Datasheet

Figure 10-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
Related Links
Low Power Crystal Oscillator on page 45
Full Swing Crystal Oscillator on page 46
Low Frequency Crystal Oscillator on page 47
10.3. 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. 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 10-3. 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:
1. 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.
2. This is the recommended CKSEL settings for the difference frequency ranges.
3. 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:
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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