Installation Manual

IRIS OEM Edition Hardware Reference Manual
When CKOPT is programmed, the Oscillator output will oscillate will 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 can not be used to
drive other clock buffers.
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.
XTAL2
XTAL1
GND
XTAL2
XTAL1
GND
Figure 4-3. Crystal Oscillator
Connections
The device has the following clock source options, selectable by Flash fuse bits as shown in
Table 4-1. The clock from the selected source is input to the AVR clock generator, and routed to
the appropriate modules.
Table 4-1. Device Clocking Options Selection
Device Clocking Option CKSEL[3..0]
Low Power Crystal Oscillator
1111—1000
Full Swing Crystal Oscillator
0111—0110
Low-Frequency Crystal Oscillator 0101-0100
Internal 128kHz RC Oscillator 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Some initial guidelines for choosing capacitors for use with crystals are given in Table 4-2. For
ceramic resonators, the capacitor values given by the manufacturer should be used.
Table 4-2. Low Power Crystal Oscillator Operating Modes
CKSEL[3..1] Freq Range (MHz) Capacitor Range
100
†
0.4 - 0.9 -
101 0.9 – 3.0 12 – 22 pF
110 3.0 – 8.0 12 – 22 pF
111 8.0 – 16.0 12 – 22 pF
†
ceramic resonators only
4.3.1 Internal RC Oscillator
The Calibrated Internal RC Oscillator provides a default 8.0 MHz clock. All frequencies are
nominal values at 3V and 25°C. This clock may be selected as the system clock by programming
the CKSEL fuses as shown in Table 4-3. If selected, it will operate with no external components.
Page 16 Doc. # 7430-0549-01 Rev. B