Datasheet

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
as shown in Table 9-9.
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 Table
28-10 on page 337 and ”Internal oscillator speed” on page 390 and page 442 for more details.
The device is shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on
page 39 for more details.
This 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
Table 28-10 on page 337.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 40, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 28-10 on page 337.
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-
bration value, see the section ”Calibration byte” on page 301.
Notes: 1. The device is shipped with this option selected.
2. If 8MHz 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.
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
(V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1K CK 14CK
(1)
000
Fast rising power 1K CK 14CK + 4.1ms
(1)
001
Slowly rising power 1K CK 14CK + 65ms
(1)
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
(1)