What capacitors for 32 kHz crystal?

[Ecir Hana]

Please, does any one know whether 32 kHz crystal oscillator needs a pair of capacitors? If so, what values should each have?

 

[RCinFLA]

Depends on the crystal spec. Many 32 kHz crystals made for low power oscillators are specified for 9 or 10 pF loads.

If it is a leaded IBP tubular package (like seen in computer motherboards) I would guess 10 pF load. Don't forget to take PCB and IC capacitance into account. For 10 pF net load, try 15-18 pF on each side to ground (3-5 pF across Xtal for IC and PCB strays).

Avoid ground plane below crystal pads, PCB runner, and IC input pins as these can add a couple pF to loading. If you are using a raw CMOS inverter you need a series resistor on output to xtal as too much drive can destroy crystal.

 

[Ecir Hana]

Great, thanks!

 

[Noggin]

It also depends on what you connect it to. For instance, at least some of the AVR micro-controllers have a timer clock input intended to be used with just the 32kHz crystal and no capacitors.