I will be using the AT32UC3C2512C, the AVR32768 Application note recommends 22pF capacitors for crystals up to 16MHz...I will be using a 20MHz crystal, should I choose a 22pF or a 15pF loading capacitor?


As requested:

20MHz: 7B-20.000MEEQ-T

16MHz: ABM3B-16.000MHZ-10-1-U-T


2 Answers 2


A section on crystal oscillator characteristics is on page 1255 of the AT32UC3C series datasheet (complete as opposed to summary).

Excerpt from datasheet

The value of CLEXT is based on the formula:

$$2(C_L - C_I) - C_{PCB}$$

The 20MHz crystal you selected, for example, has a 10pF load capacitance (\$C_L\$), denoted by the "Q" near the end of its part number (See the part numbering reference). The internal equivalent load capacitance (\$C_I\$) is 1.7 pF (per page 1256 of the AT32UC3C datasheet). This leaves just the PCB (stray) capacitance (\$C_{PCB}\$) to be determined. That can get complicated, so we'll use a "rule of thumb" of 5pF.

$$2(10 - 1.7) - 5 = 11.6pF$$

For more information see:

  • \$\begingroup\$ A difference from 15pF to 11.6pF wont matter much right? \$\endgroup\$
    – mFeinstein
    Aug 25, 2013 at 4:22
  • \$\begingroup\$ Possibly not. The stray capacitance of your board may vary by at least that amount, so you may just have to experiment. I think the Freescale document is most helpful for understanding/determining stray capacitance, but as I mentioned, it's complicated. \$\endgroup\$
    – JYelton
    Aug 25, 2013 at 4:25
  • \$\begingroup\$ what are the results of a bad capacitor selection? Just inaccurate clock frequency or something else like no clock at all or erratic behaviour? \$\endgroup\$
    – mFeinstein
    Aug 25, 2013 at 4:29
  • \$\begingroup\$ Frequency instability mostly, but no clock can also happen (the crystal will fail to resonate). \$\endgroup\$
    – JYelton
    Aug 25, 2013 at 4:33
  • 1
    \$\begingroup\$ Based on the math and the crystal's load capacitance, I'd use 10pF. But don't quote me on that, especially if you order up a thousand production units based on this. :) \$\endgroup\$
    – JYelton
    Aug 25, 2013 at 4:56

If you are going to do this for a prototype don't even bother looking it up, either one will work.

If it's for production take a deeper look at the AT32UC3C2512C datasheet.

  • \$\begingroup\$ I already took a deep look...can't find a straight answer there...only if I know the PCB capacitance..which I dont.. \$\endgroup\$
    – mFeinstein
    Aug 25, 2013 at 2:59

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