Datasheet

28
2586D–AVR–02/06
ATtiny25/45/85
quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher
frequency than OSCCAL = 0x80.
The CAL6:0 bits are used to tune the frequency within the selected range. A setting of 0x00
gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the
range. Incrementing CAL6:0 by 1 will give a frequency increment of less than 2% in the fre-
quency range 7.3 - 8.1 MHz.
Avoid changing the calibration value in large steps when calibrating the calibrated internal 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 unpredicatble behavior. Changes in OSCCAL should not
exceed 0x20 for each calibration. It is required to ensure that the MCU is kept in Reset during
such changes in the clock frequency
7.7 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure 7-
4. To run the device on an external clock, the CKSEL Fuses must be programmed to “00”.
Figure 7-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-10.
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
30 for details.
Table 7-9. 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 7-10. Start-up Times for the External Clock Selection
SUT1:0
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