Low Power Crystal Oscillator

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the lowest power consumption, but is not capable of driving other clock inputs, and may be more susceptible to noise in noisy environments. In these cases, refer to Full Swing Crystal Oscillator.

The crystal should be connected as described in Clock Source Connections.

The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL[3:1], as shown in the following table:

Table 1. Low Power Crystal Oscillator Operating Modes⁽¹⁾
Frequency Range [MHz]	CKSEL[3:1]⁽²⁾	Range for  Capacitors C1 and C2 [pF]
0.4 - 0.9	100⁽³⁾	–
0.9 - 3.0	101	12 - 22
3.0 - 8.0	110	12 - 22
8.0 - 16.0	111	12 - 22

Note:

If the crystal frequency exceeds the specification of the device (depends on V_CC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets the frequency specification of the device.
This is the recommended CKSEL settings for the difference frequency ranges.
This option should not be used with crystals, only with ceramic resonators.

The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times, as shown in the following table:

Table 2. Start-up Times for the Low Power Crystal Oscillator Clock Selection
Oscillator Source / Power Conditions	Start-up Time from Power-down and Power-save	Additional Delay from Reset  (V_CC = 5.0V)	CKSEL0	SUT[1:0]
Ceramic resonator, fast rising power	258 CK	14CK + 4.1ms⁽¹⁾	0	00
Ceramic resonator, slowly rising power	258 CK	14CK + 65ms⁽¹⁾	0	01
Ceramic resonator, BOD enabled	1K CK	14CK⁽²⁾	0	10
Ceramic resonator, fast rising power	1K CK	14CK + 4.1ms⁽²⁾	0	11
Ceramic resonator, slowly rising power	1K CK	14CK + 65ms⁽²⁾	1	00
Crystal Oscillator, BOD enabled	16K CK	14CK	1	01
Crystal Oscillator, fast rising power	16K CK	14CK + 4.1ms	1	10
Crystal Oscillator, slowly rising power	16K CK	14CK + 65ms	1	11

Note:

These options should only be used when not operating close to the maximum frequency of the device, and only if frequency stability at start-up is not important for the application. These options are not suitable for crystals.
These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency stability at start-up is not important for the application.