Datasheet

32
8011Q–AVR–02/2013
ATmega164P/324P/644P
6.3 Low Power Crystal Oscillator
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 out-
put. 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 the ”Full Swing
Crystal Oscillator” on page 33.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 6-3. The
crystal should be connected as described in ”Clock Source Connections” on page 31.
The Low Power Oscillator can operate in three different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-3.
Notes: 1. If 8 MHz 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 different frequency ranges.
3. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
6-4.
Table 6-3. Low Power Crystal Oscillator Operating Modes
(1)
Frequency Range (MHz) CKSEL3..1
(2)
Recommended 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
Table 6-4. 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 SUT1..0
Ceramic resonator, fast
rising power
258 CK 14CK + 4.1 ms
(1)
000
Ceramic resonator, slowly
rising power
258 CK 14CK + 65 ms
(1)
001
Ceramic resonator, BOD
enabled
1K CK 14CK
(2)
010
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1 ms
(2)
011
Ceramic resonator, slowly
rising power
1K CK 14CK + 65 ms
(2)
100