Hallo Azhar,

I would recommend you to have a look at archive of this forum. We discussed this stuff many times. Search for 'crystal' or 'quartz', 'oscillator', 'application note', etc. At least Raghunathan and me stated about certain application note from Intel dealing with quartz oscillator, in a very very detailed way!

If I remember correctly, built-in oscillators of microcontrollers are designed for fast and reliable start-up. Such designs do not provide highest accuracy! So, if you want to build something very stable, special RTC chip running at 32.768kHz should be used, or special stable oscillator, like Erik recommended.

Kai