29

2570L–AVR–08/07

ATmega325/3250/645/6450

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 9-7 on page 29. If selected, it will operate with no external components. During reset,

hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby

automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory

calibration in Table 28-2 on page 299.

By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on

page 31, it is possible to get a higher calibration accuracy than by using the factory calibration.

The accuracy of this calibration is shown as User calibration in Table 28-2 on page 299.

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 267.

Note:

1. The device is shipped with this option selected.

2. The frequency ranges are preliminary values. Actual values are TBD.

3. If 8 MHz frequency exceeds the specification of the device (depends on V

Fuse can be programmed in order to divide the internal frequency by 8.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 9-8 on page 29.

Note:

The device is shipped with this option selected.

9.6

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

9-3 on page 30. To run the device on an external clock, the CKSEL Fuses must be programmed

to “0000” (see Table 9-9 on page 29).

Table 9-7.

Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

Table 9-8.

Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

SUT1..0

BOD enabled

6 CK

14CK

00

Fast rising power

6 CK

14CK + 4.1 ms

01

Slowly rising power

6 CK

14CK + 65 ms(Note:)

10

Reserved

11

Table 9-9.

Crystal Oscillator Clock Frequency

Frequency Range

CKSEL3..0

0 - 16 MHz

0000