Data flow model -Lecture-4

Digital Design Using Verilog -For Absolute Beginners
LECTURE4 :DATA FLOW MODELLING

Introduction-Data Flow Modelling
•For simple circuits, the gate-level modeling
approach works fine because the number of gates is
limited and the designer can instantiate and connect
every gate individually.
•Also, the gate-level modeling is very intuitive to a
designer with a basic knowledge of digital logic
design.
•But , in complex designs where the number of gates
is very large it is not the case. (that easy)
•So, for efficient designing, the designers prefer a
higher level of abstraction than gate level .

contd
• An important point here is, as the designer is aware of
how data flows between hard ware components
(Registers) and how data is being processed in the
design, this model is more convenient to the
designers.
• Another reason for importance of Data flow modelling
is the unprecedented growth in the gate density on a
chip which made the Gate level modelling very
complicated.
• Also, presently automated tools are readily available,
to create a gate level circuit from data flow design
description easily.(This process is called Synthesis).

contd..
• This Data Flow model is also known as
Continuous Assignment model.
• This type of modelling or style is more suitable for
combinational logic circuits where clock is not
involved as any control signal .
• As the output of a logic gate is continuously driving
the input of another gate ,it is popularly known as
Continuous assignment model.
• The Assignment statement starts with the keyword
“assign”
17 June 2020 4yayavaram@yahoo.com

Continuous Assignments
• A continuous assignment is the most basic
statement in dataflow modeling, used to drive a
value onto a net.
• This assignment replaces gates in the description of
the circuit and describes the circuit at a higher level
of abstraction.
• The assignment statement starts with the keyword
assign.
• For ex: assign A = B | C ( B OR C).
• Similarly A = B & C
• Here the assignment is continuously active.

Contd
• The assign keyword creates a static binding
between RHS and LHS of the above expressions.
• So, w.r.t simulation, the output is continuously
active. i.e Continuous assignments are always
active.
• The assignment expression is evaluated as soon as
one of the right-hand-side operands changes .
• The operands on the right-hand side can be registers
or nets or function calls. Registers or nets can be
scalars or vectors.

contd
• The left hand side of an assignment must always be
a scalar or vector net or a concatenation of scalar
and vector nets. It cannot be a scalar or vector
register.
• Here net and Reg are a kind of data types.
• In a simple terminology Nets represent connection
between components. Ex: wire B;
• Nets must be continuously driven and are used
model connections between continuous assignments
& instantiations.

contd
• Where as the Reg(Register) retains the last value
assigned to it until another assignment statement
changes their value.
• Its often used to represent storage elements.
• Net and Register data types can be declared as
vectors (multiple bit width).
• For ex: wire [3:0] a;
• wire[31:0]b;
• reg[7:0] c;
• reg[31:0] d;

Ex 1: HALF ADDER
• module Half_ adder(s,c,a,b);
input a , b; // Declare input Ports
output s, c; // Declare out ports
assign s = a ^ b; // assign statement for x-or
assign c= a & b; // assign statement for logical and
endmodule.

Ex 2: Multiplexer
• The block diagram of Mux is shown below.

contd
• The out put of Mux is
here S is the select line and A,B are inputs.
• Here two logical ‘and’ operations, one NOT
operation and one OR operations are involved.
• Using the assign statements the Verilog can be
written .

Verilog code-Mux
module mymux2_1(A,B,S,Y);
output y;
input A,B,S;
assign Y= (A &(~S) | (B &S));
endmodule
In the above code, & denotes logical ‘and’, ~ Denotes
not , | denotes logical or operations.

Verilog code-Mux(Aliter)
The same code can also be written by another simple
method shown below.
module mymux2_1(A,B,S,Y);
output Y;
input A,B,S;
assign Y= S ? B:A;
endmodule
Note: here ? is the conditional Operator. Y = B when
S=1 else Y=A. (This is a ternary operator)

Conditional operator (?)
• ? Is a conditional operator which is a very useful
ternary operator.
• The conditional operator(? :) takes three operands.
• Usage is ”condition-expr ? true-expr : false-expr ;”
• The condition expression (condition-expr ) is first
evaluated.
• If the result is true (logical 1), then the true-expr is
evaluated. If the result is false (logical 0), then the
false-expression is evaluated.

Ex 3: Full Adder
• Full adder has three input bits and two output bits
sum and carry-out as shown in the diagram below.
• In the diagram A, B, Cin are inputs and Sum &
Cout are outputs.

Verilog code-Full Adder
module myFA1(S,Cout,A,B,Cin);
//Port declaration
output S,Cout;
input A,B,Cin;
assign S= (A^B^Cin); //x-or operation
assign Cout = (A&B | B & Cin | Cin & A);
endmodule

Full Adder -16 bit
module myHA_16(S,Cout,A,B,Cin);
output [15:0]S;
output Cout ;
input [15:0]A ; //here A,B ,S are defined as wire
input [15:0] B;
input Cin ;
assign S[15:0] = A[15:0]^B[15:0]^Cin ;
assign Cout = A[15:0]&B[15:0] | B[15:0]&Cin | Cin
& A[15:0];
endmodule17 June 2020 17yayavaram@yahoo.com

2-4 Decoder with Enable
• Decoder is a combinational circuit that has ‘n’ input
lines and maximum of 2n output lines.
• The diagram shows a 2 to 4 Decoder with two
inputs A & B , an enable pin E and four outputs
D3,D2,D1,D0 .

contd
• The working of the decoder can be under stood from
the truth table shown below.

Ex: 2 to 4 Line Decoder
• module decoder2_4 (A,B,E,D);
input A,B,E;
output [3:0]D;
assign D[3] =~(~A & ~B & ~E);
assign D[2] =~(~A & B & ~E);
assign D[1] =~( A & ~B & ~E);
assign D[0] =~( A & B & ~E);
endmodule

Ex: Half Subtractor
• The diagram below shows the Half Subtractor.It has
two inputs .A & B and two outputs Difference (D)
and borrow(B).
• Difference is given by = A ^ B
• Borrow is given by Bo =

Verilog Code
• module half_sub(D,Bo,A,B);
input A;
input B;
output D, Bo;
assign D = A ^ B ;
• assign borrow=(~A)&B;
• endmodule

Full Subtractor
• Similar to Full adder, Full subtractor has three input
bits and two output bits, Difference and Borrow.
• The logic diagram is shown below.

contd
• In the diagram A,B,C are inputs ;and D and Bo are
outputs.
• The Difference of the Full subtractor is given by
• D = A^(B^C)
• Bo =(B&C)|(A&(B^C)’)

Verilog Code
• module full_sub (D,Bo,A,B,C);
input A,B,C;
output D, Bo;
assign X = B^C;
• assign D = A ^ X;
• assign N =~ C;
• assign Y=B & N;
• assign N1 =~ X;
• assign z = A & N1;
• assign Bo = Y | z;
• endmodule

Verilog Code(Another way)
From the Block Diagram the assignment model is
• module full_sub (D,Bo,A,B,C);
input A,B,C;
output D, Bo;
assign D = A ^ (B^C);
assign Bo = B & C| (A&(B^C)’);
• endmodule

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PATIENTLY

Data flow model -Lecture-4

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