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Watchdog timer (WDG)

RM0352

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Watchdog timer (WDG)

The watchdog timer (WDG aka WDT) provides a way of recovering from software crashes.

The watchdog clock is used to generate a regular interrupt (WDOGINT), depending on

a programmed value. The watchdog monitors the interrupt and asserts a reset signal

(WDOGRES) if the interrupt remains unserviced for the entire programmed period. You can

enable or disable the watchdog unit as required.

The WDG is counting down at a fixed frequency of 32.768 kHz.

7.1

Functionality

The watchdog timer is a 32-bit down counter that divides the clock input to produce an

interrupt. The divide ratio is fully programmable and controls the interrupt interval, which can

be calculated using

Equation 1:

Equation 1

Interrupt interval = (WDT_LOAD + 1) / (clock frequency in Hz).

The default ratio is 4294967296 (equivalent to hex value 0xFFFFFFFF + 1). With a

32.768 kHz watchdog clock this generates an interrupt every ~131072 seconds. This

corresponds to the maximum value of WDT_LOAD: 4,294,967,295.

If zero is programmed into WDT_LOAD, an interrupt would always be generated

immediately. Therefore valid values of WDT_LOAD are 1 or higher.

Table 25 shows

examples of WDT_LOAD values and the corresponding interrupt interval when using

a 32.768 kHz watchdog clock.

A watchdog interrupt is generated each time the counter reaches 0. The counter is then

reloaded with the content of the WDT_LR register. The interrupt status should be cleared by

writing to the interrupt clear register. When the interrupt is cleared, the counter is reloaded

with WDT_LOAD value. If the interrupt status is not cleared and a new interrupt is

generated, then a watchdog reset is generated, rebooting the system.

The watchdog interrupt and reset generation can be enabled or disabled as required by the

system using the relevant bits in the control register. When the interrupt generation is

Table 25. Interrupt intervals for WDT_LOAD values using a 32 kHz clock

WDT_LOAD

Interrupt interval (ms)

4294967295

131072000

65535

2000

32767

1000

4095

125

127

3.90625

1.953125

10.0610

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