© 1999 Scenix Semiconductor, Inc. All rights reserved. - 20 - www.scenix.com SX18AC  / 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
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.  Table 9-1  shows  the  recom-
mended   capacitor   values   to   be   used   with   ceramic
resonators.  Table 9-2  shows  the  recommended  compo-
nent values to be used at different crystal frequencies. A
parallel resonant crystal type is recommended. Use of a
series resonant crystal may result in a frequency that is
outside the crystal manufacturer specifications.
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 lev-
els  (Figure 9-2).  In  this  case,  the  OSC2/CLKOUT  pin
should be left open. Note:Series  resistor  (R_S)  is  not  required  for  crystal  fre-
quencies higher than 1 MHz; use a direct connection in-
stead. 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) SX Device RF XTAL OSC2 OSC1 C₁ C₂ Internal
Circuitry SLEEP RS Figure 9-2. External Clock Input Operation (HS, XT or LP OSC Configuration) Table 9-1.  Capacitor Selection for Ceramic Resonators Clock
Mode Resonator
Frequency C1 C2 R_F R_S XT 455 kHz 220 pF 220 pF 1 MW 6.8 kW XT 1 MHz 100 pF 100 pF 1 MW 6.8 kW XT 2 MHz 100 pF 100 pF    100 kW    680 W HS 4 MHz 100 pF 100 pF    100 kW 0 HS 4 MHz Internal
(47 pF) Internal
(47 pF) 100 kW    470 W HS 8 MHz 30 pF 30 pF 1 MW 0 HS 8 MHz Internal
(47 pF) Internal
(47 pF) 1 MW 470 W HS 12 MHz 30 pF 30 pF 1 MW 0 HS 12 MHz Internal
(22 pF) Internal
(22 pF) 1 MW 0 HS 16 MHz 15 pF 15 pF 1 MW 0 HS 16 MHz Internal
(15 pF) Internal
(15 pF) 1 MW 0 HS 20 MHz 10 pF 10 pF 1 MW 0 HS 33 MHz 10 pF 10 pF 22 kW 0 HS 40 MHz 7 pF 7 pF 22 kW 0        Externally
Generated Clock OSC1 OSC2 Open SX Device