Datasheet

13.6 Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, 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 the next table. 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 Register 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

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.

Table 13-8. Internal Calibrated RC Oscillator Operating Modes

CKSEL3:0 Nominal Frequency (MHz)

0001

(1)

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 the table

below. PB6 (XTAL1/TOSC1) and PB7(XTAL2/TOSC2) can be used as either general I/O pins or Timer

Oscillator pins:

Table 13-9. 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

= 5.0V)

Recommended Usage

00 6 CK – BOD enabled

01 6 CK 4.1ms Fast rising power

(1)

6 CK 65ms Slowly rising power

11 Reserved

Note: 1. The device is shipped with this option selected.