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SX20AC/SX28AC
9.0 OSCILLATOR CIRCUITS
The device supports several user-selectable oscillator
modes. The oscillator modes are selected by program-
ming the appropriate values into the FUSE Word register.
These are the different oscillator modes offered:
9.1 XT, LP or HS modes
In XT, LP, or HS modes, you can use either an external
crystal or resonator network, or an external clock signal
as the device clock.
To use an external crystal or resonator network, connect
a crystal or ceramic resonator to the OSC1 and OSC2
pins according to the circuit configuration shown in
mended. Use of a series resonant crystal may result in a
frequency that is outside the crystal manufacturer specifi-
cations. Table 9-1 shows the recommended external
components associated with a crystal-based oscillator.
Table 9-2 shows the recommended external component
values for a resonator-based oscillator.
Bits 5, 1, and 0 of the FUSE register (FOSC2:FOSC0)
are used to configure the different external resona-
tor/crystal oscillator modes. These bits allow the selec-
tion of the appropriate gain setting for the internal driver
to match the desired operating frequency. If the XT, LP, or
HS mode is selected, the OSC1 pin can be driven by an
external clock source rather than a resonator network, as
long as the clock signal meets the specified duty cycle,
case, the OSC2 pin should be left open.
LP: Low Power Crystal
XT: Crystal/Resonator
HS: High Speed Crystal/Resonator
RC: External Resistor/Capacitor
Internal Resistor/Capacitor
Figure 9-1. Crystal Operation (or Ceramic Resonator)
(HS, XT or LP OSC Configuration)
Figure 9-2. External Clock Input Operation
(HS, XT or LP OSC Configuration)
SX Device
RF
XTAL
OSC2
OSC1
C
1
C
2
Internal
Circuitry
SLEEP
RS = 0
Externally
Generated Clock
OSC1
OSC2
Open
SX Device
Table 9-1. FUSE Settings and External Component Selection for Crystal Oscillator (Vdd=5.0V), Rs = 0
FUSE Settings
Crystal
Frequency
C1
C2
R
F
IFBD
FOSC2:FOSC0
0
010
4 MHz
15 pF
22 pF
1 M
0
011
8 MHz
56 pF
33 pF
1 M
0
011
20 MHz
33 pF
22 pF
1 M
0
011
32 MHz
15 pF
22 pF
1 M
0
100
50* MHz
15 pF
15 pF
1 M
* 50 MHz fundamental crystal