U5020M Datasheet, PDF (5/10 Pages) TEMIC Semiconductors – Digital Window Watchdog Timer

U5020M
Microcontroller in Active Mode
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, 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 5 shows the pulse diagram with a missing trigger pulse.
Figure 5. Pulse Diagram with no Trigger Pulse During the Short Watchdog Time
V_DD
t₀
t₁
Reset out
t₂
t₃
Mode
Pin 13
Pin 10
Pin 12
Trigger
Pin 11
Figure 6 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 opera-
tion 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-criti-
cal 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.
Microcontroller in Sleep Mode
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 signal 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 components 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.
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