ATMEGA8 datasheet(28/303 Pages) ATMEL

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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,

°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

(VCC = 5.0V)

Recommended Usage

6 CK

–

BOD enabled

6 CK

4.1 ms

Fast rising power

10(1)

6 CK

65 ms

Slowly rising power

Reserved

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