ATmega165 datasheet(27/336 Pages) ATMEL

27 / 336 page

ATmega165/V

2573G–AVR–07/09

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 provides a fixed 8.0 MHz clock. The frequency is

nominal value at 3V and 25

⋅C. If 8 MHz frequency exceeds the specification of the

device (depends on V

CC), the CKDIV8 Fuse must be programmed in order to divide the

internal frequency by 8 during start-up. The device is shipped with the CKDIV8 Fuse

programmed. See “System Clock Prescaler” on page 29. for more details. This clock

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

Table 8. If selected, it will operate with no external components. During reset, hardware

loads the calibration byte into the OSCCAL Register and thereby automatically cali-

brates the RC Oscillator. At 3V and 25

⋅C, this calibration gives a frequency within ± 10%

of the nominal frequency. Using calibration methods as described in application notes

available at www.atmel.com/avr it is possible to achieve ± 2% accuracy at any given V

and Temperature. When this Oscillator is used as the chip clock, the Watchdog Oscilla-

tor 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 249.

Note:

1. The device is shipped with this option selected.

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

shown in Table 9. Selecting internal RC Oscillator allows the XTAL1/TOSC1 and

XTAL2/TOSC2 pins to be used as timer oscillator pins.

Note:

1. The device is shipped with this option selected.

Table 7. 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

Table 8. Internal Calibrated RC Oscillator Operating Modes

(1)

CKSEL3..0

Nominal Frequency

0010

8.0 MHz

Table 9. Start-up times for the internal calibrated RC Oscillator clock selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4.1 ms

Fast rising power

10(1)

6 CK

14CK + 65 ms

Slowly rising power

Reserved

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