WindowWatchdog configures a timer channel to capture both edges of an
input and compare the time between edges to a duration range. In other
words, it expects a ~50% duty cycle square wave on its input.
One possible use for this is to monitor whether another device is
functional.
If VALID_THRESHOLD (currently 2) edge intervals are within range, ok()
returns true, otherwise, if an edge is outside of the valid range ok()
returns false.
state_change() runs the calling task whenever the ok() state changes.
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