I'd be more concerned about what you're wanting to do with the data, and how
big the data is.

Getting the data itself can't be that bad.  You'll need a timer that you can
set to interrupt you when you should go check the input line, and an edge
triggered int to start the timer off when you see the start of the flag
byte.

You're going to have a guaranteed maximum run time in any state, so you need
to assure that you don't drift significantly during that time.
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