SX User’s Manual Rev. 3.1 130 © 2000 Scenix Semiconductor, Inc. All rights reserved. www.scenix.com Chapter 4 Clocking, Power Down, and Reset In the “compatible” mode, the instruction rate is one-fourth of the clock rate. In this configuration, you
need to select a clock rate four times higher than the intended instruction rate. For example, if you want
to execute instructions at a rate of 1 MHz (one instruction per microsecond), you need to select a clock
rate of 4 MHz. In the “turbo” mode, the instruction rate is equal to the clock rate. For example, if you want to execute
instructions at a rate of 50 MHz (one instruction per 20 nanoseconds), you use a 50 MHz clock. This
is the preferred operating mode for new designs because you can use a slower clock to achieve a given
instruction rate, thus reducing electromagnetic interference (EMI) in the system and the cost of the
oscillator. 4.2.2 Internal RC Oscillator Using the on-chip, built-in RC (resistor-capacitor) oscillator for the device clock is the lowest-cost
option  because  no  external  components  are  required.  This  mode  is  suitable  for  lower-speed
applications (4 MHz or less) where high accuracy is not needed. For this mode, you leave the OSC1
and OSC2 pins unconnected. The internal RC oscillator operates at a nominal rate of 4 MHz and has an accuracy of plus or minus
8% over the allowed temperature range. The device can be configured to divide this clock down to
produce a lower-rate clock for device operation, with the divide-by factor selected by programming
the DIV1:DIV0 bits (SX18/20/28AC and SX18/20/28AC75) and the DIV1:DIV0 (SX48/52BD) in the
FUSE word as follows: 4.2.3 External RC Oscillator Using  an  external  RC  oscillator  network is  a low-cost option  suitable  for  applications that  do  not
require high precision. The only external components required are a resistor and a capacitor. Unlike
the internal RC oscillator, you can choose any operating frequency for which the device is rated, not
just certain frequencies between 31.25 kHz and 4 MHz. The RC oscillator operating rate is a function of the resistor and capacitor values, the supply voltage,
and the operating temperature. The operating rate will vary from unit to unit due to normal variations
in component values, and from time to time due to fluctuations in temperature and voltage. Therefore,
an application that requires high precision (for example, a system with a real-time clock) should use
an external resonator or crystal rather than an RC oscillator. Figure 4-1 shows how the resistor and capacitor are connected to the device. The operating frequency can be adjusted by choosing the values for R and C. In  this  operating  mode,  OSC1  is  the  clock  input.  A  resistor  value  between  3  kW  and  100  kW  is
recommended.  For  resistor  values  below  this  range,  the  oscillator  might  become  unstable  or  stop
completely.  For  resistor  values  higher  than  this  range,  the  oscillator  becomes  sensitive  to  noise,
humidity, and capacitor leakage. Although  the  device  will  operate  without  a  capacitor  (C  =  0  pF),  a  capacitor  of  at  least  20  pF  is recommended for noise immunity and stability. For capacitance values lower than this, the oscillator