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# Moore and Mealy machines

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### Moore and Mealy machines

1. 1. Finite-State Machine Muhammad Irfan Anjum 1
2. 2. OUTLINE • Introduction • Types of Finite State Machines • • • Mealy State Machine Moore State Machine Example 2
3. 3. Finite State Machine • FSMs are different from counters in the sense that they have external I/Ps, and state transitions are dependent on these I/Ps and the current state. • Simple forms of pattern matching. • Models for sequential logic circuits, of the kind on which every present-day computer and many device controllers is based. 3
4. 4. Mealy and Moore Machines External I/Ps External I/Ps m1 External O/Ps m1 Comb. Logic n FFs m2 Next State Comb. Logic n even ↓ odd n Output Logic CLK m2 FFs n CLK External Outputs Moore Machine Model Mealy Machine Model 4
5. 5. Difference Between Mealy and Moore Machine Mealy (1) O/Ps depend on the present state and present I/Ps (2) The O/P change asyn -chronously with the enabling clock edge (3) (4) Moore O/Ps depend only on the present state Since the O/Ps change when the state changes, and the state change is synchronous with the enabling clock edge, O/Ps change synchronously with this clock edge A counter is a Moore machine A counter is not a Mealy machine A Mealy machine will have the same # or fewer states than a Moore machine 5
6. 6. Finite State Machine (FSM) Design Example: There is a one bit-serial I/P line. Design an FSM that outputs a ‘0’ if an even # of 1’s have been received on the I/P line and the outputs a ‘1’ otherwise. x FSM O/p y CLK CLK x # of 1s even (0) odd (1) even (2) odd (3) 6 odd (3)
7. 7. Solution 1: (Mealy) Solution 2: (Moore) 0 0/0 Reset Even Transition Arc Input 1/1 1/0 Odd Reset Output Even Output [0] O/P is dependent on current state and input in Mealy 1 1 Odd [1] 0 0/1 Mealy Machine: Output is associated with the state transition, and appears before the state transition is completed (by the next clock pulse). Input Output is dependent only on current state Moore Machine: Output is associated with the state and hence appears after the state transition take place. 7
8. 8. State Transition Table (Even-Parity Checker) Even State: 0 ; Present State Input A Odd State: 1; Next State x State Variable A Mealy O/P Moore O/P A+ y1 D-FF Excit. y2 DA 0 0 0 0 0 0 0 1 1 0 1 1 1 1 0 1x 1 0 Input variables to comb. logic 1 1 DA= A⊕x ; y1 = A for Moore y2 = A⊕x for Mealy Output functions 1 1 y2 0 N.S. & O/P Logic Q A FF x N.S. Logic Or Q DA CLK 8 0 A O/P Logic FFs y1 DA
9. 9. Moore M/C Implementation a) D-FF 0 D x=1 R CLK A Q y1 Q Reset Moore O/P is synchronized with clock. Mealy M/C Implementation y2 0 1 D x=1 CLK Q R A Q Reset a) D-FF Mealy O/P is not synchronized with clock. 9
10. 10. Moore M/C Implementation a) D-FF 0 D x=1 R CLK A Q y1 Q Reset Moore O/P is synchronized with clock. Mealy M/C Implementation y2 0 1 D x=1 CLK Q R A Q Reset a) D-FF Mealy O/P is not synchronized with clock. 9