K60P100M100SF2RM Datasheet, PDF (518/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Functional Overview
to be serviced periodically, failing which it resets the system. This ensures that the
software is executing correctly and has not run away in an unintended direction. Software
can adjust the period of servicing or the time-out value for the watchdog timer to meet the
needs of the application.
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 a little
later 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.
23.3.1 Unlocking and Updating the Watchdog
You can unlock the write-once-only control and configuration registers for updating
them. As a pre-condition, the ALLOW_UPDATE bit in the watchdog control register
must be set. The actual unlock is accomplished by writing 0xC520 followed by 0xD928
within 20 bus clock cycles to a specific unlock register (WDOG_UNLOCK). This opens
up an update window equal in length to the watchdog configuration time (WCT) within
which you can update the configuration and control register bits. You can not update
registers on the bus clock cycle immediately following the write of the unlock sequence,
but one cycle later. 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 (or interrupt-then-reset) to the system. Trying to unlock the
watchdog within the WCT time after an initial unlock, has no effect. During the update
operation, the watchdog timer is not paused and keeps running in the background. After
the update window closes, the watchdog timer restarts and the watchdog functions as per
the new configuration.
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
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Freescale Semiconductor, Inc.