I have a few of these Xtals
http://www.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=300-8303-nd
It's loading capacitance is 12.5 pf
So to calculate the load capacitor
C=(25)-(5) or 20pF
5 being the parasitic capacitance and 25 = 2x12.5 the load capacitance.
I found this calculation here:
http://www.piclist.com/techref/clocks.htm
Does this seem correct?
Some other clarifications on the article:
If I read it correctly, it suggests taking one capacitor lead to ground and the other to VCC. But every circuit I have ever seen takes both caps to ground, including the PICS datasheet. Which is it?
And do I read it correctly that both caps to go straight to the ground and VCC pin of the PIC.
The datasheet shows an "RS" resistor and says "RS may be required to avoid overdriving crystals with low drive level specifications."
How do I know if the XTAL has a low drive spec? and how do I calculate the value of RS?
I am going to take the advice of Wookie in this thread and add an adjustable cap.
http://forum.allaboutcircuits.com/showthread.php?t=9215
But what is a good way of timing the accuracy of my adjustment? Other than the obvious, eye balling it off of a known good timing source.
http://www.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=300-8303-nd
It's loading capacitance is 12.5 pf
So to calculate the load capacitor
C=(25)-(5) or 20pF
5 being the parasitic capacitance and 25 = 2x12.5 the load capacitance.
I found this calculation here:
http://www.piclist.com/techref/clocks.htm
Does this seem correct?
Some other clarifications on the article:
If I read it correctly, it suggests taking one capacitor lead to ground and the other to VCC. But every circuit I have ever seen takes both caps to ground, including the PICS datasheet. Which is it?
And do I read it correctly that both caps to go straight to the ground and VCC pin of the PIC.
The datasheet shows an "RS" resistor and says "RS may be required to avoid overdriving crystals with low drive level specifications."
How do I know if the XTAL has a low drive spec? and how do I calculate the value of RS?
I am going to take the advice of Wookie in this thread and add an adjustable cap.
http://forum.allaboutcircuits.com/showthread.php?t=9215
But what is a good way of timing the accuracy of my adjustment? Other than the obvious, eye balling it off of a known good timing source.