ATMEGA8_14 Datasheet, PDF (30/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

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-
bration value, see the section “Calibration Byte” on page 218.
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
4.1ms
65ms
Fast rising power
Slowly rising power
11
Reserved
Note: 1. The device is shipped with this option selected
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