TC1784
System Control Unit (SCU)
User´s Manual
3-136
V1.1, 2011-05
32-bit SCU, V1.18
•
Normal Mode
•
Disable Mode
•
Prewarning Mode
The following overview describes these modes and how the WDT changes from one
mode to the other.
Time-Out Mode
The Time-Out Mode is entered after an Application Reset or when a valid Password
Access to register WDT_CON0 is performed (see
). The Time-Out Mode
is indicated by bit WDT_SR.TO = 1. The timer is set to FFFC
H
and starts counting
upwards. Time-Out Mode can only be exited properly by setting ENDINIT = 1 with a
correct access sequence. If an improper access to the WDT is performed, or if the timer
overflows before ENDINIT is set, a WDT_NMI is requested, and Prewarning Mode is
entered.
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 WDT_CON1.DR.
Normal Mode
In Normal Mode (DR = 0), the WDT operates in a standard Watchdog fashion. The timer
is set to WDT_CON0.REL, and begins counting up. It has to be serviced before the
counter overflows. Servicing is performed through a proper access sequence to the
control register WDT_CON0. This enters the Time-Out Mode.
If the WDT is not serviced before the timer overflows, a system malfunction is assumed.
Normal Mode is terminated, a WDT_NMI is requested, and Prewarning Mode is entered.
Disable Mode
Disable Mode is provided for applications which truly do not require the WDT function. It
can be requested from Time-Out Mode when the disable request bit WDT_CON1.DR is
set. The Disable Mode is entered when was requested AND bit WDT_CON0.ENDINIT
is set. The timer is stopped in this mode. However, disabling the WDT only stops it from
performing the standard Watchdog function, 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.
Prewarning Mode
Prewarning Mode is entered always when a Watchdog error is detected. This can be due
to an overflow of the timer in Normal or Time-Out Mode, or an invalid access to register
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