Verilog HDL

The Verilog HDL

Th.S Nguyễn Thế Hoàng

Bộ môn Viễn thông, Khoa Kỹ Thuật Điện Tử

Trường đại học Công Nghiệp TP. Hồ Chí Minh

M.E Hoang Nguyen, Telecommunications Department

Ho Chi Minh City University of Industry, 2011The Verilog HDL

What is HDLs ?• HDLs - Hardware Description Languages • HDL : a language used to describe a digital system

The Verilog HDL

•Verilog is convenient, device-independent representation of digital logic

The Verilog HDL (cont.)

• Originally a modeling language for a very efficient event-driven digital logic simulator

• Later pushed into use as a specification language for logic synthesis

• Now, one of the two most commonly-used languages in digital hardware design (VHDL is the other)

• Virtually every chip (FPGA, ASIC, etc.) is designed in part using one of these two languages

• Combines structural and behavioral modeling styles

Verilog : Synthesis and Simulation

• Simulation / synthesis• Design / Testbench

Structural Modeling

• When Verilog was first developed (1984) most logic simulators operated on netlists

• Netlist: list of gates and how they’re connected

• A natural representation of a digital logic circuit

• Not the most convenient way to express

test benches

Behavioral Modeling

• A much easier way to write testbenches• Also good for more abstract models of circuits

– Easier to write– Simulates faster

• More flexible• Provides sequencing• Verilog succeeded in part because it allowed both

the model and the testbench to be described together

Verilog - overview

Verilog – Module definition

module and2();

endmodule

module and2(A,B,OUT);Input A;Input B;Output OUT;

endmodule

module and2(A,B,OUT);Input A;Input B;Output OUT;

assign OUT=A&B;//your comment

endmodule

Verilog – Port declaration

To make code easy to read, use self-explanatory port names For the purpose of conciseness, use short port names In vector port declaration, MSB can be smaller index. e.g. output [0:3] result (result[0] is the MSB)

Verilog – Examples

not2 modulemodule not2 (a , b);input a;output b;assign b = ~a;endmodule

and2 module;module and2 (in_a , in_b , out);input in_a; input in_b;output out;

assign out = in_a & in_b;endmodule

or2 modulemodule or2 (in_a , in_b , out);input in_a; input in_b;output out;

assign out = in_a | in_b;endmodule

nand2 modulemodule nand2 (in_a , in_b , out);input in_a; input in_b;output out;

assign out = ~ (in_a & in_b);endmodule

Verilog – Hierarchical Module

Top-Level ALU Declaration

Wiring Example 1 and2 modulemodule and2 (a , b , c)input a; input b;output c;

assign c = a & b;endmodule

and3 module sử dụng module and2module and3 (x , y , z , t )input x; input y; input z;output t;

wire temp;and2 u1(.a(x) , .b(y) , .c(temp) );and2 u2(.a(temp) , .b(z) , .c(t) );

endmodule

Wiring Example 2 or2 module

module or2 (in_a , in_b , out)

endmodule

or3 module sử dụng module or2

module or3 (in_x , in_y , in_z , out3 )

endmodule

More on Module Interconnection

Problems

Cho module mux 2 to 1 (mux2_1.v), viết module mux4 to 1 cấu tạo từ hai mux 2 to 1 như hình sau

Verilog – syntax (1)

Numbers<size>’<base><value> size is the size in bits base can be b(binary), o(octal), d(decimal), or h(hexadecimal) value is any legal number in the selected base, including x and z Default value is 32 bits decimal number.Example: 1’b1, 1’b0, 0 8’b10000111 or 8’b1000_0111 8’d135 8’h87 8’bxxxx_xxxx 8’bzzzz_zzzz

Verilog – syntax (1A)Example:

a[2:0] = 3’d9; //a=?

b[3:0] = 4’d18; //b=?

Example:a[5:0] = 3’d19; //a[0]=? ,a[1]=? ,…a[5]=?

