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Dataflow Modelin 1 2

but not for complex designs.

Desi ners can desi n more effectivel if the concentrate on

implementing the function at a level of abstraction higher

than gate level.

Dataflow modeling provides a powerful way to implement a

design.

Verilog allows a circuit to be designed in terms of the data

flow between registers and how a design processes data

rather than instantiation of individual gates.

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Dataflow Modelin 2 2

With ate densities on chi s increasin ra idl , dataflow

modeling has assumed great importance.

Currently, automated tools are used to create a gate-level

circuit from a dataflow design description. This process is

called logic synthesis.

The data flow modeling allows the designer to concentrate on

optimizing the circuit in terms of data flow.

In the digital design community, the term RTL (Register

Transfer Level) design is commonly used for a combination of

dataflow modeling and behavioral modeling.

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This level of abstraction level resembles like that of boolean

equation representation of digital systems.

The dataflow assignments are called Continuous Assignments.

Continuous assignments are always active.

Syntax: assign #(delay) target = expression;

A Verilog module can contain any number of continuous

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ass gnmen s a emen s, an a s aemen s execu e concurren y.

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LHS target -> always a net, not a register.

RHS -> registers, nets or function calls.

Delay values can be specified in terms of time units.

Whenever an event (change of value) occurs on any operand

used on the RHS of an expression, the expression is

evaluated and assigned to the LHS target.

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Regular continuous assignment.

wire mux_out, a_in, b_in, sel_in;

ass gn mux_ou = se _ n _ n : a_ n;

.

wire a_in, b_in, sel_in;

wire mux_out = sel_in ? b_in : a_in;

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wire and_out;

assign and_out = i1 & i2; // regular continuous assignment

wire exor_out = i1[31:0] ^ i2[31:0] // implicit assignment

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Example

module enerate_mux

(data,select,out);input [0:7] data;

OUTMUXATA

input [0:2]select;

output out;w re ou ;

assign out = data[select];SELECT

Note: Non- constant index in expression on RHS generates MUX

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Exam le

module enerate decoder

(data,select,out);input data;

DECODERAT

A

input [0:1]select;

output [0:3] out;

OUT

w re : ou ;

assign out[select]=data;SELECT

-

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Example

module enerate_mux

(a,b,f,s);input a,b;

fMUX

a

b

input s;

output f;w re ;

assign f = s? a : b;s

Note: Conditional Operator generate MUX

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Delay values control the time between the change in a right

hand-side operand and when the new value is assigned to

- .

Regular assignment delay

mp c con nuous ass gnmen e ay

Net declaration dela

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The delay value is specified after the keyword assign.

Any change in values of in1 or in2 will result in a delay of 10 time units before

recomputation of the expression in1 & in2, and the result will be assigned to out.

Example:

module and_ex(and_out , a_in, b_in);

in ut a in b in_ _

output and_out;

wire and_out;

ass gn an _ou = a_ n _ n;endmodule

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wire out;

ass gn out = ; egu ar ass gnment e ay

The inertial delay of real circuits is modeled through

regular assignment delay.

An event on the RHS si nals which is not lastin for the

amount of inertial delay specified will not have any effect on

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.

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in1in1

in2in2

outout

1010 2020 3030 6060 7070 8080 8585

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An equivalent method is to use an implicit continuous assignment to

Exam le:

specify both a delay and an assignment on the net.

module and_ex(and_out , a_in, b_in);

npu a_ n, _ n;

output and_out;

wire #10 and_out = a_in & b_in;

endmodule

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A delay can be specified on a net when it is declared without

putting a continuous assignment on the net.

If a delay is specified on a net out, then any value change applied

Example:

o e ne ou s e aye accor ng y.

_ _ , _ , _

input a_in, b_in;

output and_out;

wire #10 and_out;assign and_out = a_in & b_in;

endmodule

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