Difference Between Mealy And Moore Machine: In the synchronous sequential circuits, the combinational circuit consists of a combination of logic gates whereas the flip flops are used as memory elements. The synchronous sequential circuit is called a Moore circuit if the output depends only on the present state of flip flops. The circuit is called a Mealy circuit if the output is dependent on the present state of flip-flops and the external inputs. The basic Difference Between the Mealy And Moore Machine is based on the dependency of output on the present state and input.
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Mealy Circuit
In the Mealy Machine, the output is a function of both the present state and input. The generalized block diagram of a Mealy circuit has been shown in figure below.
The input combinational circuit which is also called the next state decoder receives inputs from the outputs of the memory element and from the external inputs. But, the most important point to be noted is that the output combinational circuit C2 (also called as output decoder) receives signals from the output of memory elements as well as the external inputs. Hence, the outputs of the Mealy circuit are dependent on the present state of memory elements as well as the external inputs.
Moore Circuit
In the Moore Machine, the output is a function of the present state only. The general form of Moore circuit is shown in below figure.
Here, it may be noted that the most important thing for a Moore circuit that its output depends only on the present state of the memory elements. The external inputs do not influence the outputs. The combinational circuit C1 on the input side is sometimes called the next state decoder. The external inputs and the feedback signals from the memory elements are applied to this combinational circuit. The outputs of the memory elements are applied to another combinational circuit C2 which is also called an output decoder.
Difference Between Mealy And Moore Machine
| Mealy Machine | Moore Machine | |
| 1. | The output is a function of both the present state and input. | The output is a function of the present state only. |
| 2. | The state graph for Mealy Machine has the output associated with the arrow going between states. | The state graph for Moore Machine has the output associated with the state. |
| 3. | In Mealy Machine one state may have more than one output. | In Moore Machine one state is associated with only one output. |
| 4. | The output may change either when input changes or when the flip-flop change state. | During the analysis, the output will only change when the state change. |
| 5. | In Mealy Machine less number of states are require for implementing state graph for sequence detector. | In Moore Machine more states require for the implementation of state graphs for sequence detectors. |
| 6. | Its design has more flexibility. | Moore circuit is less flexible for design. |
| 7. | The output is observed after applying each input. | The output is observed after the entire input sequence has been applied. |
| 8. | The output may have momentary false values when the state has changed to its next value but the input has not yet changed. | No false output appears because the output is not a function of the input. Changing the inputs at the same time the state change occurs will eliminate false outputs. |
| 9. | In this model, there is no separate column for output. Instead, the outputs are written along with states. | In-state table, the output states are written in a separate column. |
| 10. | Output can change in between the clock edges if the external inputs change. | The output changes only after the active clock edge. |
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Flip Flop | Types, Truth Table, Applications
