Generated Resets and Interrupts
watchdog before enabling it. A system reset brings the watchdog out of the disabled
mode.
24.3.6 Debug modes of operation
You can program the watchdog to disable in debug modes through DBG_EN in the
watchdog control register. This results in the watchdog timer pausing for the duration of
the mode. Register read/writes are still allowed, which means that operations like refresh,
unlock, and so on are allowed. Upon exit from the mode, the timer resumes its operation
from the point of pausing.
The entry of the system into the mode does not excuse it from compulsorily configuring
the watchdog in the WCT time after unlock, unless the system bus clock is gated off, in
which case the internal state machine pauses too. Failing to do so still results in a reset, or
interrupt-then-reset, if enabled, to the system. Also, all of the exception conditions that
result in a reset to the system, as described in
Generated Resets and Interrupts
, are still
valid in mode. So, if an exception condition occurs and the system bus clock is on, a reset
occurs, or interrupt-then-reset, if enabled.
The entry into Debug mode within WCT after reset is treated differently. The WDOG
timer is kept reset to zero and there is no need to unlock and configure it within WCT.
You must not try to refresh or unlock the WDOG in this state or unknown behavior may
result. Upon exit from mode, the WDOG timer restarts and the WDOG has to be
unlocked and configured within WCT.
24.4 Testing the watchdog
For IEC 60730 and other safety standards, the expectation is that anything that monitors a
safety function must be tested, and this test is required to be fault tolerant. To test the
watchdog, its main timer and its associated compare and reset logic must be tested. To
this end, two tests are implemented for the watchdog, as described in
and
. A control bit is provided to put the watchdog into functional test mode. There
is also an overriding test-disable control bit which allows the functional test mode to be
disabled permanently. After it is set, this test-disable bit can only be cleared by a reset.
These two tests achieve the overall aim of testing the counter functioning and the
compare and reset logic.
Chapter 24 Watchdog Timer (WDOG)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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