what the right way to determine the correct capacitors value needed for a avr-chip with a crystal oscillator?

Let say the crystal's load capacitance is: 20pF

and i want to use: atmega8-16pu

what should i look for and what the math behind it?

please don't just tell me to use 2x22pF capacitor. I'm not here to do my homework, but to

understand it for future use.


From AVR042, "AVR Hardware Design Considerations", section 4, "Using crystal and ceramic resonators", subsection 2, "General about crystals and ceramic resonators":

\$C = 2 ⋅ C_L − C_S\$

(CS can be estimated to be 5pF - 10pF)

So, about 30pF.

  • \$\begingroup\$ Wh it estimated to 5pF - 10pF? why not something more accurate ? why would i pick 5 instead of 10 or opposite ? \$\endgroup\$ – user1022734 Dec 7 '14 at 2:02
  • \$\begingroup\$ That would depend on how you've laid out the board and what it's made of. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 7 '14 at 2:04
  • \$\begingroup\$ okay, how did you guess about 30pF? \$\endgroup\$ – user1022734 Dec 7 '14 at 2:07
  • \$\begingroup\$ I took the high end of the estimate of the stray capacitance and plugged it into the equation. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 7 '14 at 2:08
  • \$\begingroup\$ what is the reason behind that choice? \$\endgroup\$ – user1022734 Dec 7 '14 at 2:12

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