Datasheet

27
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
7.4 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 7-2. Either a quartz crystal or
a 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 Table 7-3 on page 27. For ceramic resonators, the capacitor
values given by the manufacturer should be used.
Figure 7-2. 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 7-3 on
page 27.
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
Table 7-4 on page 28.
Table 7-3. Crystal Oscillator Operating Modes
CKSEL3..1 Frequency Range (MHz)
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
100
(1)
0.4 - 0.9 –
101 0.9 - 3.0 12 - 22
110 3.0 - 8.0 12 - 22
111 8.0 - 12 - 22
XTAL2
XTAL1
GND
C2
C1