Default Clock Source; Crystal Oscillator - Atmel ATtiny25 Manual

6.3

Default Clock Source

6.4

Crystal Oscillator

ATtiny25/45/85
24
Table 6-2. Number of Watchdog Oscillator Cycles
Typ Time-out
4 ms
64 ms
The device is shipped with CKSEL = "0010", SUT = "10", and CKDIV8 programmed. The default
clock source setting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up
time and an initial system clock prescaling of 8. This default setting ensures that all users can
make their desired clock source setting using an In-System or High-voltage Programmer.
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
figured for use as an On-chip Oscillator, as shown in
ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in
the manufacturer should be used.
Figure 6-3. Crystal Oscillator Connections
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3..1 as shown in
Table 6-3. Crystal Oscillator Operating Modes
CKSEL3..1 Frequency Range (MHz)
(1)
100
101
110
111
Notes: 1. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in
6-4.
Table 6-3. For ceramic resonators, the capacitor values given by
C2
C1
Recommended Range for Capacitors C1 and
0.4 - 0.9
0.9 - 3.0
3.0 - 8.0
8.0 -
Number of Cycles
512
8K (8,192)
Figure 6-3. Either a quartz crystal or a
XTAL2
XTAL1
GND
Table
C2 for Use with Crystals (pF)
–
12 - 22
12 - 22
12 - 22
6-3.
Table
7598H–AVR–07/09