Datasheet

30
8197C–AVR–05/11
ATtiny261A/461A/861A
6.2.6 Crystal Oscillator / Ceramic Resonator
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 Figure 6-4. Either a quartz crystal or a
ceramic resonator may be used.
Figure 6-4. Crystal Oscillator Connections
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 6-11. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Notes: 1. This option should not be used with crystals, only with ceramic resonators.
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by fuses CKSEL[3:1] as shown in Table 6-11.
The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times as shown in Table
6-12.
Table 6-11. Crystal Oscillator Operating Modes
CKSEL[3:1] Frequency Range (MHz) Recommended C1 and C2 Value (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
Table 6-12. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT[1:0]
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
0 00 258 CK
(1)
14CK + 4 ms
Ceramic resonator,
fast rising power
0 01 258 CK
(1)
14CK + 64 ms
Ceramic resonator,
slowly rising power
0 10 1K (1024) CK
(2)
14CK
Ceramic resonator,
BOD enabled
XTAL2
XTAL1
GND
C2
C1