29
ATmega8A [DATASHEET]
8159E–AVR–02/2013
9.6
Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0MHz clock. All frequencies are nominal
values at 5V and 25
C. This clock may be selected as the system clock by programming the CKSEL Fuses as
shown in
Table 9-8. If selected, it will operate with no external components. The CKOPT Fuse should always be
unprogrammed when using this clock option. During reset, hardware loads the 1MHz calibration byte into the OSC-
CAL Register and thereby automatically calibrates the RC Oscillator. At 5V, 25
C and 1.0MHz Oscillator frequency
selected, this calibration gives a frequency within ± 3% of the nominal frequency. Using run-time calibration meth-
ods as described in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accuracy at
any given V
CC and Temperature. 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 calibration
Note:
1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-9. PB6
(XTAL1/TOSC1) and PB7(XTAL2/TOSC2) can be used as either general I/O pins or Timer Oscillator pins:
Note:
1. The device is shipped with this option selected.
Table 9-8.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Table 9-9.
Start-up Times for the Internal Calibrated RC 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
6 CK
–
BOD enabled
01
6 CK
4.1 ms
Fast rising power
6 CK
65 ms
Slowly rising power
11
Reserved