U5021M Datasheet, PDF (5/11 Pages) ATMEL Corporation – DIGITAL WINDOW WATCHDOG TIMER

U5021M
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
Reset out
Mode
t₀
t₁
t₃
t₂
Pin 6
Pin 5
Pin 3
Trigger
Pin 2
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 active.
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 3). In con-
trast 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 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.
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4756C–AUTO–09/04