I remember reading long ago that I don't need filter caps for a watch (32768 Hz) crystal. Is this true? Or will I need to put some capacitors to ground on either end of the crystal? I will be using the crystal as the primary clock source.

  • \$\begingroup\$ This may be helpful: arduino.stackexchange.com/a/9039/37523 \$\endgroup\$ – VE7JRO Jan 12 '19 at 1:54
  • \$\begingroup\$ @VE7JRO That covers using the crystal for the asynchronous mode of timer2. I'm talking about using the crystal for the CPU clock. \$\endgroup\$ – Alexander M Jan 12 '19 at 2:01
  • \$\begingroup\$ all parallel crystals have a rated load pF that you almost double when in split on either side with CMOS input pF added to Cin. This tunes it to the rated 25'C tolerance error. otherwise it runs faster by 100 ppm or so,. Cload ranges from 6pF to 12.5pF to achieve 10 to 20 ppm error at 25'C \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 12 '19 at 2:12
  • \$\begingroup\$ Perhaps "9.5 Low Frequency Crystal Oscillator" in the datasheet may be helpful: ww1.microchip.com/downloads/en/DeviceDoc/… \$\endgroup\$ – VE7JRO Jan 12 '19 at 2:16
  • \$\begingroup\$ avrfreaks.net/forum/xtal-load-caps \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 12 '19 at 2:18

Atmel says Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied as described in Figure 9-2 on page 38.

6pF Xtals do exist but will not be as stable unless your layout includes a ground plane under the XTAL to create this added capacitance, the tolerance of the crystal cannot be met. But 50 ~100 ppm error may be close enough for you.

With no ground plane under the tracks or Xtal there is much less stray capacitance.

A Saturn PCB design simulation (free) can prove a layout capacitance if you really want to eliminate the caps.

0.127mm 1oz tracks over 0.127 thick FR4 is 0.92 pF/mm so 6.5mm track length would give 6pF on each track. Cin on chip is <1pF.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.