian.

 

[Sam Cristtina]

Thanks for help...

 

[godfreyl]

Can't you set the crystal to the correct frequency by adjusting the load capacitor?

 

[Sam Cristtina]

Not need to connect capacitar because when i use battery than not need to connect load capacitor as per datasheet of bq32000. so another way to get perfact time without calibration?

 

[godfreyl]

The datasheet says not to use a super-capacitor with a battery. A super-capacitor is different to crystal load capacitor.

A super-capacitor can be used instead of a battery to store energy.
A crystal load capacitor can be used to adjust the oscillator frequency.

 

[Sam Cristtina]

Ok..
Thanks for inform me..