Verilog – syntax (2)

IdentifiersIdentifiers are user-defined words for variables, module names, block names and instance names. Syntax

Allowed symbols: ABCDE . . . abcdef. . . 1234567890 _$ Not allowed: anything else especially: - & # @

Example adder // use underscores to make your meaning by_8_shifter // identifiers more meaningful _ABC_ /* is not the same as */ _abc_ read_ // is often used for NOT Read* Identifiers in Verilog are case-sensitive.

Verilog – Operators(1)

Arithmetic Operators These perform arithmetic operations. The + and - can be used as either sign (-z) or operator (x-y) .Operators

+ (addition)- (subtraction)* (multiplication)/ (division)% (modulus)

Does synthesis tool support?

module adder (a, b, s); parameter n = 7; input [n:0] a, b; output [n+1:0] s;

assign s = a + b;

endmodule

Bit-wise Operators Bit-wise operators do a bit-by-bit comparison between two operands.Operators

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

Examplemodule and2 (a, b, c)input [1:0] a, b;output [1:0] c;assign c = a & b;endmodule

c[0] = a[0] & b[0]c[1] = a[1] & b[1]

Verilog – Operators(3)• Relational Operators

• Relational operators compare two operands and return a single bit 1 or 0.

• Operators

< (less than)

<= (less than or equal to)

> (greater than)

>= (greater than or equal to)

== (equal to)

!= (not equal to)

module and_or (a, b, sel, f); input a, b, sel; output f; reg f; always @ (a or b or sel) if (sel == 1'b1) f = a & b; else f = a | b;endmodule

Verilog – Operators(4)• Logical Operators• Logical operators return a single bit 1 or

0. They are the same as bit-wise operators only for single bit operands. They can work on expressions groups of bits, and treat all values that are nonzero as “1”.

• Operators

! (logical NOT)

&& (logical AND)

|| (logical OR)

module and_or (a, b, sel1, sel2, f); input a, b; input sel1, sel2; output f; reg f; always @ (a or b or sel1 or sel2) if (sel1 == 1'b1 && sel2 == 1'b1) f = a & b; else f = a | b;endmodule

Verilog – Operators(5)• Reduction Operators• Reduction operators operate on all the bits of an

operand vector and return a single-bit value. These are the unary (one argument) form of the bit-wise operators above.

• Operators& (reduction AND)

| (reduction OR)

~& (reduction NAND)

~| (reduction NOR)

^ (reduction XOR)

~^ or ^~ (reduction XNOR)M.E Hoang Nguyen, Telecommunications Department

Example module chk_zero (a, z); input [2:0] a; output z; assign z = ~| a; endmodule

z = ~ ( a[0] | a[1] | a[2] )a=3’b010 => z=~(0 | 1 | 0)=0

Verilog – Operators(6)• Shift Operators• Shift operators shift the first operand by the number of bits

specified by the second operand. Vacated positions are filled with zeros for both left and right shifts (There is no sign extension).

• Operators– << (shift left)– >> (shift right)

• Example – assign c = a << 2; – // c = a shifted left 2 bits; vacant positions are filled with 0’s – // a = 8’b1111_0000, c = 8’b1100_0000

Verilog – Operators(6)• Concatenation Operator• The concatenation operator combines two or more operands to

form a larger vector.• Operators

– { } (concatenation)• Example

– wire cout;

– wire [1:0] a, b, s;

– wire [2:0] x;

– wire [3:0] y;

– assign x = {1’b0, a}; // x[2]=0, x[1]=a[1], x[0]=a[0]

– assign y = {a, b}; // y[3]=a[1], y[2]=a[0], y[1]=b[1], y[0]=b[0]

– assign {cout, s} = a + b; // Concatenation of a result

Verilog – Operators(6A)Example 1:

a[3:0] = 4’b1011;

b[3:0] = 4’b1100;

Wire y;

assign y = {a , b}; //y=?

Example 2:a[7:0]=8’b10100011; b[7:0]=8’b11001101;wire [7:0] result;wire c;assign {c,result}=a+b; //c = ?,result=?

Example 3:a[7:0]=8’b10100011; assign a={a[3:0],a[7:4]}; //a = ?

