MSC8144E Reference Manual, Rev. 3
21-14
Freescale
Semiconductor
Timers
The software watchdog timer is enabled after reset to cause a hard reset or non-maskable
interrupt (MCP) if it times out. If the software WDT is not needed, you must clear
SWCRR[SWEN] to disable it. If it is used, the software WDT requires a special service sequence
that executes periodically. Without this periodic servicing, the software watchdog timer times out
and issues a reset or a non-maskable interrupt, as programmed in SWCRR[SWRI]. Once
software writes SWRI, the state of SWEN cannot be changed. Figure 21-1 shows the high level
WDT block diagram.
Figure 21-1. Software Watchdog Timer High Level Block Diagram
21.3.1 Features
The key features of the WDT include the following:
•
Based on 16-bit prescaler and 16-bit down-counter.
•
Provide a selectable range for the time-out period.
•
Provide ~21.47 s maximum software time-out delay for 200 MHz input clock.
21.3.2 Modes of Operation
The WDT unit can operate in the following modes:
•
WDT enable/disable mode:
If the software watchdog timer is not needed, user can disable it. SWCRR[SWEN] bit
enables the watchdog timer. It should be cleared by software after a system reset to disable
the software watchdog timer. When the watchdog timer is disabled, the watchdog counter
and prescaler counter are held in a stopped state.
— WDT enable mode (SWCRR[SWEN] = 1)
This is the default value after soft reset.
— WDT disable mode (SWCRR[SWEN] = 0)
If the software watchdog timer is not needed, the user must clear SWCRR[SWEN] to
disable it.
•
WDT reset/interrupt output mode
Without software periodic servicing, the software watchdog timer times out and issues a
reset or a nonmaskable interrupt (MCP), programmed in SWCRR[SWRI].
According to SWCRR[SWRI] programming, WDT timer causes a hard reset or machine
check interrupt to the core.
Software
Reset
or MCP
System
Watchdog
Register Interface
clock
Timer
Содержание MSC8144E
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