Verilogforlab

FIRST CYCLE – PROGRAMMING USING VERILOG

E-CAD & VLSI - LAB

What is Verilog HDL? Why using Hardware Description Language?

Design abstraction: HDL ←→ layout by human Hardware modeling(design a hardware) Reduce cost and time to design hardware

Two Popular HDLs VHDL Verilog

What is Verilog HDL? Key features of Verilog

Supports various levels of abstraction Behavior level Register transfer level Gate level Switch level

Simulate design functions

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Description of digital systems only

Basic Limitation of Verilog

Different Levels of Abstraction Architectural / Algorithmic Level

Implement a design algorithm inhigh-level language constructs.

Register Transfer Level Describes the flow of data

between registers and how a design processthese data.

System

Algorithm

Architecture

Register Transfer Level

Gate Level

Transistor Level

Different Levels of Abstraction Gate Level

Describe the logic gates and theinterconnections between them.

Switch (Transistor) Level Describe the transistors and

the interconnectionsbetween them.

System

Algorithm

Architecture

Register Transfer Level

Gate Level

Transistor Level

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Verilog Value Set 0 represents low logic level or false condition

1 represents high logic level or true condition

x represents unknown logic level

z represents high impedance logic level

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Numbers in Verilog (i)

<size>’<radix> <value>

No of bits

No of bits

Binary b or BOctal o or ODecimal d or DHexadecimal h or H

Binary b or BOctal o or ODecimal d or DHexadecimal h or H

Consecutive chars 0-f, x, z

Consecutive chars 0-f, x, z

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Numbers in Verilog (ii)

You can insert “_” for readability 12’b 000_111_010_100 12’b 000111010100 12’o 07_24

Represent the same number

Number Representation

Examples: 6’b010_111 gives 010111 8’b0110 gives 00000110 4’bx01 gives xx01 16’H3AB gives 0000001110101011 24 gives 0…0011000 5’O36 gives 11110 16’Hx gives xxxxxxxxxxxxxxxx 8’hz gives zzzzzzzz

Data Type: Register Register

Keyword: reg, integer, time, real Event-driven modeling Storage element (modeling sequential circuit) Assignment in “always” block (LHS of expressions)

Data Type: Net Net

Keyword: wire, wand, wor, tri, triand, trior, supply0, supply1

Doesn’t store value, just a connection Input, output and inout ports are default “wire”

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Nets

AB

Ywire Y; // declarationassign Y = A & B;

A Y

dr

tri Y; // declarationassign Y = (dr) ? A : z;

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Vectors Represent buses

wire [3:0] busA;

reg [1:4] busB;

reg [1:0] busC;

Left number is MS bit Slice management

busC[1] = busA[2];

busC[0] = busA[1];busC = busA[2:1];

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Logical Operators && logical AND || logical OR ! logical NOT Operands evaluated to ONE bit value: 0, 1 or x Result is ONE bit value: 0, 1 or x

A = 6; A && B 1 && 0 0B = 0; A || !B 1 || 1 1C = x; C || B x || 0 x

but C&&B=0but C&&B=0

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Bitwise Operators (i) & bitwise AND | bitwise OR ~ bitwise NOT ^ bitwise XOR ~^ or ^~ bitwise XNOR

Operation on bit by bit basis

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Bitwise Operators (ii)

c = ~a; c = a & b;

a = 4’b1010;

b = 4’b1100;

a = 4’b1010;

b = 2’b11;

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Reduction Operators

& AND | OR ^ XOR ~& NAND ~| NOR ~^ or ^~ XNOR

One multi-bit operand One single-bit resulta = 4’b1001;

..

c = |a; // c = 1|0|0|1 = 1

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Shift Operators >> shift right << shift left

Result is same size as first operand, always zero filleda = 4’b1010;

...

d = a >> 2; // d = 0010

c = a << 1; // c = 0100

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Concatenation Operator

{op1, op2, ..} concatenates op1, op2, .. to single number Operands must be sized !!

reg a;

reg [2:0] b, c;

..

a = 1’b 1;

b = 3’b 010;

c = 3’b 101;

catx = {a, b, c}; // catx = 1_010_101

caty = {b, 2’b11, a}; // caty = 010_11_1

catz = {b, 1}; // WRONG !!

Replication ..catr = {4{a}, b, 2{c}}; // catr = 1111_010_101101

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Relational Operators

> greater than < less than >= greater or equal than <= less or equal than

Result is one bit value: 0, 1 or x1 > 0 1’b1x1 <= 0 x10 < z x

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Equality Operators

== logical equality != logical inequality === case equality !== case inequality

4’b 1z0x == 4’b 1z0x x 4’b 1z0x != 4’b 1z0x x 4’b 1z0x === 4’b 1z0x 1 4’b 1z0x !== 4’b 1z0x 0

Return 0, 1 or x

Return 0 or 1

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Conditional Operator cond_expr ? true_expr : false_expr

Like a 2-to-1 mux ..

