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TC1796
System Units (Vol. 1 of 2)
Watchdog Timer
User’s Manual
16-24
V2.0, 2007-07
WDT, V2.0
16.5.4
Handling the User-Definable Password Field
WDT_CON0.WDTPW is an 8-bit field that can be set by software to any arbitrary value
during a Modify Access. Settings of this bit field have no effect on the operation of the
WDT, other than the role it plays in forming the password bit pattern, as discussed on
The purpose of this bit field is to support further enhancements to the password
protection scheme. For the following description, it is assumed that software does at
least not fully compute the value for the Password Access from the contents of registers
WDT_CON0 and WDT_CON1, but uses a predefined constant, embedded in the
instruction stream, for the password (this is at least necessary for the user-definable
password field WDTPW). For example, software can modify this bit field each time it
executes a Watchdog service sequence. The next service sequence needs to take this
new value into account for its Password Access, and it again changes the value during
its Modify Access. Up to 256 different password values can be used. In this way, each
service sequence is unique. If a malfunction occurs that, for instance, would result in the
omission of one or more of these service sequences, the next service sequence would
most probably not write the correct password. This service sequence would rely on the
password value programmed during the normally preceding service sequence.
However, if this one was skipped, the password value required by the contents of the
Watchdog registers is the one programmed at the last service sequence executed before
the malfunction had occurred. A Watchdog error condition would be detected in this
case.
In the same manner, the Watchdog would detect the malfunction if a service sequence
would be executed twice due to a falsely performed jump.
illustrates these
examples.