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# LEC-11 - 11 Finite State Machines I Finite State Machines...

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11 Finite State Machines I 3/22/2010 © Copyright Kenneth Short 2003, 2006 1 3/22/2010 © Copyright Kenneth Short 2003, 2006 1 Finite State Machines Kenneth Short 3/22/2010 © Copyright Kenneth Short 2003, 2006 2 Finite State Machine A sequential machine, also known as a finite state machine (FSM), is a digital system whose outputs are a function of both its present and past inputs A synchronous FSM’s inputs are examined and its outputs change at the triggering edge of its clock Accordingly, its input and output sequences are ordered in time by its clock A FSM’s present state provides all the information concerning its past history of input values that is needed is to determine its next state and output The present state is stored as a binary value in a state register

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11 Finite State Machines I 3/22/2010 © Copyright Kenneth Short 2003, 2006 2 3/22/2010 © Copyright Kenneth Short 2003, 2006 3 FSM Block Diagram Combinational logic Register Inputs Outputs next_state present_state clock 3/22/2010 © Copyright Kenneth Short 2003, 2006 4 Present State – Next State A FSM’s outputs are computed by a combinational logic subcircuit from its external inputs and present state. Combinational logic computes the next state from the FSM’s inputs and present state The next state is the value clocked into the state register at the next triggering clock edge
11 Finite State Machines I 3/22/2010 © Copyright Kenneth Short 2003, 2006 3 3/22/2010 © Copyright Kenneth Short 2003, 2006 5 Separation of Output and Next State Combinational Logic Combinational Next State Logic Register Inputs Outputs next_state present_state clock Combinational Output Logic 3/22/2010 © Copyright Kenneth Short 2003, 2006 6 Synchronous FSM Operation state 0 state 2 state 1 input a input b input c state 2 state 1 state 3 inputs present state next state clock output q output p output r output

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