Example 4:a[7:0]=8’b10100011; wire [7:0] b,c,d;assign b={a[7:4],1’b0}; //b = ?assign c={1’b1, a[7:4]}; //c = ?assign d={a[7:1],1’b0}; //d = ?

Verilog – Operators(7)• Replication Operator• The replication operator makes multiple copies of an item.• Operators

– {n{item}} (n fold replication of an item)• Example

wire [1:0] a, b;

wire [3:0] x;

assign x = {2{1’b0}, a}; // Equivalent to x = {0, 0, a[1], a[0]}

x[3] x[2] x[1] x[0]

assign y = {2{a}, 3{b}}; //Equivalent to y = {a, a, b, b, b}

Verilog – Operators(7A)Example 1:

a[1:0] = 2’b01;

b[2:0] = 3’b110;

wire [9:0] y;

assign y = {2{a} , 3{b} }; //y=?

Example 2:

a[3:0]=4’b1011; wire [7:0] y;assign y={4{a[3]} , a}; //y = ?

Verilog – Operators(8)• Conditional Operator: “?”• Conditional operator is like those in C/C++. They

evaluate one of the two expressions based on a condition. It will synthesize to a multiplexer (MUX).

• Operators– (cond) ? (result if cond true) : (result if cond false)

• Exampleassign a = (g ) ? x : y;

assign a = (inc == 2) ? a +1 : a -1;

Verilog – Operators(8A)

Example :a[1:0] = 2’b01;b[2:0] = 3’b110;wire [2:0] y,z;assign y = (a==1)?9:5; //y=?assign z = (b==2)?3:(a==1)?2:1; //z=?

Verilog – Operators(10)• Bit-Selects

• Bit-selects and part-selects are a selection of a single bit and a group of bits, respectively, from a wire, reg or parameter vector using square brackets “[ ]”.

• Syntax

– variable_name [index]

– variable_name [msb:lsb]

• Example

– wire [7:0] a, b;

– wire [3:0] ls;

– wire c;

– assign c = a[7] & b[7]; // bit-selects

– assign ls = a[7:4] + b[3:0]; // part-selects

Two Main Components of Verilog

• Concurrent, event-triggered processes (behavioral)– Initial and Always blocks– Imperative code that can perform standard data manipulation

tasks (assignment, if-then, case)– Processes run until they delay for a period of time or wait for a

triggering event• Structure

– Verilog program build from modules with I/O interfaces– Modules may contain instances of other modules– Modules contain local signals, etc.– Module configuration is static and all run concurrently

Two Main Data Types

• Wires represent connections between things– Do not hold their value– Take their value from a driver such as a gate or other module– Cannot be assigned in an initial or always block

• Regs represent data storage – Behave exactly like memory in a computer– Hold their value until explicitly assigned in an initial or always block– Never connected to something– Can be used to model latches, flip-flops, etc., but do not

correspond exactly– Shared variables with all their attendant problems

Two Main Data Types

• Wires represent connections between things– Use by assign statement, can not assigned in always/initial block

wire a;

assign a = 2; //correct

always @(a) begin

assign a = 2; //incorrect

a=2; //incorrect

Two Main Data Types

• Regs represent data storage – Can be assigned in always/initial block

reg a;

always @(a) begin

a=2; //correct

Four-valued Data

• Verilog’s nets and registers hold four-valued data• 0, 1

– Obvious

• Z– Output of an undriven tri-state driver– Models case where nothing is setting a wire’s value

• X– Models when the simulator can’t decide the value– Initial state of registers– When a wire is being driven to 0 and 1 simultaneously– Output of a gate with Z inputs

