ATmega16M1 datasheet(33/365 Pages) ATMEL

33 / 365 page

7647F–AVR–04/09

ATmega16/32/64/M1/C1

5.5

Calibrated Internal RC Oscillator

By default, the Internal RC OScillator provides an approximate 8.0 MHz clock. Though voltage

and temperature dependent, this clock can be very accurately calibrated by the user. 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 5-1 on page 30. 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 calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory

calibration in Table 26-1 on page 319.

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

page 34, 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 “Clock Characteristics” 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 cali-

bration value, see the section.

Notes:

1. The device is shipped with this option selected.

2. 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 5-6 on page 33.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

The device is shipped with this option selected.

Table 5-5.

Internal Calibrated RC Oscillator Operating Modes(1)(2)

Frequency Range (MHz)

CKSEL3..0

7.3 - 8.1

0010

Table 5-6.

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(1)

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms(2)

Reserved

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