A

BY

sel

Y = (sel)? A : B;0

1

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Hierarchical Design

Top LevelModule

Top LevelModule

Sub-Module1

Sub-Module1

Sub-Module2

Sub-Module2

Basic Module3

Basic Module3

Basic Module2

Basic Module2

Basic Module1

Basic Module1

Full AdderFull Adder

Half AdderHalf Adder Half AdderHalf Adder

E.g.

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Module

f

in1

in2

inN

out1

out2

outM

my_module

module my_module(out1, .., inN);

output out1, .., outM;

input in1, .., inN;

.. // declarations

.. // description of f (maybe

.. // sequential)

endmodule

Everything you write in Verilog must be inside a moduleexception: compiler directives

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Example: Half Adder

module half_adder(S, C, A, B);output S, C;input A, B;

wire S, C, A, B;

assign S = A ^ B;assign C = A & B;

endmodule

HalfAdder

HalfAdder

A

B

S

C

A

B

S

C

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Example: Full Adder

module full_adder(sum, cout, in1, in2, cin);output sum, cout;input in1, in2, cin;

wire sum, cout, in1, in2, cin;wire I1, I2, I3;

half_adder ha1(I1, I2, in1, in2);half_adder ha2(sum, I3, I1, cin);

assign cout = I2 || I3;

endmodule

Instancename

Modulename

HalfAdder ha2

HalfAdder ha2

A

B

S

C

HalfAdder 1ha1

HalfAdder 1ha1

A

B

S

C

in1

in2

cin

cout

sumI1

I2 I3

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Hierarchical Names

ha2.A

Remember to use instance names,not module names

HalfAdder ha2

HalfAdder ha2

A

B

S

C

HalfAdder 1ha1

HalfAdder 1ha1

A

B

S

C

in1

in2

cin

cout

sumI1

I2 I3

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Structural Model (Gate Level) Built-in gate primitives:

and, nand, nor, or, xor, xnor, buf, not, bufif0, bufif1, notif0, notif1

Usage:nand (out, in1, in2); 2-input NAND without delayand #2 (out, in1, in2, in3); 3-input AND with 2 t.u. delaynot #1 N1(out, in); NOT with 1 t.u. delay and instance namexor X1(out, in1, in2); 2-input XOR with instance name

Write them inside module, outside procedures

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Example: Half Adder, 2nd Implementation

Assuming:• XOR: 2 t.u. delay• AND: 1 t.u. delay

module half_adder(S, C, A, B);output S, C;input A, B;

wire S, C, A, B;

xor #2 (S, A, B);and #1 (C, A, B);

endmodule

A

B

S

C

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Behavioral Model - Procedures (i)

Procedures = sections of code that we know they execute sequentially

Procedural statements = statements inside a procedure (they execute sequentially)

e.g. another 2-to-1 mux implem:begin

if (sel == 0)

Y = B;

else

Y = A;

end

ExecutionFlow Procedural assignments:

Y must be reg !!

Procedural assignments:Y must be reg !!

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Behavioral Model - Procedures (ii)

Modules can contain any number of procedures

Procedures execute in parallel (in respect to each other)

and ..

.. can be expressed in two types of blocks: initial they execute only once

always they execute for ever (until simulation finishes)

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“Initial” Blocks Start execution at sim time zero and finish when their

last statement executesmodule nothing;

initial

$display(“I’m first”);

initial begin

#50;

$display(“Really?”);

end

endmodule

Will be displayedat sim time 0

Will be displayedat sim time 0

Will be displayedat sim time 50

Will be displayedat sim time 50

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“Always” Blocks Start execution at sim time zero and continue until

sim finishes

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Events (i) @

always @(signal1 or signal2 or ..) begin

..

end

always @(posedge clk) begin

..

end

always @(negedge clk) begin

..

end

execution triggers every time any signal changes

execution triggers every time any signal changes

execution triggers every time clk changes from 0 to 1

execution triggers every time clk changes from 0 to 1

execution triggers every time clk changes from 1 to 0

execution triggers every time clk changes from 1 to 0

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Examples

3rd half adder implemmodule half_adder(S, C, A, B);

output S, C;

input A, B;

reg S,C;

wire A, B;

always @(A or B) begin

S = A ^ B;

C = A && B;

end

endmodule

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Timing (i)

initial begin#5 c = 1;#5 b = 0;#5 d = c;end

initial begin#5 c = 1;#5 b = 0;#5 d = c;end

0 5 10 15

Time

b

c

d

Each assignment isblocked by its previous one

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Timing (ii)

initial beginfork#5 c = 1;#5 b = 0;#5 d = c;joinend

initial beginfork#5 c = 1;#5 b = 0;#5 d = c;joinend

0 5 10 15

Time

b

c

d

Assignments are not blocked here

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Procedural Statements: if

if (expr1)true_stmt1;

else if (expr2)true_stmt2;

