NXP Semiconductors Kinetis KE1xZ256, Reference Manual

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The unlock sequence is similar to the refresh sequence but uses different values.
29.4.3.2.1 Unlocking the Watchdog
The unlock sequence is a write to the CNT register of 0xC520 followed by 0xD928
within 16 bus clocks at any time after the watchdog has been configured. On completing
the unlock sequence, the user must reconfigure the watchdog within 128 bus clocks;
otherwise, the watchdog closes the unlock window.
NOTE
Due to the 128 bus clocks requirement for reconfiguring the
watchdog, some delays must be inserted before executing
STOP or WAIT instructions after reconfiguring the watchdog.
This ensures that the watchdog's new configuration takes effect
before the MCU enters low power mode. Otherwise, the MCU
may not be waken up from low power mode.
The example codes can be found at end of this chapter.
29.4.4 Using interrupts to delay resets
• When interrupts are enabled (CS[INT] = 1): After a reset-triggering event (like a
counter timeout or invalid refresh attempt), the watchdog first generates an interrupt
request. Next, the watchdog delays 128 bus clocks (from the interrupt vector fetch,
not the reset-triggering event) before forcing a reset, to allow the interrupt service
routine (ISR) to perform tasks (like analyzing the stack to debug code).
• When interrupts are disabled (CS[INT] = 0): the watchdog does not delay the
forcing a reset.
29.4.5 Backup reset
NOTE
A clock source other than the bus clock must be used as the
reference clock for the counter; otherwise, the backup reset
function is not available.
The backup reset function is a safeguard feature that independently generates a reset in
case the main WDOG logic loses its clock (the bus clock) and can no longer monitor the
counter. If the watchdog counter overflows twice in succession (without an intervening
reset), the backup reset function takes effect and generates a reset.
Chapter 29 Watchdog timer (WDOG)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors 617