ATMEGA649 datasheet(28/375 Pages) ATMEL

28 / 375 page

ATmega329/3290/649/6490

2552H–AVR–11/06

Notes:

1. These options should only be used when not operating close to the maximum fre-

quency 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 fre-

quency stability at start-up. They can also be used with crystals when not operating

close to the maximum frequency of the device, and if frequency stability at start-up is

not important for the application.

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 13. When this Oscillator is selected,

start-up times are determined by the SUT Fuses as shown in Table 7 and CKSEL1..0 as

shown in Table 8.

Note:

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

the application

Calibrated Internal RC

Oscillator

The calibrated Internal RC Oscillator by default provides a 8.0 MHz clock. The fre-

quency is nominal 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.

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

shown in Table 9 on page 29. If selected, it will operate with no external components.

During reset, hardware loads the pre-programmed calibration value into the OSCCAL

calibration is shown as Factory calibration in Table 143 on page 319.

By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Reg-

ister” on page 29, 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

143 on page 319.

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” on page 284.

Table 7. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1..0

Additional Delay from Reset (V

CC = 5.0V)

Recommended Usage

14CK

Fast rising power or BOD enabled

14CK + 4.1 ms

Slowly rising power

14CK + 65 ms

Stable frequency at start-up

Reserved

Table 8. 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(1)

1K CK

0111

32K CK

Stable frequency at start-up

ATMEGA649 Datasheet(PDF) 28 Page - ATMEL Corporation

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