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TC1784
System Control Unit (SCU)
User´s Manual
3-130
V1.1, 2011-05
32-bit SCU, V1.18
3.8
Watchdog Timer
This section describes the TC1784 Watchdog Timer (WDT). Topics include an overview
of the WDT function and descriptions of the registers, the password-protection scheme,
accessing registers, modes, and initialization.
3.8.1
Watchdog Timer Overview
The WDT provides a highly reliable and secure way to detect and recover from software
or hardware failure. The WDT helps to abort an accidental malfunction of the TC1784 in
a user-specified time period. When enabled, the WDT can cause the TC1784 system to
be reset if the WDT is not serviced within a user-programmable time period. The CPU
must service the WDT within this time interval to prevent the WDT from causing a
TC1784 System or Application Reset. Hence, routine service of the WDT confirms that
the system is functioning properly.
In addition to this standard “Watchdog” function, the WDT incorporates the End-of-
Initialization (Endinit) feature and monitors its modifications.
Because servicing the Watchdog and modifications of the ENDINIT bit are critical
functions that must not be allowed in case of a system malfunction, a sophisticated
scheme is implemented that requires a password and guard bits during accesses to the
WDT control register. Any write access that does not deliver the correct password or the
correct value for the guard bits is regarded as a malfunction of the system, and a
Watchdog reset is requested. In addition, even after a valid access has been performed
and the ENDINIT bit has been cleared to provide access to the critical registers, the
Watchdog imposes a time limit for this access window. If bit ENDINIT has not been
properly set again before this limit expires, the system is assumed to have
malfunctioned, and a Watchdog reset is requested. These stringent requirements,
although not guaranteed, nonetheless provide a high degree of assurance of the
robustness of system operation.
A further enhancement in the TC1784’s WDT is its reset prewarning operation. Instead
of immediately resetting the device on the detection of an error (the way that standard
Watchdogs do), the WDT first issues a Non-Maskable Interrupt (NMI) to the CPU before
finally resetting the device at a specified time period later.
3.8.2
Features of the Watchdog Timer
The main features of the WDT are summarized here.
•
16-bit Watchdog counter
•
Selectable input frequency:
f
FPI
/256 or
f
FPI
/16384
•
16-bit user-definable reload value for normal Watchdog operation, fixed reload value
for Time-Out and Prewarning Modes
•
Incorporation of the ENDINIT bit and monitoring of its modifications
Summary of Contents for TC1784
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