PXS20RM Datasheet, PDF (1303/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Chapter 45
Software Watchdog Timer (SWT)
Software Watchdog Timer (SWT)
45.1 Introduction
45.1.1 Overview
The Software Watchdog Timer (SWT) is a peripheral module that can prevent system lockup in situations
such as software getting trapped in a loop or if a bus transaction fails to terminate. When enabled, the SWT
requires periodic execution of a watchdog servicing operation. The servicing operation resets the timer to
a specified time-out period. If this servicing action does not occur before the timer expires the SWT
generates an interrupt or hardware reset. The SWT can be configured to generate a reset or interrupt on an
initial time-out, a reset is always generated on a second consecutive time-out.
45.1.2 Features
The SWT has the following features:
• 32-bit time-out register to set the time-out period
• Fixed selection of IRCOSC clock for timer operation
• Programmable selection of window mode or regular servicing
• Programmable selection of reset or interrupt on an initial time-out
• Programmable selection of fixed or keyed servicing
• Master access protection
• Hard and soft configuration lock bits
On this device, the SWT is always driven by the IRCOSC.
45.1.3 Modes of Operation
The SWT supports three device modes of operation: normal, debug and stop. When the SWT is enabled
in normal mode, its counter runs continuously. In debug mode, operation of the counter is controlled by
the FRZ bit in the SWT_CR. If the FRZ bit is set, the counter is stopped in debug mode, otherwise it
continues to run. In stop mode, operation of the counter is controlled by the STP bit in the SWT_CR. If
the STP bit is set, the counter is stopped in stop mode, otherwise it continues to run.
45.2 External Signal Description
The SWT module does not have any external interface signals.
45.3 Memory Map and Register Definition
The SWT programming model has seven 32-bit registers. The programming model can only be accessed
using 32-bit (word) accesses. References using a different size are invalid. Other types of invalid accesses
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
45-1