1. I have to connect two crystal , one for RTC & other for main clock, i have selected below crystals:
http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=370-1191-ND
http://www.digikey.com/product-search/en?vendor=0&keywords=SA315H16.00MF15XA-ND

2. Datasheet of both caps says 12.5pf load capacitance.

3. Does that mean I have to selected both CL1 & CL2 as 12.5pF each?

Edit: If yes , 12.5pf caps are not available on digikey , can I use 10pF instead with same cyrstal



4. Datasheet of MCU for 16Mhz says:
For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the 5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match the requirements of the crystal or resonator (see Figure 30). CL1 and CL2 are usually the same size. The crystal manufacturer typically specifies a load capacitance which is the series combination of CL1 and CL2. PCB and MCU pin capacitance must be included (10 pF can be used as a rough estimate of the combined pin and board capacitance) when sizing CL1 and CL2.

5. Datasheet of MCu for 32.768Khz says:
For CL1 and CL2 it is recommended to use high-quality external ceramic capacitors in the 5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see Figure 31). CL1 and CL2, are usually the same size. The crystal manufacturer typically specifies a load capacitance which is the series combination of CL1 and CL2.
Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray where Cstray is the pin capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF.

 

I usually start with 10-22pf then tune as described in AN849 and AN826 (from Microchip but applicable to most others). AN849 in particular is an excellent resource.

Good luck.

 

@JohnInTX , even if datasheet says 12.5pf, I have used 10pf & its working.
Thanks

 

I've found the exact value of cap not to be too critical. Of course, I've also never measured the exact frequency the thing was running at either.

 

The capacitance given is the series capacitance of the 2 capacitors + stray capacitance. So typically you would use about 2 x C - x, where x is a few pF. In this case I would use 22pF.
Bob

 

@JohnInTX , even if datasheet says 12.5pf, I have used 10pf & its working.
Thanks

Did you measure the frequency?