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U5020M-MFPY

Digital Window Watchdog Timer

Atmel Corporation 

U5020M

4.5 Microcontroller in Active Mode

4.5.1

Monitoring with the “Short” Trigger Window

After the switch-over mode the watchdog operates in short watchdog mode and expects a trig-

ger pulse from the microcontroller within the defined time window, t3, (enable time). The

watchdog generates a reset pulse which resets the microcontroller if

• the trigger pulse duration is too long

• the trigger pulse is within the disable time, t2

• there is no trigger pulse

Figure 4-3 shows the pulse diagram with a missing trigger pulse.

Figure 4-3. Pulse Diagram with no Trigger Pulse During the Short Watchdog Time

VDD

t0

t1

Reset out

t2

t3

Mode

Pin 13

Pin 10

Pin 12

Trigger

Pin 11

Figure 4-4 on page 6 shows a correct trigger sequence. The positive edge of the trigger signal

starts a new monitoring cycle with the disable time, t2. To ensure correct operation of the micro-

controller, the watchdog needs to be triggered three times correctly before it sets its enable

output. This feature is used to activate or deactivate safety-critical components which have to be

switched to a certain condition (emergency status) in the case of a microcontroller malfunction.

As soon as there is an incorrect trigger sequence, the enable signal is reset and it takes a

sequence of three correct triggers before enable is reset.

4.6 Microcontroller in Sleep Mode

4.6.1

Monitoring with the “Long” Trigger Window

The long watchdog mode allows cyclical wake-up of the microcontroller during sleep mode. As

in short watchdog mode, there is a disable time, t4, and an enable time, t5, in which a trigger sig-

nal is accepted. The watchdog can be switched from the short trigger window to the long trigger

window with a “high” potential at the mode pin (pin 12). In contrast to the short watchdog mode,

the time periods are now much longer and the enable output remains inactive so that other com-

ponents can be switched off to effect a further decrease in current consumption. As soon as a

wake-up signal at one of the 6 wake up inputs (pins 3 to 8) is detected, the long watchdog mode

ends, a reset pulse wakes-up the sleeping microcontroller and the normal monitoring cycle starts

with the mode switch-over time.

By means of a low or high potential at pin 16 (time switch), two values for the long watchdog

time can be selected.

5

4755B–AUTO–11/05

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