TC1796
System Units (Vol. 1 of 2)
Watchdog Timer
User’s Manual
16-8
V2.0, 2007-07
WDT, V2.0
16.4.2.1 Time-Out Mode
The Time-Out Mode is the default mode after a reset. It is also always entered when a
Valid Password Access to register WDT_CON0 is performed (see
). The
timer is set to a predefined value and starts counting upwards. Time-Out Mode can only
be exited properly by setting ENDINIT to 1 with a correct access sequence. If an
improper access to the WDT is performed, or if the timer overflows before ENDINIT is
set to 1, a WDT Non-Maskable Interrupt request (WDT_NMI) is requested, and
Prewarning Mode is entered. A reset of the TC1796 is imminent and can no longer be
stopped.
A proper exit from Time-Out Mode can either be to the Normal or the Disable Mode,
depending on the state of the disable request bit, WDTDR, in register WDT_CON1.
16.4.2.2 Normal Mode
In Normal Mode (WDTDR = 0), the WDT operates in a standard Watchdog fashion. The
timer is set to a user-defined start value, and begins counting up. It has to be serviced
before the counter overflows. Servicing is performed through a proper access sequence
to the WDT control register WDT_CON0. This reloads the timer with the start value, and
normal operation continues.
If the WDT is not serviced before the timer overflows, or if an invalid access to the WDT
is performed, a system malfunction is assumed. Normal Mode is terminated, a WDT NMI
request (WDT_NMI) is requested, and Prewarning Mode is entered. A reset of the
TC1796 is imminent and can no longer be stopped.
Because servicing the WDT is an access sequence, first requiring a Valid Password
Access to register WDT_CON0, the WDT will enter Time-Out Mode until the second
proper access is performed.
16.4.2.3 Disable Mode
Disable Mode is provided for applications which truly do not require the WDT function. It
can be entered from Time-Out Mode when the disable request bit WDTDR is set to 1.
The timer is stopped in this mode. However, disabling the WDT does only stop it from
performing the standard Watchdog function (Normal Mode), eliminating the need for
timely service of the WDT. It does not disable Time-Out and Prewarning Mode. If an
access to register WDT_CON0 is performed in Disable Mode, Time-Out Mode is entered
if the access was valid, and Prewarning Mode is entered if the access was invalid. Thus,
the ENDINIT monitor function as well as (a part of) the system malfunction detection will
still be active.