00 -> 01 -> 10 -> 11 -> 00 -> 01 -> ..... so on.
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Step-1: In the 1st step we draw a State Diagram representing the above sequential circuit.
The State Diagram representing the above counter is shown below:
Figure – State Diagram for 2-bit UP Counter -
Step-2: In the next step we derive the State Table from the above given State Diagram
The State Table is as given below:
Present State Q(n) Next State Q(n+1) Output 00 01 01 01 10 10 10 11 11 11 00 00 -
Step-3: In the third step we need to choose the type of flip flop we will be using to store the state of the circuit, for simplicity, we will be considering the Positive Edge Triggered D-type Flip-Flop.We also need to determine the number of Flip-Flops required to represent the internal state of the circuit. The general formula for the number of Flip-Flops required:
Total Number of Flip-Flops =
Then we need to note down the Excitation Table for chosen Flip-Flop. The Excitation Table for the D-type Flip-Flop is shown below:Where, N = Total Number of States in State Table Present State Q(n) Next State Q(n+1) D X 0 0 X 1 1 -
Step-4: In this step we combine the State Table from the 2nd step with the excitation table of the previous step as follows:
0 0 0 1 0 1 0 1 1 0 1 0 1 0 1 1 1 1 1 1 0 0 0 0 - Step-5: Next, from the above table we try to express
as boolean functions of . In this case the expression for both are trivial.
The Final Sequential Circuit is shown below:
- From the above example we observe that the Synchronous Sequential Logic Design process is a fairly involved process and requires us to go through a sequence of well defined steps even for simple circuits like the one above.
- Secondly, if the number of states become large then this process becomes cumbersome and time-consuming, and sometimes even impossible.
RTL Design | Sequential Logic Design |
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In RTL Design the basic building blocks are registers, Multiplexers, Adders. | In Sequential Logic Design the basic building blocks are the Logic Gates, Flip-Flops. |
RTL Design is much closer to the Behavioural Design of a Logic Circuit as it models the data flow among different registers, and hence is much more intuitive. | Sequential Logic Design process is more mechanical in nature as compared to RTL design process |
Finally, RTL Modelling allows us to synthesize complex circuits with a large number of States with much more ease as compared to Sequential Logic Design. | The Sequential Logic Design techniques are only applicable to circuits having a small number of states. |
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