© 2000 Scenix Semiconductor, Inc. All rights reserved. - 23 - www.scenix.com SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75 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. 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   typically   +/–8%
accuracy over the allowed temperature range. The inter-
nal 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  fre-
quency of the internal RC oscillator may not be 4 MHz.
This is due to the fact that the SX device requires trim-
ming to obtain 4 MHz operation. The parts shipped out of
the factory are not trimmed. The device relies on the pro-
gramming tool provided by the third party vendors to sup-
port trimming. 10.0    REAL TIME CLOCK
(RTCC)/WATCHDOG TIMER The  device  contains  an  8-bit  Real  Time  Clock/Counter
(RTCC)  and  an  8-bit  Watchdog  Timer  (WDT).  An  8-bit
programmable prescaler extends the RTCC to 16 bits. If
the prescaler is not used for the RTCC, it can serve as a
postscaler  for  the  Watchdog  Timer.  Figure 10-1  shows
the RTCC and WDT block diagram. 10.1    RTCC RTCC is an 8-bit real-time timer that is incremented once
each instruction cycle or from a transition on the RTCC
pin.  The  on-board prescaler  can  be  used  to  extend  the
RTCC counter to 16 bits.
The RTCC counter can be clocked by the internal instruc-
tion cycle clock or by an external clock source presented
at the RTCC pin.
To select the internal clock source, bit 5 of the OPTION
register should be cleared. In this mode, RTCC is incre-
mented at each instruction cycle unless the prescaler is
selected to increment the counter.
To select the external clock source, bit 5 of the OPTION
register must be set. In this mode, the RTCC counter is
incremented with each valid signal transition at the RTTC
pin. By using bit 4 of the OPTION register, the transition
can be programmed to be either a falling edge or rising
edge.  Setting  the  control  bit  selects  the  falling  edge  to
increment the counter. Clearing the bit selects the rising
edge.   
The RTCC generates an interrupt as a result of an RTCC
rollover from 0FF to 000. There is no interrupt pending bit
to  indicate  the overflow  occurrence.  The RTCC register
must be sampled by the program to determine any over-
flow occurrence. Figure 9-3. RC Oscillator Mode V_dd R C Internal
Circuitry OSC2 OSC1 N ~ SX Device