Module : FSM Topic : types of FSM

Two types of FSM The instant of transition from the present to the next can be completely controlled by a clock; additionally, changes in the inputs may also dictate such transitions. FSMs can be broadly classified into two categories – Moore machines and Mealy machines.

Moore machine Two aspects characterizes Moore machine : Next state of the output is decided fully by the present state. All changes in the output are brought about only at the active edge of the clock. Hence the Moore machine is inherently synchronous. Edge-triggered flip flops, synchronous counters, etc, are typical example of Moore machines. Moore machine are also known as synchronous machine.

Generate a state diagram from the problem statement. Minimize the number of states Select a binary encoding for the states. Generate an encoded state table. Select the memory device – T flip-flop or D flip-flop Generate a next-state K map for each memory device. Generate a K map for each output. Implement memory and combinational logic using PLAs or other devices.

Example of sequence detector A sequence generator is to sequence through eight distinct states. The states are represented by a set of four binary variables – W, X, Y and Z. the states and the sequence are as follows ( the 4 bits represent values of W, X, Y and Z, respectively) : 1000  1100  0100  0110  0010  0011  0001  1001  1000 … Each transition is to take place at the positive edge of the clock. Since the scheme has no external primary input to affect the output, it is realized as a Moore machine.

Example 1 : solution A 3-bit state machine suffices to generate the eight independent states specified. Continued to next page….

Test Bench

Mealy Machine Mealy machine differs from the Moore machine at the output stage. The inputs can affect the outputs directly in Mealy Machine. The changes in the input reflect as corresponding changes in the outputs without the clock being directly involved : to that extent the behavior is asynchronous. Counters with asynchronous preset and clear and shift registers with preset are examples of Mealy machines. A Mealy machine has to respond to changes in input in addition to the response to the active edges of the clock.

Example of Mealy machine A sequence generator is to have four binary outputs designated W, X, Y, and Z. they are to follow either of two sequences depending on the value of a Boolean variable A: If A = 0, the sequence to be followed is 1000  1100  0100  0110  0010  0011  0001  1001  1000 … where, W is the most significant bit and Z the least significant bit. If A=1, the sequence to be followed is : 1001  0001  0011  0010  0110  0100  1100  1000  1001 ….

Example 2 : Solution

Diagram for Moore & mealy machine Moore machineMealy machine