Note:
1. These options should only be used if frequency stability at start-up is not important for the
application.
10.7 Calibrated Internal RC Oscillator
By defaylt, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 31-1
on page 374 and “Internal Oscillator Speed” on page 406 for more details. The device is shipped
with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 49 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 10-8. 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 31-1 on page 374.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 50, 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 31-1 on page 374.
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 340.
Table 10-8. Internal Calibrated RC Oscillator Operating Modes(1)(2)
Frequency Range (MHz)
CKSEL3:0
7.3 - 8.1
0010
Notes: 1. The device is shipped with this option selected.
2. If 8 MHz 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.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 10-9 on page 47.
46
ATmega640/1280/1281/2560/2561
2549L–AVR–08/07
ATMEL [ ATMEL ]
