29.4 Functional description
The WDOG module provides a fail safe mechanism to ensure the system can be reset to a
known state of operation in case of system failure, such as the CPU clock stopping or
there being a run away condition in the software code. The watchdog counter runs
continuously off a selectable clock source and expects to be serviced (refreshed)
periodically. If it is not, it generates a reset triggering event.
The timeout period, window mode, and clock source are all programmable but must
be configured within 128 bus clocks after a reset.
29.4.1 Clock source
The watchdog counter has the following clock source options selected by programming
CS[CLK]:
• bus clock
• internal Low-Power Oscillator clock (LPO_CLK) (This is the default source.)
• internal 8 MHz clock (SIRC)
• external clock (SOSC)
The options allow software to select a clock source independent of the bus clock for
applications that need to meet more robust safety requirements. Using a clock source
other than the bus clock ensures that the watchdog counter continues to run if the bus
clock is somehow halted; see
.
An optional fixed prescaler for all clock sources allows for longer timeout periods. When
CS[PRES] is set, the clock source is prescaled by 256 before clocking the watchdog
counter.
The following table summarizes the different watchdog timeout periods available.
Table 29-2. Watchdog timeout availability
Reference clock
Prescaler
Watchdog time-out availability
Internal LPO_CLK
Pass through
~1 ms–65.5 s (if LPO_CLK = 1 kHz); (~1 ms–65.5 s)/128 (if
÷256
~256 ms–16,777.2 s (if LPO_CLK = 1 kHz); ~2 ms–131.1 s (if
LPO_CLK = 128 kHz).
Internal 8 MHz (SIRC)
Pass through
125 ns–8.1925 ms
÷256
32 µs–2.09728 s
1 MHz (from bus or external)
Pass through
1 µs–65.54 ms
Table continues on the next page...
Chapter 29 Watchdog timer (WDOG)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors
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