# Verilog hdl - II

VERILOG HDL II MOORE MACHINE REVISITED A good way to specify Moore is to use separate procedural

blocks for Next State Combinational Logic For the state update Sequential Logic

Output Combinational Logic THE THREE STATES OF MOORE:

MOORE DUAL SEQUENCE OVERLAPPING: A Moore type sequence FSM that reads a binary sequence w

and sets z = 1if either a 110 or a 101 pattern with overlaps is detected. The state diagram for such a machine is as follows.

VERILOG FOR MOORE 1MODEL: VERILOG CODE FOR MOORE 2:

VERILOG CODE FOR MOORE 3: Cannot put z in the always block else it would have been

edge sensitive

Z is combinational and by putting it in always an extra flop would be required leading to extra clock cycle MEALY DESIGN OF THE SEQUENCE

I/O DECLARATION IS THE SAME AS MOORE 1-BIT FULL ADDER/SUBTRACTOR:

EQUATIONS FOR THE ABOVE CIRCUIT: (W = sub XOR y); 1s compliment of w

(S = X XOR W XOR Cin); Computing the sum using Binary Arithmetic (Cout = X * W + X * Cin + W * Cin); Choosing W chooses if

4-BIT ADDER SUBTRACTOR FROM 1-BIT MODULE, 4 INSTANTIATIONS VECTORED SIGNALS:

Specifying all bits individually is cumbersome. Use vectored signals for a cleaner code.

input [3:0] x; (Makes x a 4 bit quantity) x[3] MSB, x[2], x[1], x[0] LSB Input [15:0] x; (Represents vector x with16 bit quantity x [15:8]; [Represents an 8 bit vector with the 8 most MSBs],

LSB[7:0] x [7:6]; Represents a 2 bit vector consisting of x[7], x[6]

4-BIT VECTORED ADDER/SUBTRACTOR BEHAVIORAL 1 BIT MODULE TO BE INSTANTIATED:

GENERALIZING THE SIZE OF A LARGE MODULE The ripple carry adders we created have a fixed length. If a 32-bit or a 64- bit adder was needed, we would need as many instantiations of FullAddSub.

Desirable from designers perspective to define an AddSub module that n number of bits can be set to any value.

In Verilog such a construct exists, can specify n as : parameter n = 15 and then write input [n-1:0] x. This case is for 16 bit vector.

GENERAL DESCRIPTION OF PARAMETERIZED I BIT ADDER Wk = sub XOR Yk, sub is 1 for subtraction and zero for addition

Sk = Xk XOR Wk XOR Ck Ck = Xk * Wk + Xk * Ck + Wk * Ck For k = 0, 1, 2---k-1

THE FOR LOOP: In Verilog the repetitive structure for the of the ripple carry adder/subtractor can be specified using for procedural statement, (for k=1; k

The syntax is same as C/C++ except that increments and decrements have to specified explicitly as k = k+1 and k = k-1, since ++, -- operators doesnt exist in Verilog.

For is a procedural statement and needs to be put inside a procedural block such as always block. Outputs of procedural blocks goes to registers.

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