30
2486AA–AVR–02/2013
ATmega8(L)
Calibrated Internal
RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0MHz, 2.0MHz, 4.0MHz, 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. 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 OSCCAL Reg-
ister 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 methods 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 cali-
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 10. 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. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Table 10. 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
(VCC = 5.0V)
Recommended Usage
00
6 CK
–
BOD enabled
01
6 CK
4.1ms
Fast rising power
6 CK
65ms
Slowly rising power
11
Reserved