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A down counter is a counter which decreases the count value instead of increasing of "Up Counter."
For a timer, if you assign the length to count,
it will be easier (or cheaper) in terms of speed and number of logic gates required
to check if the count value is 0 or not
instead of comparing with each particular value.
I've seen this approach even in "C" programs, some (or many?) programmers who care about speed tend to write a for loop just like
for(i = max-1 ; i ; --i)
where I would usually write
for (i = 0 ; i < max ; i++)