ATmega169P datasheet(32/393 Pages) ATMEL

32 / 393 page

8018N–AVR–08/09

ATmega169P

8.3

Default Clock Source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default

clock source setting is the Internal RC Oscillator with longest start-up time and an initial system

clock prescaling of 8. This default setting ensures that all users can make their desired clock

source setting using an In-System or Parallel programmer.

8.4

Calibrated Internal RC Oscillator

B 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 user. See Table 28-2

on page 330 and ”Internal Oscillator Speed” on page 363 for more details. The device is shipped

with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on page 37 for more details.

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

Table 8-3. 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 cali-

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

28-2 on page 330.

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

page 38, 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 330.

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 298.

Notes:

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 VCC), 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 8-4

Note:

1. The device is shipped with this option selected

Table 8-3.

Internal Calibrated RC Oscillator Operating Modes(1)(3)

Frequency Range(2) (MHz)

CKSEL3:0

7.3 - 8.1

0010

Table 8-4.

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 (V

CC = 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(1)

Reserved

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