Question

Mealy type and moore type difference in computer organization

Answer 1

Design of sequential circuit can be composed of designing combinational circuit and state register. Sequential circuits are implemented in two different ways:

Mealy Machine

Moore Machine

We can represent a sequential machine as M = <I,O,S,f,g>; where

I: Input set O: Output set S: State space f: A function mapping IxS => O g: A function mapping IxS => S

Let’s understand both type of sequential machines and compare them.

Mealy Machine

In case of Mealy machine, output is a function of not only the present inputs but also past inputs. In other words we can say; in case of Mealy, both output and the next state depends on the present input and the present state.



Mealy Machine: (a) Block Diagram, (b) State Transition Diagram

PS: Present State NS: Next State [NS becomes PS after every sensitive edge of clock pulse] CLK: Clock Signal

f: IxS => O

So, output function:

o(t) = f(i(t),s(t))

       = f(i(t),PS(t))

Similarly

g: IxS => S

So, Next state function:

s(t+1) = g(i(t),s(t))

NS(t)  = g(i(t),PS(t))

Moore Machine

In case of Moore machine, present output is not a function of present inputs but is a function of past inputs. The next state is a function of both the present input and the present state.

In this case the output is not associated with the transition but are associated with the state unlike the Mealy machine. This is because the output “O” is a function of present state (PS) and independent of present input “I”.

Of-course present input I influences the next state (NS) and that’s how it is going to influence the output but there is a time lag between the input and output. The present inputs are going to influence the outputs that are going to come after the next clock.



Moore Machine: (a) Block Diagram, (b) State Transition Diagram

f: S => O

So, output function:

o(t) = f(s(t))

       = f(PS(t))

Similarly

g: IxS => S

So, Next state function:

s(t+1) = g(i(t),s(t))

NS(t)  = g(i(t),PS(