ATmega8(L)
Calibrated Internal 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 25C. This clock may be selected as the
RC Oscillator
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, 25C and 1.0MHz Oscillator
frequency selected, this calibration gives a frequency within 3ꢀ 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 1ꢀ accuracy at any given VCC 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-
bration value, see the section “Calibration Byte” on page 218.
Table 9. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
0001(1)
0010
Nominal Frequency (MHz)
1.0
2.0
4.0
8.0
0011
0100
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
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
SUT1..0
00
(VCC = 5.0V)
Recommended Usage
BOD enabled
6 CK
6 CK
6 CK
–
01
4.1ms
65ms
Fast rising power
Slowly rising power
10(1)
11
Reserved
Note:
1. The device is shipped with this option selected
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2486AA–AVR–02/2013
ATMEL [ ATMEL ]
