14.3 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.

The crystal should be connected as described in Clock Source Connections. When selecting crystals, load capacitance must be taken into consideration. The capacitance (C_e+C_i) needed at each TOSC pin can be calculated by using:

C_{e} + C_{i} = 2 C_{L} - C_{s}

where:

C_e - is optional external capacitors. (= C₁, C₂ as shown in Figure 14-2)
C_i - is the pin capacitance in Table 14-3.
C_L - is the load capacitance specified by the crystal vendor.
C_S - is the total stray capacitance for one XTAL pin.

Table 14-3. Internal capacitance of Low-Power Oscillator
32kHz Osc. Type	Internal Pad Capacitance (XTAL1)	Internal Pad Capacitance (XTAL2)
C_i of system oscillator (XTAL pins)	6pF	6pF

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 14-4. Low Power Crystal Oscillator Operating Modes⁽¹⁾
Frequency Range [MHz]	CKSEL[3:1]⁽²⁾	Range for total capacitance of C1 and C2 [pF]⁽⁴⁾
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:

This is the recommended CKSEL settings for the difference frequency ranges.
This option should not be used with crystals, only with ceramic resonators.
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.
When selecting the external capacitor value, the stray capacitance from the PCB and device should be deducted.

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

Table 14-5. 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	19CK + 4ms⁽¹⁾	0	00
Ceramic resonator, slowly rising power	258 CK	19CK + 65ms⁽¹⁾	0	01
Ceramic resonator, BOD enabled	1K CK	19CK⁽²⁾	0	10
Ceramic resonator, fast rising power	1K CK	19CK + 4ms⁽²⁾	0	11
Ceramic resonator, slowly rising power	1K CK	19CK + 65ms⁽²⁾	1	00
Crystal Oscillator, BOD enabled	16K CK	19CK	1	01
Crystal Oscillator, fast rising power	16K CK	19CK + 4ms	1	10
Crystal Oscillator, slowly rising power	16K CK	19CK + 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.

ATmega48PB/88PB/168PB

14.3 Low Power Crystal Oscillator