Verilog Basics

Nattha JindapetchNovember 2008

Agenda

Logic design review Verilog HDL basics LABs

Logic Design Review

Basic Logic gates

Additional Logic Gates

Two Types Combinational circuit

No "memory" Its output depends only upon the current

state of its inputs. Sequential circuit

Contains memory elements Its output depends not only upon the

current state of its inputs, but also on the current state of the circuit itself.

Two Styles: Synchronous and Asynchronous

Combinational Circuits: examples

Multiplexers 2n x 1 multiplexer receives 2n input bits and n

selector bits, and outputs exactly one of the input bits, determined by the pattern of the selector bits.

Combinational Circuits: examples

Demultiplexers 1 x 2n DMUX is the

inverse of MUX It takes 1 input and

transmits that input on exactly one of its outputs, determined by the pattern of its n selector bits.

Combinational Circuits: examples

Encoders 2n x n encoder takes

2n inputs and sets each of its n outputs, based upon the pattern of its inputs.

an encoder is the inverse of a decoder.

Combinational Circuits: examples

Decoders An n x 2n

decoder takes n inputs and sets exactly one of its 2n outputs, based upon the pattern of its inputs.

Sequential Circuits Circuits with feedback

Cross-coupled NOR gates

R S Q Q’

0 0 No change

1 0 0 1 Reset

0 1

1 0 Set

1 1 Not allowed

R

S

Q

Q'

Sequential Elements

Latches vs Flip-Flops Latches work on Level-sensitive Flip-Flops work on Edge-

triggered RS, JK, D, T

S R Q Q’

0 0 No change

0 1 0 1 Reset

1 0

1 0 Set

1 1 Not allowed

J K Q Q’

0 0 No change

0 1 0 1

1 0

1 0

1 1 Toggle

D Q

0 0

1 1

T Q

0 1

1 0

Counters

Synchronous counter Clock of synchronous cou

nter are same clock. Asynchronous counter

Clock of second flip-flop is Q of first flip-flop.

The clock of asynchronous counter are different source.

Registers

A register is used for storing several bits of digital data.

It basically consists of a set of flip-flops. each flip-flop representing one bit of the register. Thus, an n-bit register has n flip-flops.

A Simple Shift Register Consisting of D-type Flip-flops

Verilog HDL basics

HDL (Hardware Description Language)

Big picture: Two main HDLs out there VHDL 　

Designed by committee on request of the Department of Defense 　

Based on Ada Verilog HDL

Designed by a company for their own use Based on C

Both now have IEEE standards Both are in wide use 　

Verilog Description Styles

Verilog supports a variety of descriptio n styles

Structural 　 explicit structure of the circuit e.g., each logic gate instantiated and connecte

d to others Behavioral

program describes input/output behavior of circuits

Mixed

Verilog Module

module module-name (list-of-port); input/output declarations local net declarations

parallel statements endmodule

Structural Module: example1

module half_adder (s, a, b, co);inputa, b;output s, co;

wire w0, w1, w2; assign w0 = a & b,

w1 = ~w0,w2 = a | b,s = w1 & w2,co = w0;

endmodule

a

b w0w1

w2

co

s

a b s co

0 0 0 0

0 1 1 0

1 0 1 0

1 1 1 1

HAa

b

i0co

s

HAa

b

i1co

s

w0

ci

a

b

w1

w2

s

co

Structural Module: example1

module full_adder (s, a, b, co, ci);inputa, b, ci;output s, co;

wire w0, w1, w2; assign co = w1 | w2;half_adder i0

(.co(w1), .s(w0), .a(a), .b(b));half_adder i1

(.co(w2), .s(s), .a(w0), .b(ci));endmodule

Structural Module: example1

module adder4 (s, a, b, co, ci);input [3:0] a, b;input ci;output [3:0] s, output co;

wire w0, w1, w2; full_adder i0

(.co(w0), .s(s[0]), .a(a[0]), .b(b[0]), .ci(ci));full_adder i1

(.co(w1), .s(s[1]), .a(a[1]), .b(b[1]), .ci(w0));full_adder i2

(.co(w2), .s(s[2]), .a(a[2]), .b(b[2]), .ci(w1));full_adder i3(.co(co), .s(s[3]), .a(a[3]), .b(b[3]), .ci(w2));

endmodule

FAa

b

i0

co

sci

FAa

b

i1

co

sci

FAa

b

i2

co

sci

FAa

b

i3

co

sci

w0

w1

w2

ci

a[0]

b[0]

a[1]

b[1]

a[2]

b[2]

a[3]

b[3]

s[0]

s[2]

s[1]

s[3]

co

Behavioral Module: example1module adder4 (s, a, b, co, ci);

input [3:0] a, b;input ci;output [3:0] s, output co;

reg [3:0] s;reg co;

always @(a or b or ci) {co, s} = a + b + ci;

endmodule

Verilog Data Types

Possible Values: 0 0: logic , false 1 1: logic , true X : unknown logic value Z H HHH HH HHHHHHH HHHHH:

