Datasheet

24
ATtiny13
2535E–AVR–10/04
write may fail. Note that the Oscillator is intended for calibration to 9.6 MHz or 4.8 MHz.
Tuning to other values is not guaranteed, as indicated in Table 6.
Avoid changing the calibration value in large steps when calibrating the calibrated inter-
nal RC Oscillator to ensure stable operation of the MCU. A variation in frequency of
more than 2% from one cycle to the next can lead to unpredictable behavior. Changes in
OSCCAL-register should not exceed 0x20 for each calibration.
External Clock To drive the device from an external clock source, CLKI should be driven as shown in
Figure 12. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “00”.
Figure 12. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 7.
When applying an external clock, it is required to avoid sudden changes in the applied
clock frequency 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 25 for details.
Table 6. Internal RC Oscillator Frequency Range
OSCCAL Value
Min Frequency in Percentage of
Nominal Frequency
Max Frequency in Percentage of
Nominal Frequency
0x00 50% 100%
0x3F 75% 150%
0x7F 100% 200%
Table 7. 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
CLKI
GND