crystal capacitor

[hayee]

Hi,
I am using pic16f628A and with this i am using 20Mhz crystal (ECX-200-20-1X).
In the datasheet of crystal(ECX-200-20-1X) load capacitance of 20pF is given.

I also read from different websites that if load capacitance is of 20pF then use 32pF capacitor on both sides.
they give the formula to calculate capacitance like that
C.load=(C.actual value + C.parasitic capacitance)/2
also C.parasitic capacitance is usually 5pF.
now if we put values in equation then
20pF=(C.actual value + 5pF)/2 is equal to 32pF.

Now
1- I wanna know that i directly use 20pF capacitor as given in the datasheet or i go with this formula
2- Which ceramic capacitor i can use for this.give me any model no.

[PCM programmer]

See this thread for a discussion on it:
https://www.ccsinfo.com/forum/viewtopic.php?p=103324

Look at page 2 of this document, on the right-hand side:
http://www.crystek.com/microwave/appnotes/Pierce-GateIntroduction.pdf

However, 22 pf load capacitors are commonly used by Microchip on their
development boards. Search for this part number at the Digikey website
and you will see what they look like:

[Ttelmah]

The formula, calculates the required capacitance to _give_ a particular load capacitance. Basically, the individual capacitors are paralleled with the parasitic capacitance, and then these two resulting capacitors are seen _in series_to give the atual load on the crystal. Hence the /2.
The parasitic capacitance, will depend _massively_ on the board layout. 5 to 6pF, is a 'good' guide figure, _provided_ the board has been properly laid out, _without_ a power plane, round the oscillator pins. See Microchip's application note on this.
The data sheet for the crystal, is saying that the crystal requires a load capacitance of 20pF, and to give this, provided the board is well laid out, the 32pF capactors are the way to go, and the formula you have is correct.
Microchip, uses crystals that only require 14pF load capacitance. Hence their use of 22pF load capactors.
This becomes 'critical', if you want to get really good accuracy out of your oscillator (things like USB, are much less forgiving than things like RS232....).

Best Wishes