Vhdl Code For Serial Adder Using Finite State Machine

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This tutorial is about implementing a finite state machine is vhdl. I will go through each and every step of designing a finite state machine and simulating it. Xilinx is used as a tool to construct finite state machine and for simulation and testing purpose. I suppose you know what is finite state machine and why it is used? I will give a short introduction of finite state machines and then move on to the designing phase.
Finite state machine is a graphical model/representation of sequential activities or events. After representing and modeling the events they can be implemented easily in case of sequential logic designs.
Finite state machines can be utilized in many fields of study e.g neural networks, artificial intelligence, mathematics, games, robotics and sequential flow of data. Since we are dealing with the sequential circuits so i will explain their use in sequential circuit design in this tutorial.
There are many fsm(finite state machines) in existence. The two most popular used in digital combinational and sequential circuits are
  • Melay Machine
  • Moore Machine
The main difference between melay and moore is the computation of the next state. In melay machine the output depends on the current state and the input variables. Where as in moore machine the output depends on the current state only. There are also other differences which are hardly highlighted any where.
​Vhdl Code For Serial Adder Using Finite State Machine

State diagram for serial adder: Let S0 and S1 are the states where the carry in values is '0' and '1' respectively. Figure shows the suitable state diagram defined as a mealy model. The output value sum depends on both state and the present value of the inputs a and b, each transition is labeled using the notation ab / sum which indicates the. Register the block diagram is attached, vhdl code for serial adder using moore type fsm serial adder verilog 4 bit serial adder when doing digital system design it is very common to begin by defining how the system works with a finite state machine model this design step allows the designer to think. Keywords— D-latch, Finite state machine, Mealy Model, Multisim, Serial adder. INTRODUCTION TO FINITE STATE MACHINE Based on the state table we can construct the state diagram. The state diagram is as shown in Fig 2. As observed in the state A finite state machine can be represented by a state transition table or a state diagram. Assume that i want to add 4 signals a,b,c,d and i want to add them using adder2 which adds to signals only.and i want to implement them in adder tree format so in the first state a, and b will be added and saved at signal y and c and d will be added as well and saved at signal z then in the next state y and z will be added and and the result will be at the the output x.


Vhdl Code For Serial Adder Using Finite State Machine

Moore Machine
  1. More number of states in moore compared to melay for same fsm.
  2. States changes after 1 clock cycle. Latency = 1.
  3. Synchronous output. Because the states are determined in a process.
  4. States are output.
VhdlUsing

Mealy Machine

Vhdl Code For Serial Adder Using Finite State Machine Learning

  1. Less number of states in mealy compared to moore for same fsm.
  2. State transition on the same clock cycle. Latency = 0.
  3. Asynchronous output.
  4. Transition are output.
Serial adder circuit

Serial Adder Circuit

In the below figure you can see a melay machine fsm. Fsm has four states S0, S1, S2 and S3. Outputs can be seen on the edges. Inputs are also on the edges. Transitions from one state to another take place on the bases of current state and the inputs. Fsm below is actually a counter. When input is 1 the state moves to next and when the input is 0 state jumps to previous. Counter is 4-bit but only one bit is manipulated in the counter. ‘1’ in the 4-bit output moves forward or backward depending on the state and input.
The top level entity of melay machine fsm is below. Output is 4-bit named count. Clock and reset are necessary signals for finite state machine. UpDw is a single bit input. When UpDw is 1 state jumps from current to next and when 0 it scroll back to previous state.
Melay machine fsm counter – vhdl top level entity
Output of the melay machine can be seen in the figure below. If you match the behavior of the simulation with the actual state machine above you will find the behavior of the two similar. Which means that the finite state machine is implemented correctly. The simulation below is run on ISim simulator by xilinx.
Filed Under: Microcontroller Projects, VHDLSerial adder circuit