# MCU crystal capacitor selection

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?

EDIT:

As requested:

20MHz: 7B-20.000MEEQ-T

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

• can you please share the datasheets of the crystals, you are going to use, usually loading capacitor values are specified in datasheet, check this electronics.stackexchange.com/questions/75704/… – AKR Aug 25 '13 at 3:24
• Yes, I have all the values, but the formula in the MCU datasheet asks for the PCB capacitance also..which I have no clue... – mFeinstein Aug 25 '13 at 3:30
• The datasheet specifies the load cap to be 18pf for the ABM3B crystal – AKR Aug 25 '13 at 3:38
• – JYelton Aug 25 '13 at 4:00
• – JYelton Aug 25 '13 at 4:09

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

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.

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