28

2570L–AVR–08/07

ATmega325/3250/645/6450

Note:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

9.4

Low-frequency Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal

Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111”. The crystal should

be connected as shown in Figure 9-2 on page 27. When this Oscillator is selected, start-up times

are determined by the SUT Fuses as shown in Table 9-5 on page 28 and CKSEL1..0 as shown

in Table 9-6 on page 28.

Note:

This option should only be used if frequency stability at start-up is not important for the application

9.5

Calibrated Internal RC Oscillator

The calibrated Internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-

inal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See

“System Clock Prescaler” on page 31 for more details.

011

1K CK(2)

14CK + 4.1 ms

Ceramic resonator,

fast rising power

100

1K CK(2)

14CK + 65 ms

Ceramic resonator,

slowly rising power

1

01

16K CK

14CK

Crystal Oscillator,

BOD enabled

1

10

16K CK

14CK + 4.1 ms

Crystal Oscillator, fast

rising power

1

11

16K CK

14CK + 65 ms

Crystal Oscillator,

slowly rising power

Table 9-4.

Start-up Times for the Crystal Oscillator Clock Selection (Continued)

CKSEL0

SUT1:0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(V

Recommended

Usage

Table 9-5.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1..0

Recommended Usage

00

14CK

Fast rising power or BOD enabled

01

14CK + 4.1 ms

Slowly rising power

10

14CK + 65 ms

Stable frequency at start-up

11

Reserved

Table 9-6.

Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3..0

Start-up Time from

Power-down and Power-save

Recommended Usage

0110(Note:)

1K CK

0111

32K CK

Stable frequency at start-up