2012年11月3日 星期六

字序比對設計法Sequence Recognizer



Example:  Sequence Recognizer


How to Design States

ãCircuit has input, X, and output, Z
ãRecognizes sequence 1101 on X
lSpecifically, if X has been 110 and next bit is 1, make Z high


ãStates remember past history
lClearly must remember weve seen 110 when next 1 comes along
ã
lTell me one necessary state for this example?


1)Beginning State




ãStart state:  let’s call it A

lif 1 appears on input, move to next state B  output remains at 0 













ã

New state, C

ãTo reach C, must have seen 11








ã

If 110 has been received, go to D

ãNext 1 will generate a 1 on output Z







ã

What happens to arrow on right?

ãMust go to some state.
ãWhere?








ã

Here we have to interpret the problem statement
ãWe’ve just seen 01
lIs this beginning of new 1101?
lOr do we need to start over w/ another 1?
ãTextbook:  decides that it’s beginning (01…)








ã



Well, must have every possibility out of every state
ãIn this case, just two: X = 0 or 1
ãYou fill in other cases





















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