29
2552K–AVR–04/11
ATmega329/3290/649/6490
Note:
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.
8.4
Low-frequency Crystal Oscillator
To use a 32.768kHz watch crystal as the clock source for the device, the low-frequency crystal
Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111”. The crystal should
be connected as shown in
Figure 8-2. When this Oscillator is selected, start-up times are deter-
Note:
1. This option should only be used if frequency stability at start-up is not important for the
application
8.5
Calibrated Internal RC Oscillator
The calibrated Internal RC Oscillator by default provides a 8.0MHz clock. The frequency is nom-
inal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
011
14CK + 4.1ms
Ceramic resonator,
fast rising power
100
14CK + 65ms
Ceramic resonator,
slowly rising power
1
01
16K CK
14CK
Crystal Oscillator,
BOD enabled
1
10
16K CK
14CK + 4.1ms
Crystal Oscillator, fast
rising power
1
11
16K CK
14CK + 65ms
Crystal Oscillator,
slowly rising power
Table 8-4.
Start-up Times for the Crystal Oscillator Clock Selection (Continued)
CKSEL0
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC = 5.0V)
Recommended
Usage
Table 8-5.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
Additional Delay from Reset (V
CC = 5.0V)
Recommended Usage
00
14CK
Fast rising power or BOD enabled
01
14CK + 4.1ms
Slowly rising power
10
14CK + 65ms
Stable frequency at start-up
11
Reserved
Table 8-6.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3..0
Start-up Time from
Power-down and Power-save
Recommended Usage
1K CK
0111
32K CK
Stable frequency at start-up