25
ATtiny26(L)
1477G–AVR–03/05
each time-out is shown in Table 5. The frequency of the Watchdog Oscillator is voltage
dependent as shown in the Electrical Characteristics section.
Default Clock Source
The deviced is shipped with CKSEL = “0001”, SUT = “10”, and PLLCK unprogrammed.
The default clock source setting is therefore the internal RC Oscillator with longest star-
tup time. This default setting ensures that all users can make their desired clock source
setting using an In-System or Parallel Programmer.
Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can
be configured for use as an On-chip Oscillator, as shown in Figure 22. Either a quartz
crystal or a ceramic resonator may be used. The maximum frequency for resonators is
12 MHz. The CKOPT Fuse should always be unprogrammed when using this clock
option. C1 and C2 should always be equal. The optimal value of the capacitors depends
on the crystal or resonator in use, the amount of stray capacitance, and the electromag-
netic noise of the environment. Some initial guidelines for choosing capacitors for use
with crystals are given in Table 6. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Figure 22. Crystal Oscillator Connections
The Oscillator can operate in three different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in
Table 6.
Note:
1. This option should not be used with crystals, only with ceramic resonators.
Table 5. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V)
Typ Time-out (V
CC
= 3.0V)
Number of Cycles
4.1 ms
4.3 ms
4K (4,096)
65 ms
69 ms
64K (65,536)
Table 6. Crystal Oscillator Operating Modes
CKSEL3..1
Frequency
Range (MHz)
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
101
0.4 - 0.9
–
110
0.9 - 3.0
12 - 22
111
3.0 - 16
12 - 22
16 -
12 - 15
XTAL2
XTAL1
GND
C2
C1
