© 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. 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.
Bits 0, 1 and 5 of the FUSE register (FOSC1:FOSC2) are
used  to  configure  the  diffrent  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.The  recommended  target  values  for  the  external
component values are available at Scenix website.. 9.2 External RC Mode The external RC oscillator mode provides a cost-effective
approach  for  applications  that  do  not  require  a  precise
operating frequency. In this mode, the RC oscillator fre-
quency is a function of the supply voltage, the resistor (R)
and capacitor (C) values, and the operating temperature.
In addition, the oscillator frequency will vary from unit to
unit due to normal manufacturing process variations. Fur-
thermore,   the   difference   in   lead   frame   capacitance
between  package  types  also  affects  the  oscillation  fre-
quency, especially for low C values. The external R and
C   component   tolerances   contribute   to   oscillator   fre-
quency variation as well.
Figure 9-3  shows  the  external  RC  connection  diagram.
The recommended R value is from 3kW to 100kW. For R
values below 2.2kW, the oscillator may become unstable,
or may stop completely. For very high R values (such as
1 MW), the oscillator becomes sensitive to noise, humid-
ity, and leakage.
Although   the   oscillator   will   operate   with   no   external
capacitor (C = 0pF), it is recommended that you use val-
ues above 20 pF for noise immunity and stability. With no
or  small  external  capacitance,  the  oscillation  frequency
can  vary  significantly  due  to  variation  in  PCB  trace  or
package lead frame capacitances.
In  the  external  RC  mode,  the  OSC2/CLKOUT  pin  pro-
vides  an  output  frequency,  which  the  input  frequency
divided by four. 9.3 Internal RC Mode The internal RC mode uses an internal oscillator, so the
device  does  not  need  any  external  components.  At  4
MHz,   the   internal   oscillator   provides   +/–8%   accuracy
over  the  allowed  temperature  range.  The  internal  clock
frequency  can  be  divided  down  to  provide  one  of  eight
lower-frequency choices by selecting the desired value in
the  FUSE  Word  register.  The  frequency  range  is  from
31.25 KHz to 4 MHz. The default operating frequency of
the internal RC oscillator may not be 4 MHz. This is due 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)        Externally
Generated Clock OSC1 OSC2 Open SX Device