36
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
To find suitable load capacitance for a 32.768kHz 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.
9.6
Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the the user. See
TableThis clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 9-10. If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
page 40, it is possible to get a higher calibration accuracy than by using the factory calibration.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the
Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-
Notes:
1. The device is shipped with this option selected.
2. If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
Table 9-9.
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
(VCC = 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 + 4.1ms
1
01
Slowly rising power
32K CK
14CK + 65ms
1
10
Reserved
1
11
Table 9-10.
Internal Calibrated RC Oscillator Operating Modes.
Frequency range (2) [MHz] CKSEL3..0
7.3 - 8.1
0010