5

4756D–AUTO–11/05

U5021M

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, t

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, t

2

• 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

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, t

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 active.

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, t

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 3). 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 the wake-up input (pins 1) 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.

t

0

V

DD

Mode

Trigger

Reset out

Pin 2

Pin 3

Pin 5

Pin 6

t

1

t

2

t

3