ATtiny13A datasheet(25/174 Pages) ATMEL

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8126D–AVR–11/09

ATtiny13A

When this clock source is selected, start-up times are determined by the 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. It is required to ensure that the

MCU is kept in Reset during such changes in the clock frequency.

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 “System Clock Prescaler” on page

26 for details.

6.2.2

Calibrated Internal 4.8/9.6 MHz Oscillator

The calibrated internal oscillator provides a 4.8 or 9.6 MHz clock source. The frequency is nomi-

nal at 3V and 25

⋅C. If the frequency exceeds the specification of the device (depends on V

CC),

the CKDIV8 fuse must be programmed so that the internal clock is divided by 8 during start-up.

See “System Clock Prescaler” on page 26. for more details.

The internal oscillator is 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.

Note:

1. The device is shipped with this option selected.

During reset, hardware loads the calibration data into the OSCCAL register and thereby auto-

matically calibrates the oscillator. There are separate calibration bytes for 4.8 and 9.6 MHz

operation but only one is automatically loaded during reset (see section “Calibration Bytes” on

page 105). This is because the only difference between 4.8 MHz and 9.6 MHz mode is an inter-

nal clock divider.

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

page 27, it is possible to get a higher calibration accuracy than by using the factory calibration.

See “Calibrated Internal RC Oscillator Accuracy” on page 119.

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 Bytes” on page 105.

Table 6-3.

Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from

Power-down and Power-save

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

CKSEL1..0

Nominal Frequency

10(1)

9.6 MHz

4.8 MHz

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