
41
2490R–AVR–02/2013
ATmega64(L)
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.
Low-frequency
Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency crystal
Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-
nected as shown in
Figure 19. By programming the CKOPT Fuse, the user can enable internal
capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter-
nal capacitors have a nominal value of 36 pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Note:
1. These options should only be used if frequency stability at start-up is not important for the
application.
Table 9. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
000
4.1 ms
Ceramic resonator, fast
rising power
001
65 ms
Ceramic resonator,
slowly rising power
010
–
Ceramic resonator, BOD
enabled
011
4.1 ms
Ceramic resonator, fast
rising power
100
65 ms
Ceramic resonator,
slowly rising power
1
01
16K CK
–
Crystal Oscillator, BOD
enabled
1
10
16K CK
4.1 ms
Crystal Oscillator, fast
rising power
1
11
16K CK
65 ms
Crystal Oscillator, slowly
rising power
Table 10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
Start-up Time
from Power-down
and Power-save
Additional Delay
from Reset
(V
CC = 5.0V)
Recommended Usage
00
4.1 ms
Fast rising power or BOD enabled
01
65 ms
Slowly rising power
10
32K CK
65 ms
Stable frequency at start-up
11
Reserved