ATmega8(L)
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 11. Either a quartz
crystal or a ceramic resonator may be used. The CKOPT Fuse selects between two dif-
ferent Oscillator amplifier modes. When CKOPT is programmed, the Oscillator output
will oscillate a full rail-to-rail swing on the output. This mode is suitable when operating
in a very noisy environment or when the output from XTAL2 drives a second clock
buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the
Oscillator has a smaller output swing. This reduces power consumption considerably.
This mode has a limited frequency range and it cannot be used to drive other clock
buffers.
For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and
16 MHz with CKOPT programmed. 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 environ-
ment. Some initial guidelines for choosing capacitors for use with crystals are given in
Table 4. For ceramic resonators, the capacitor values given by the manufacturer should
be used.
Figure 11. Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
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 4.
Table 4. Crystal Oscillator Operating Modes
Frequency
Range(MHz)
Recommended Range for Capacitors
C1 and C2 for Use with Crystals (pF)
CKOPT
CKSEL3..1
101(1)
1
1
1
0
0.4 - 0.9
0.9 - 3.0
3.0 - 8.0
1.0 ≤
–
110
12 - 22
12 - 22
12 - 22
111
101, 110, 111
Note:
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 5.
25
2486M–AVR–12/03
ATMEL [ ATMEL ]
