Datasheet
26
8183F–AVR–06/12
ATtiny24A/44A/84A
6.2.1 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure 6-2
on page 26. To run the device on an external clock, the CKSEL Fuses must be programmed to
“0000”.
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 6-3.
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. See “System Clock Prescaler” on page 30
for details.
6.2.2 Calibrated Internal 8 MHz Oscillator
By default, the Internal Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 20-2
on page 175 and “Internal Oscillator Speed” on page 236 for more details. The device is shipped
with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 30 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 175.
Table 6-3. Start-up Times for the External Clock Selection
SUT[1:0]
Start-up Time
from Power-down
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
CLKI
GND