AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Brian" <blue_eagle74@...> wrote:
> Under impedance is says 18pf and series.
The specified impedance affects the size of the external load 
capacitors that you must use.  There is a formula given in the 
crystal's datasheet that you can use to compute the size of the load 
capacitors.  When computing the load capacitor size, you must take 
into account both the stray capacitance of the crystal/capacitor 
circuit and the the impedance of the X1/X2 pins of the AVR.

With the crystal and capacitors located very close to the AVR on my 
circuit boards, I use 27pF capacitors with an 18pF crystal.  Others 
seem to prefer to use 22pF capacitors but my calculations indicate 
that the latter value may be a bit low.  For many applications, I 
suspect that either size, or anything in between, would work just as 
well.

The load capacitance does affect the actual oscillator frequency.  
If the load is too high or too low the oscillator frequency will be 
too low or too high.  That is the reason that you should calculate 
the load capacitor value carefully.

Don Kinzer
ZBasic Microcontrollers
http://www.zbasic.net