Datasheet

28
ATmega8(L)
2486M–AVR–12/03
Calibrated Internal RC
Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All
frequencies are nominal values at 5V and 25°C. This clock may be selected as the sys-
tem 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 unpro-
grammed when using this clock option. During reset, hardware loads the 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 possible to achieve ± 1% accu-
racy 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 sec-
tion “Calibration Byte” on page 221.
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)
0001
(1)
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
(V
CC
= 5.0V) Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1 ms Fast rising power
10
(1)
6 CK 65 ms Slowly rising power
11 Reserved