© 2000 Scenix Semiconductor, Inc. All rights reserved. - 21 - www.scenix.com SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75 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
resonator  network  or  an  external  clock  signal  as  the
device clock.
To  use  an  external  resonator  network,  you  connect  a
crystal  or  ceramic  resonator  to  the  OSC1/CLKIN  and
OSC2/CLKOUT  pins  according  to  the  circuit  configura-
tion shown in Figure 9-1. A parallel resonant crystal type
is  recommended.  Use  of  a  series  resonant  crystal  may
result in a frequency that is outside the crystal manufac-
turer  specifications.  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 0, 1 and 5 of the FUSE register (FOSC1:FOSC2) are
used to configure the different external resonator/crystal
oscillator  modes.  These  bits  allow  the  selection  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/CLKIN 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, rise and fall times, and input levels (Figure 9-2). In
this case, the OSC2/CLKOUT 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 C₁ C₂ Internal
Circuitry SLEEP RS        Externally
Generated Clock OSC1 OSC2 Open SX Device Table 9-1.  External Component Selection for Crystal Oscillator(Vdd=5.0V) FOSC2:FOSC0    Crystal
Frequency C1 C2 R_F R_S 010 4 MHz 15 pF 22 pF 1 MW 0 W 011 8 MHz 56 pF 33 pF 1 MW 0 W 011 20 MHz 33 pF 22 pF 1 MW 0 W 011 32 MHz 15 pF 22 pF 1 MW 0 W 100 50* MHz 15 pF 15 pF 1 MW 0 W * 50 MHz fundamental crystal