Datasheet

8246A–AVR–11/09

ATtiny2313A/4313

Figure 6-2. External Clock Drive Configuration

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

Table 6-2.

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. See “System Clock Prescaler” on page 29

for details.

6.2.3 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

), the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 dur-

ing start-up. The device is shipped with the CKDIV8 Fuse programmed. This clock may be

selected as the system clock by programming the CKSEL Fuses as shown in Table 6-3. If

selected, it will operate with no external components. During reset, hardware loads the calibra-

tion byte into the OSCCAL Register and thereby automatically calibrates 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 possi-

ble to achieve ± 2% accuracy at any given V

and Temperature. When this Oscillator is used

as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the

Table 6-2. 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

00 6 CK 14CK BOD enabled

01 6 CK 14CK + 4 ms Fast rising power

10 6 CK 14CK + 64 ms Slowly rising power

11 Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND