ATtiny43U datasheet(25/182 Pages) ATMEL

25 / 182 page

8048B–AVR–03/09

When this clock source is selected, start-up times are determined by SUT Fuses as shown in

Table 6-3.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is

required, ensure that the MCU is kept in Reset during the changes.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal

clock frequency while still ensuring stable operation. Refer to “Power Management and Sleep

Modes” on page 31 for details.

6.2.2

Calibrated Internal 8 MHz 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. See Table

20-2 on page 157 for more details. The device is shipped with the CKDIV8 Fuse programmed.

See “System Clock Prescaler” on page 27 for more details.

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

Table 6-4. 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 20-2 on page 157.

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

page 28, 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 20-2 on page 157.

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 142.

Notes:

1. The device is shipped with this option selected.

Table 6-3.

Start-up Times for the External Clock Selection

SUT1..0

Start-up Time

from Power-down

Additional Delay

from Reset

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

6 CK

14CK + 64 ms

Slowly rising power

Reserved

Table 6-4.

Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0

Nominal Frequency (MHz)

0010

(1)

8.0

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