Logically, code should look like this:

WAIT FOR LOW LEVEL AND MEASURE IT'S DURATION
IF LOW LEVEL LENGTH=10MS THEN MEASURE HIGH LEVEL DURATION AFTER IT
IF HIGH LEVEL DURATION WAS 2MS, THEN GO TO NEXT STEP, ELSE, LOOP THIS CYCLE AGAIN
NEXT LEVEL:
CONSIDER 600US LOW LEVEL AS 1, AND 2MS LOW LEVEL AS 0, WRITE THEM SEQUENTALY TO 32 BIT ARRAY, UNTIL 32 BITS WRITTEN
EXTRACT MSB AND LSB FROM ARRAY
CONVERT TO WORD