External Clock - Atmel ATtiny25 Manual

6.7

External Clock

7598H–AVR–07/09
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the
lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-
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
Table 6-8. Internal RC Oscillator Frequency Range
Min Frequency in Percentage of
OSCCAL Value
0x00
0x3F
0x7F
To drive the device from an external clock source, CLKI should be driven as shown in
6-4. To run the device on an external clock, the CKSEL Fuses must be programmed to "00".
Figure 6-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-9.
Table 6-9. Start-up Times for the External Clock Selection
Start-up Time from
SUT1..0 Power-down and Power-save
00
01
10
11
Nominal Frequency
50%
75%
100%
EXTERNAL
CLOCK
SIGNAL
Additional Delay from
6 CK
6 CK
6 CK
ATtiny25/45/85
Max Frequency in Percentage of
Nominal Frequency
CLKI
GND
Reset Recommended Usage
14CK BOD enabled
14CK + 4 ms Fast rising power
14CK + 64 ms Slowly rising power
Reserved
100%
150%
200%
Figure
27