SX User’s Manual Rev. 3.1 129 © 2000 Scenix Semiconductor, Inc. All rights reserved. www.scenix.com Chapter 4 Clocking, Power Down, and Reset 4.1     Introduction The SX device can be configured to operate in any one of several clocking modes. You can use the
built-in oscillator, an external oscillator circuit, or an external clock signal to drive the device. Each
type of clock has its advantages and disadvantages with respect to clock rate choices, rate accuracy,
and cost. The SX device supports a “power down” mode, which reduces power consumption to a very low level
during  periods  of  inactivity.  This  mode  is  invoked  by  executing  the  “SLEEP”  instruction.  During
power down,  the device is completely inactive  (except  for  the  Watchdog  timer, if  enabled).  Upon
“wakeup” from the power down mode, the device is reset. A reset occurs for any of the following conditions: initial power-up, wakeup from the power down
mode, brown-out, Watchdog timeout, or assertion of the MCLR input signal (Master Clear Reset).
When a reset occurs, the program counter is initialized to the highest program address (7FFh or FFFh),
where the application program should have a “jump” instruction to its initialization routine. 4.2 Clocking Options You can configure the SX device to use an on-chip RC oscillator, an external RC oscillator, an external
crystal/resonator, or an externally generated clock signal. This choice depends on the required speed
and precision of the clock, as well as cost considerations. There are two device pins used for clocking, called OSC1 and OSC2. The functions of these pins
depend on the device configuration and the chosen clocking mode. You  select  the  desired  clocking  mode  by  programming  the  FUSE  word  register,  a  12-bit  register
mapped into the program memory. This register is accessible only when you are programming the
instruction memory of the device, not at run time. For information on the specific bit fields in the
register and the corresponding clocking modes, see Section 2.8. 4.2.1 Clock/Instruction Rate Option (Compatible or Turbo Mode) When you select the clock type, you need to consider the clock/instruction rate option. This option lets
you select one of two instruction clocking modes, called the “compatible” mode and the “turbo” mode.
The “compatible” mode is available only in the SX18/20/28AC and SX18/20/28AC75 devices.