6

9145D–AUTO–05/10

ATA5021

3.5

Microcontroller in Active Mode

3.5.1

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 3-2 shows the pulse diagram with a missing trigger pulse.

Figure 3-2.

Pulse Diagram with no Trigger Pulse during the Short Watchdog Time

Figure 3-3 shows a correct trigger sequence. The positive edge of the trigger signal starts a new

monitoring cycle with the disable time, t

watchdog needs to be triggered three times correctly before it sets its enable output. This fea-

ture 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. For proper operation, the trigger pulse duration must be

longer than the input signal debounce time (see item 4.2 in Section 7. “Electrical Characteristics”

on page 9) and must not exceed the maximum duration of 45 clock cycles (see item 4.4 in Sec-

tion 7. “Electrical Characteristics” on page 9).

Figure 3-3.

Pulse Diagram of a Correct Trigger Sequence during the Short Watchdog Time

Reset out

Mode

Trigger

Pin 2

Pin 3

Pin 5

Pin 6

Reset out

Mode

Trigger

Enable

Pin 4

Pin 2

Pin 3

Pin 5

Pin 6