What you learn today
- Count heartbeats
- Detect failure
- Take over emergency mode
A chip that runs past the end of the program is silent and dead – without an error message. A chip somebody takes out is too. If the living module's heating depends on it, you only notice when it gets cold. A second chip can keep watch: the watchdog.
Heartbeats
The monitored chip counts a pulse up in every main loop and writes it into a logic memory:
add pulse pulse 1
sbn MEMORY HASH("Puls") Setting pulse # heartbeatAs long as it runs, the value changes every tick. If it stands still, the chip has stopped, is stuck in a loop without this code or is gone.
Detecting failure
The watchdog compares the pulse with the last value. Same: a counter counts up. Different: counter to 0. Only when the counter exceeds the patience does the chip count as failed – a single missed tick is not a failure yet.
seq tmp pulse old
add still still tmp
mul still still tmp
move old pulse
sgt dead still PATIENCEmul still still tmp is the trick from watering: if tmp is 0, the counter drops to 0.
NaN is never equal to NaN, not even to itself. If the pulse memory is missing, seq tmp pulse old gives 0 every tick, the counter stays at 0 – and the watchdog never reports a failure. A separate snan is therefore mandatory.
Taking over emergency mode
When the watchdog reports a failure, it must not just warn. It takes over the bare essentials itself: an emergency heater, a vent, a valve closed. When the pulse comes back, it hands over again.
Choose the patience to match the monitored chip. A chip with sleep 5 only pulses every ten ticks – a patience of 10 would be too tight.
Watchdogs work in both directions: two chips can watch each other. If one fails, the other takes over – that is the basic idea of every redundancy.
Diagram
Exercise
Task
Write the watchdog for the pulse in the memory “Puls”:
- if the pulse stays the same for more than PATIENCE = 10 ticks, the chip counts as failed
- if the memory is missing, failed at once
- on failure: light “Ausfall” on and heater “Notheizung” on
- when the pulse comes back, both off again
Hints
Hint 1
seq tmp pulse old, add still still tmp, mul still still tmp, then move old pulse.
Hint 2
Failure: sgt dead still PATIENCE.
Hint 3
Plus snan tmp pulse and or dead dead tmp.
Quiz
3 questions. You pass from 60 % – the server does the grading.
Complete the day
- Passed the simulator exercise
- Passed the quiz with at least 60 %
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