..else

def_stmt;

E.g. 4-to-1 mux:module mux4_1(out, in, sel);output out;input [3:0] in;input [1:0] sel;

reg out;wire [3:0] in;wire [1:0] sel;

always @(in or sel)if (sel == 0)

out = in[0];else if (sel == 1)

out = in[1];else if (sel == 2)

out = in[2];else

out = in[3];endmodule

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Procedural Statements: case

case (expr)

item_1, .., item_n: stmt1;item_n+1, .., item_m: stmt2;..default: def_stmt;

endcase

E.g. 4-to-1 mux:module mux4_1(out, in, sel);output out;input [3:0] in;input [1:0] sel;

reg out;wire [3:0] in;wire [1:0] sel;

always @(in or sel)case (sel)0: out = in[0];1: out = in[1];2: out = in[2];3: out = in[3];endcase

endmodule

System Tasks $monitor

$monitor ($time,"%d %d %d",address,sinout,cosout); Displays the values of the argument list whenever

any of the arguments change except $time.

$display $display ("%d %d %d",address,sinout,cosout); Prints out the current values of the signals in the argument list

$finish $finish Terminate the simulation

Thanasis OikonomouVerilog HDL Basics42

Compiler Directives

`include “filename” inserts contents of file into current file; write it anywhere in code ..

`define <text1> <text2> text1 substitutes text2; e.g. `define BUS reg [31:0] in declaration part: `BUS data;

`timescale <time unit>/<precision> e.g. `timescale 10ns/1ns later: #5 a = b;

50ns50ns

Test MethodologyTest Methodology

Systematically verify the functionality of a model.

Procedure of simulation Detect syntax violations in source code Simulate behavior Monitor results

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Test MethodologyTest Methodology

TestbenchHardware Design

(Design Under Test)

Stimulus

Response

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Testbench for Full AdderTestbench for Full Adder

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module t_full_add();reg a, b, cin; // for stimulus waveformswire sum, c_out;

full_add M1 (sum, c_out, a, b, cin); //DUT

initial #200 $finish; // Stopwatch

initial begin // Stimulus patterns#10 a = 0; b = 0; cin = 0; // Execute in sequence#10 a = 0; b = 1; cin = 0; #10 a = 1; b = 0; cin = 0; #10 a = 1; b = 1; cin = 0; #10 a = 0; b = 0; cin = 1; #10 a = 0; b = 1; cin = 1; #10 a = 1; b = 0; cin = 1; #10 a = 1; b = 1; cin = 1;endendmodule

bin2gray.v

module bin2gray ( B ,G );

input [3:0] B ;wire [3:0] B ;

output [3:0] G ;wire [3:0] G ;

assign G[3] = B[3];assign G[2:0] = B[3:1] ^ B[2:0];

endmodule

Convert Binary to Gray: Copy the most significant bit. For each smaller iG[i] = B[i+1] ^ B[i]

Binary coding {0...7}: {000, 001, 010, 011, 100, 101, 110, 111}

Gray coding {0...7}: {000, 001, 011, 010, 110, 111, 101, 100}

Binary - Gray Code Conversions

Gray code: G[i], i = n – 1 : 0Binary code: B[i], i = n – 1 : 0

Convert Binary to Gray: Copy the most significant bit. For each smaller iG[i] = B[i+1] ^ B[i]

Convert Gray to Binary: Copy the most significant bit. For each smaller iB[i] = B[i+1] ^ G[i]

Gray Code

000000

001001

010011

011010

100110

101111

110101

111100

BinaryB[2:0]

Gray CodeG[2:0]

Gray code to Binary

B[2] = G[2];B[1:0] = B[2:1] ^ G[1:0];

module gray2bin6 ( G ,B );

input [5:0] G ;wire [5:0] G ;

output [5:0] B ;wire [5:0] B ;

assign B[5] = G[5];assign B[4:0] = B[5:1] ^ G[4:0];

endmodule

B(msb) = G(msb);for(j = msb-1; j >= 0; j=j-1) B(j) = B(j+1) ^ G(j);

Gray code to Binary

gray2bin.v

module gray2bin ( G ,B );

input [3:0] G ;wire [3:0] G ;

output [3:0] B ;reg [3:0] B ;integer i;

always @(G) begin

B[3] = G[3];for(i=2; i >= 0; i = i-1) B[i] = B[i+1] ^ G[i];

end

endmodule

Convert Gray to Binary: Copy the most significant bit. For each smaller iB[i] = B[i+1] ^ G[i]