Datasheet

28
8246B–AVR–09/11
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 31
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 speed specification of the device
(depends on V
CC
), the CKDIV8 Fuse must be programmed in order to divide the internal fre-
quency by 8 during 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 calibration 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 fre-
quency. 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
CC
and Temperature.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used 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
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND