© 2000 Scenix Semiconductor, Inc. All rights reserved. - 26 - www.scenix.com SX48BD/SX52BD/SX52BD75/SX52BD100 8.2 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. Figure8-3  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. 8.3 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.  During  internal  RC  mode,  OSC1  is  high impedance and OSC2 pulled low. Figure 8-3. RC Oscillator Mode V_dd R C Internal Circuitry OSC2 OSC1 N ~ SX Device