ATmega644PA datasheet(37/468 Pages) ATMEL

37 / 468 page

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

8.6

Calibrated Internal RC Oscillator

By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and

temperature dependent, this clock can be very accurately calibrated by the the user. See Table

27-6 on page 333 and ”Internal Oscillator Speed” on page 359 and page 384 for more details.

The device is shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on

page 39 for more details.

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

Table 8-10. 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 cal-

ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in

Table 27-6 on page 333.

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

page 41, 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 27-6 on page 333.

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 299.

Notes:

1. The device is shipped with this option selected.

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

CC), the CKDIV8

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 8-11 on page 37.

Note:

The device is shipped with this option selected.

Table 8-10.

Internal Calibrated RC Oscillator Operating Modes

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

(1)

Table 8-11.

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

Reset (VCC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms

10(1)

Reserved

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