5.3 Recommended Capacitor Values

Important:

Recommendations given in this chapter are guiding values only. Always check the data sheet, and calculate values accordingly.

When using the external crystal oscillator on non-PB AVR devices, crystals with a nominal frequency range starting from 400 kHz can be used. For the standard high-frequency crystals, the recommended capacitor value range is in the range of 22 pF - 33 pF. For the newer AVR PB, the recommended capacitor value range is 12 pF - 22 pF, the total capacitance (Ce + Ci + Cs) for each pin must not exceed 22 pF.

The external low-frequency crystal is intended for 32.768 kHz crystals. When selecting this clock source, the internal oscillator circuit might provide the required capacitive load. By programming the CKOPT Fuse (1), the user can enable internal capacitors on XTAL1 and XTAL2. The value of the internal capacitor is typical 20 pF but can vary. External capacitors are not required when using a 32.768 kHz crystal that does not require more load. Then the value of the external capacitor can be determined using the Equation - 2. The CKOPT Fuse should not be programmed when using external capacitors.

In other cases, an external capacitive load specified by the manufacturer of the crystal must be used.

When using the external ceramic resonator, refer to the device data sheet for determining the capacitors values. Always use the recommended capacitive load, as the resonant frequency of the ceramic resonator is very sensitive to capacitive load.

Note:
  1. Some AVR devices may not come with internal capacitors. Some AVR devices may not have the CKOPT fuse, instead they have dedicated pins (TOSC1-TOSC2), to connect the 32.768 kHz crystal.
  2. Refer to the device data sheet for specific details related to oscillator connections.