25
8126F–AVR–05/12
ATtiny13A
When this clock source is selected, start-up times are determined by the SUT fuses as shown in
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
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.
The internal oscillator is selected as the system clock by programming the CKSEL fuses as
shown in
. 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
). 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
, it is possible to get a higher calibration accuracy than by using the factory calibration.
“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
SUT[1:0]
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
Table 6-4.
Internal Calibrated RC Oscillator Operating Modes
CKSEL[1:0]
Nominal Frequency
10
9.6 MHz
01
4.8 MHz