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You can select a windowed mode of operation that expects the servicing to be done only
in a particular window of the time-out period. An attempted servicing of the watchdog
outside this window results in a reset. By operating in this mode, you can get an
indication of whether the code is running faster than expected. The window length is also
user programmable.
If a system fails to update/refresh the watchdog due to an unknown and persistent cause,
it will be caught in an endless cycle of resets from the watchdog. To analyze the cause of
such conditions, you can program the watchdog to first issue an interrupt, followed by a
reset. In the interrupt service routine, the software can analyze the system stack to aid
debugging.
To enhance the independence of watchdog from the system, it runs off an independent
LPO oscillator clock. You can also switch over to an alternate clock source if required,
through a control register bit.
25.4.1 Unlocking and updating the watchdog
As long as ALLOW_UPDATE in the watchdog control register is set, you can unlock
and modify the write-once-only control and configuration registers:
1. Write 0xC520 followed by 0xD928 within 20 bus clock cycles to a specific unlock
register (WDOG_UNLOCK).
2. Wait one bus clock cycle. You cannot update registers on the bus clock cycle
immediately following the write of the unlock sequence.
3. An update window equal in length to the watchdog configuration time (WCT) opens.
Within this window, you can update the configuration and control register bits.
These register bits can be modified only once after unlocking.
If none of the configuration and control registers is updated within the update window,
the watchdog issues a reset, that is, interrupt-then-reset, to the system. Trying to unlock
the watchdog within the WCT after an initial unlock has no effect. During the update
operation, the watchdog timer is not paused and continues running in the background.
After the update window closes, the watchdog timer restarts and the watchdog functions
according to the new configuration.
The update feature is useful for applications that have an initial, non-safety critical part,
where the watchdog is kept disabled or with a conveniently long time-out period. This
means the application coder does not have to frequently service the watchdog. After the
critical part of the application begins, the watchdog can be reconfigured as needed.
The watchdog issues a reset, that is, interrupt-then-reset if enabled, to the system for any
of these invalid unlock sequences:
Freescale Semiconductor, Inc.
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Chapter 25 Watchdog Timer (WDOG)
465
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