Registers and Nets (wires) are the main dat a types

Integer, time, and real are used in behavior al modeling, and in simulation

HHHH HHHH HHHH are not synt hesi zed !

Verilog Registers

Abstract model of a data storage element

A reg holds its value from one assignme nt to the next

The value “sticks” Register type declarations

reg a; // a scalar register reg [3:0] b; // a 4-bit vector register

Verilog Nets

Nets (wires ) model physical connections

They don’t hold their value They must be driven by a “driver” (i .e. a

gate output o r a continuous assignment) Their value is Z if not driven

Wire declarations wire d; // a scalar wire wire [3:0] e; // a 4- bit vector wire

Verilog Parameters

Used to define constantsparameter size = 16, value 8= ;wire - [ 1:0] ; // defines a 15:0 bus

Verilog Operators

Arithmetic operators: +, -, *, /, % Logical operators: &&, ||, ! Bitwise operators: &, |, ~, ^, ^~ Equality operators: ==, !=, Relational operators: >, <, >=, <= Reduction operators: &, ~&, |, ~|, ^ Shift operators: >>, << Conditional: ?:

Example2: 4:1 multiplexer

module mux4 (s, d, z); //bitwise operators

input [1:0] s;input [3:0] d;output z;assign z =(~s[1] & ~s[0] & d[0]) |

(~s[1] & s[0] & d[1]) | ( s[1] & ~s[0] & d[2]) | ( s[1] & s[0] & d[3]) ;

endmodule

Example2: 4:1 multiplexer

module mux4 (s, d, z); //using conditional operatorsinput [1:0] s;input [3:0] d;output z;assign z = s[1] ? (s[0] ? d[3] : d[2]) : (s[0] ? d[1] : d[0]);endmodule

Verilog Assignments

Two types: Continuous Assignments

assign values to nets This means combinational logic

Procedural Assignments assign values to registers Only allowed inside procedural blocks

(initial and always)

Continuous Assignments

Models combinational logic using a logical expression instead of gates

Assignment is evaluated whenever any signal changes

wire a, b, out; assign out = ~(a & b);

wire [15:0] sum, a, b;wire cin, cout;assign {cout,sum} = a + b + cin;

Procedural Assignments

Assigns values to register types They do not have a duration

The register holds the value until the next procedural assignment to that variable

They occur only within procedural blocks initial and always

They are triggered when the flow of execution reaches them

always Blocks When is an always block executed?

always Starts at time 0

always @(a or b or c) Whenever there is a change on a, b, or c Used to describe combinational logic

always @(posedge foo) Whenever foo goes from low to high Used to describe sequential logic

always @(negedge bar) Whenever bar goes from high to low

Inside always blocks

Procedural assignments if statements case statements for, while, forever statements wait statements

Blocking vs Non-blocking

Blocking vs Non-blocking

beginq1 =

x1;q2 =

q1;z1 =

q2;end

Blocking

beginq1 <=

x1;q2 <=

q1;z1 <=

q2;end

Non-Blocking

Quick Review

Continuous assignments to wires assign variable = exp; Result in combinational logic

Procedural assignment to regs Always inside procedural blocks (always blocks

in particular for synthesis) blocking

variable = exp; non-blocking

variable <= exp; Can result in combinational or sequential logic

Quick Review

module name (args…);input …; // define inputsoutput …; // define outputs

wire… ; // internal wiresreg …; // internal regs, possibly output

// the parts of the module body are// executed concurrently<continuous assignments><always blocks>

endmodule

comment in Verilog_HDL // Single-line comment /*………….

…………..*/ multiple-line comment

module mux2_1(out,a,b,sel); // port declare. input a,b,sel; output out; wire sel_,a1,b1 /* structural design using logic operator mux2_1 */ not (sel_,sel); and (a1,a,sel),(b1,b,sel); or (out,a1,b1); endmodule;

Naming in Verilog_HDL

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