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32
2593O–AVR–02/12
ATmega644
Notes:
1. 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.
2. 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 fre-
quency of the device, and if frequency stability at start-up is not important for the application.
7.5
Low Frequency Crystal Oscillator
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-
quency Crystal Oscillator. The crystal should be connected as described in
”Clock SourceTo find suitable load capacitance for a 32 kHz crysal, please consult the crystal datasheet.
When this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0
Note:
1. These options should only be used if frequency stability at start-up is not important for the
application.
Ceramic resonator, slowly
rising power
1K CK
100
Crystal Oscillator, BOD
enabled
16K CK
14CK
1
01
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
1
10
Crystal Oscillator, slowly
rising power
16K CK
14CK + 65 ms
1
11
Table 7-6.
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
SUT1..0
Table 7-7.
Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC = 5.0V)
CKSEL0
SUT1..0
BOD enabled
1K CK
000
Fast rising power
1K CK
001
Slowly rising power
1K CK
010
Reserved
0
11
BOD enabled
32K CK
14CK
1
00
Fast rising power
32K CK
14CK + 4.1 ms
1
01
Slowly rising power
32K CK
14CK + 65 ms
1
10
Reserved
1
11