Datasheet

6Atmel ATA5021 [DATASHEET]
9145G–AUTO–09/12
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 trigger pulse from the microcontroller
within the defined time window, t
3
, (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, 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
2
. To ensure correct operation of the microcontroller, 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. 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 Section 7. “Electrical
Characteristics” on page 9).
Figure 3-3. Pulse Diagram of a Correct Trigger Sequence during the Short Watchdog Time
V
DD
Reset out
Mode
Trigger
Pin 2
Pin 3
Pin 5
Pin 6
t
0
t
1
t
2
t
3
V
DD
Reset out
Mode
Trigger
Enable Pin 4
Pin 2
Pin 3
Pin 5
Pin 6
t
trig
t
0
t
1
t
2
t
3
t
2