Verilog Tutorial

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Verilog Tutorial

Chao-Hsien, Hsu

Computer Architecture Date: 2011/4/12

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Outline

• Verilog & Example

• Major Data Type

• Operators

• Conditional & Looping Statements

• Behavior Modeling

• Structure Modeling

• Verification Methodology

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IC Design Flow

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Example: 32 bits Full Adder

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Outline

• Major Data Type

• Operators

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Wire vs. Register (1/4)

• Wire

– Physical wire in the circuit

– A wire does not store its value, it must be driven by

• connecting the wire to the output of a gate or module

• assigning a value to the wire in a continuous assignment

– Can not use “wire” in left-hand-side of assignment in procedural – Can not use “wire” in left-hand-side of assignment in procedural

block

• Register

– Not “register” of CPU

– No guarantee to be a DFF(D-flip flop) – Maybe a physical wire

– Holding its value until a new value is assigned to it.

• It is event-driven.

– Can not use “reg” in left-hand side of continuous assignment

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Wire vs. Register (2/4)

Wire (net)

a_reg

out_reg sel_reg

b_reg

Register

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Wire vs. Register (3/4)

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Wire vs. Register (4/4)

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Integer & Real Numbers

X: unknown

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Outline

• Major Data Type

• Operators

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Operators

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Operators: Example

• Example

– A = 4’b1101, B = 4’b1010 – Logical

• A||B = 1

• A&&B = 1

– Bit-wised

• A|B = 4’b1111

• A&B = 4’b1000

– Unary

• |A = 1, &A = 0

• |B = 1, &B = 0

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Equality vs. Identity

• “=” is the assignment operator.

• “==” is the equality operator

– A = 2’b1x; B = 2’b1x;

• A == B (?)

• “===” is the identity operator

– A = 2’b1x; B = 2’b1x;

• A === B (?)

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Concatenation & Replication

// 8’b01010101

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Outline

• Major Data Type

• Operators

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Behavior Model (1/3)

• At system level, system’s function view is more important than implementation.

– You do not have any idea about how to implement your net-list.

– The data flow of this system is analyzed.

– You may need to explore different design options.

• Behavior modeling enables you to describe the system at a high-level of abstraction.

• All you need to do is to describe the behavior of your design.

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Behavior Model (2/3)

• Describing the behavior of your design(circuits).

– Action

• How do you model your circuit’s behaviors?

– Timing control – Timing control

• What time to do what thing

• What condition to do what thing

– You may need to explore different design options.

• Behavior modeling enables you to describe the system at a high-level of abstraction.

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Behavior Model (3/3)

• All you need to do is to describe the behavior of your design.

– Procedural block

– Procedural assignment – Procedural assignment – Timing control

– Control statement

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Procedural Blocks

• Procedural block is the basic of behavior modeling.

– One behavior in one procedural block

• Two types

– “Initial” procedural block

• Execute only once

“Always” procedural block

Execute only once

– “Always” procedural block

• Always executing in a loop

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Procedural Block: Example

200

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Timing Control

• Simple Delay

– #10 rega = regb;

– #(cycle/2) clk = ~clk; // cycle is declared as parameter

• Edge-Trigger Timing Control

– @(r or q) rega = regb; // Controlled by “r” or “q”

– @(posedge clk) rega = regb; // positive edge – @(negedge clk) rega = regb; // negative edge

• Level-Triggered Timing Control

– wait (!enable) rega = regb; // will wait until enable = 0

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Example

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Procedural Assignment

• Continuous Assignment

– Cannot be inside procedural block

• Procedural Assignment

– Must be inside procedural block – Blocking

– Non-blocking – Non-blocking

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Blocking and Non-blocking Procedural Assignment (1/2)

• Blocking

• Non-blocking

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Blocking and Non-blocking Procedural Assignment (2/2)

• Usage Policies

– Non-blocking is only used in “Always” block with “clock”

– Blocking is used in “Always” block without “clock”

– Blocking is used in continuous assignment

– Only one assignment is used in “Always” block

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Conditional Statements (1/2)

• If and if-else statement

if (expression) begin

// statement end

else begin

// statement

if (a >= b) begin

result <= 1;

end else begin

result <= 0;

// statement end

if (expression) begin

// statement end

else if(expression) begin

// statement End

else begin

// statement end

result <= 0;

end

if (a > b) begin

result <= 2;

end

else if (a < b) begin

result <= 1;

end else begin

result <= 0;

end

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Conditional Statements (2/2)

• Case statement

`define pass_accum 4’b0000

`define pass_data 4’b0001

`define ADD 4’b0010

`define AND 4’b0011

`define XOR 4’b0100 case(opcode)

`pass_accum: alu_out = accum;

`pass_data: alu_out = data;

`ADD: alu_out = accum + data;

`AND: alu_out = accum & data;

`XOR: alu_out = accum ^ data;

default: alu_out = 8’b11111111;

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Looping Statements (1/2)

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Looping Statements (2/2)

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Outline

• Major Data Type

• Operators

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Structure Modeling (1/4)

• In structural modeling, you connect

components with each other to create a more complex component.

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Structure Modeling (2/4)

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Structure Modeling (3/4)

module HA(a, b, sum, co);

input a, b;

output sum, co;

assign sum = a ^ b;

assign co = a & b;

endmodule

module FA(A, B, CarryIn, Sum, CarryOut);

input A, B, CarryIn;

output Sum, CarryOut;

wire sum0, sum1, co0;

HA ha0(.a(A), .b(B), .sum(sum0), co(co0));

HA ha1(.a(co0), .b(CarryIn), .sum(sum1), co(CarryOut));

assignSum = sum0 ^ sum1;

endmodule

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Structure Modeling (4/4)

module FA(A, B, CarryIn, Sum, CarryOut);

input A, B, CarryIn;

output Sum, CarryOut;

wiresum0, sum1, co0;

HA ha0(.a(A), .b(B), .sum(sum0), co(co0));

HA ha0(.a(co0), .b(CarryIn), .sum(sum1), co(CarryOut));

assignSum = sum0 ^ sum1;

endmodule

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Outline

• Major Data Type

• Operators

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Verification Methodology

• Simulation

– Detect syntax violations in source code – Simulate behavior

– Monitor results – Monitor results

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Verilog Simulator (ModelSim)

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VHDL & Verilog

• Process Block

• Signal Assignment

• Interface Declaration

• Buses

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Process Block

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Signal Assignment

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Interface Declaration

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Buses

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Reference

• Some contents are referenced from the below materials

– The slides of the “VLSI System Design” course by Prof. Kuen-Jong Lee

Prof. Kuen-Jong Lee

– The slides of the “Digital System Design” course by Prof. An-Yeu (Andy) Wu

– http://www.ece.msstate.edu/~reese/EE4743/lect ures/verilog_intro_2002/verilog_intro_2002.pdf

Verilog Tutorial

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