ATMEGA8_08 Datasheet, PDF (30/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Calibrated Internal
RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All frequen-
cies 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 1 MHz calibration byte into the OSCCAL Register and thereby
automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0 MHz 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 possi-
ble 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 value, see the section
“Calibration Byte” on page 225.
Table 9. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
0001⁽¹⁾
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
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..
Table 10. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
00
Start-up Time from
Power-down and
Power-save
6 CK
Additional Delay
from Reset
(V_CC = 5.0V)
–
Recommended Usage
BOD enabled
01
10⁽¹⁾
6 CK
6 CK
4.1 ms
65 ms
Fast rising power
Slowly rising power
11
Reserved
Note: 1. The device is shipped with this option selected.
30 ATmega8(L)
2486T–AVR–05/08