Full Swing Crystal Oscillator

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving other clock inputs and in noisy environments. The current consumption is higher than for the Low Power Crystal Oscillator. Note that the Full Swing Crystal Oscillator will only operate for V_CC=2.7-5.5V.

Some initial guidelines for choosing capacitors for use with crystals are given in Table 2. The crystal should be connected as described in Clock Source Connections”.

The operating mode is selected based on the fuses CKSEL[3:1] as shown in the table:

Table 1. Full Swing Crystal Oscillator operating modes
Frequency Range⁽¹⁾ [MHz]	CKSEL[3:1]	Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 20	011	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.

For the Crystall Oscillator connections refer to Low Power Crystal Oscillator.

Table 2. Start-Up Times for the Full Swing 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.