Structural Modeling

• Nets and Registers

• Modules and Instances

• Gate-level Primitives

• Delays on Primitive Instances

• A Sequential Primitive

• Continuous Assignment

Behavioral Modeling

• Initial and Always Blocks

• Procedural Assignment

• Imperative Statements

• Blocking vs. Nonblocking

Structual Modeling

Nets and Registers

Wires and registers can be bits, vectors, and arrays

wire a; // Simple wire

tri [15:0] dbus; // 16-bit tristate bus

tri #(5,4,8) b; // Wire with delay

reg [-1:4] vec; // Six-bit register

trireg (small) q; // Wire stores a small charge

integer imem[0:1023]; // Array of 1024 integers

reg [31:0] dcache[0:63]; // A 32-bit memory

Gate-level Primitives

Verilog provides the following:

and nand // logical AND/NAND

or nor //logical OR/NOR

xor xnor //logical XOR/XNOR

buf not //buffer/inverter

bufif0 notif0 //Tristate with low enable

bifif1 notif1 //Tristate with high enable

Delays on Primitive Instances

Instances of primitives may include delays

`timescale 1 ps / 1 ps //define simulation time unit

buf b1(a, b); // Zero delay

buf #3 b2(c, d); // Delay of 3

buf #(4,5) b3(e, f); // Rise=4, fall=5

buf #(3:4:5) b4(g, h); // Min-typ-max delay

Ex : Led 7 segments display module

Led 7 segments decoder module

BCD7SegBCD7Seg

Led 7 segments display module

Truth table

BCD7SegBCD7Seg

BOOLEAN FUNCTIONBOOLEAN FUNCTION

S[0] = B3 B2 B1 B0 + B2 B1 B0

B2 B1 B0

00 01 11 10

00 11 10

B3 B2 B1 B0

Using KARNAUGH Map to minimal expression of a boolean function

Boolean Function

Verilog code – Structural Style

file : bcd7seg_struc.vmodule bcd7seg(b,S); input [3:0] b; output [6:0] S; wire [6:0] S;

Behavioral Modeling

Initial and Always Blocks

Basic components for behavioral modeling

initial

// … your code here …

//Runs when simulation starts

//Terminates when control

//reaches the end

//Good for providing stimulus

always

//… your code here …

//Runs when simulation starts

//Restarts when control

//reaches the end

//Good for modeling/specifying

// hardware

Initial and Always

Run until they encounter a delayinitial begin #10 a = 1; b = 0; //a,b must be a regs #10 a = 0; b = 1;end

or a wait for an eventalways @(posedge clk)

q = d; //q must be a regs

always begin wait(i); a = 0; //a must be a regswait(~i); a = 1;

Conditional statement

If/else statement

if (select == 1) y = a;else y = b;

case/endcase statement

case (op) 2’b00: y = a + b; //if (op==0) then y = a+b 2’b01: y = a – b; 2’b10: y = a ^ b; default: y = ‘hxxxx;endcase

If/else vs case

if…else case

For Loop

reg [3:0] i, output;

for ( i = 0 ; i <= 3 ; i = i + 1 ) begin

output = i;

#10;output = i;

While Loop

while ( i <= 3) begin

output = i;

i=i+1;

output = i;

#10;output = i;

Ex : mux 4 to 1 module

Mux 4 to 1 module

Sel[1:0]

module mux4_1 (i0,i1,i2,i3,sel,out);input i0,i1,i2,i3;output reg out;input [1:0] sel;always @(sel or i0 or i1 or i2 or i3)begin

if(sel==0) out=i3;else if(sel==1) out=i2;else if(sel==2) out=i1;else out=i0;

endendmodule

Led 7 segments display module

Truth table

BCD7SegBCD7Seg

Verilog code – Behavioral Style

file : bcd7seg_behav.vmodule bcd7seg(b,S); input [3:0] b; output [6:0] S; reg [6:0] S;Always @(b) begin

case (b)4’h0: S= 7’b1000000; 4’h1: S= 7’b1111001;4’h2: S= 7’b0100100;…4’h9: S= 7’b0010000;default : S = 7’bxxxxxxx;

endcaseendendmodule

References• [1] David Harris, Structural Design with Verilog,

2010• [2] Prof. Stephen A. Edwards, CS dept.,

Columbia University• [3] John F. Warkerly, Digital Logic Design:

Practice and Principles, 3rd Edition, Prentice Hall International Inc., 2002.

• [4] J. Bhasker, A Verilog HDL, Star Galaxy Publishing, 1999.

• [5] World of ASIC,www.asic.com

• Thực hiện mạch chuyển đổi số nhị phân 6bit thành hai số bcd-4bit

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