Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-1

Chapter 3: Dataflow Modeling

Department of Electronic Engineering

National Taiwan University of Science and Technology

Prof. Ming-Bo Lin

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-2

Syllabus

ObjectivesDataflow modelingOperandsOperators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-3

Objectives

After completing this chapter, you will be able to:Describe what is the dataflow modelingDescribe how to use continuous assignmentsDescribe how to specify delays in continuous assignmentsDescribe the data types allowed in Verilog HDLDescribe the operation of the operators used in Verilog HDLDescribe the operands may be used associated with a specified operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-4

Syllabus

ObjectivesDataflow modeling

IntroductionContinuous assignmentExpressionsDelays

OperandsOperators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-5

Why Dataflow ?

Any digital system interconnecting registers combinational logic circuits

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-6

Features of Dataflow

A powerful way to implement a designLogic synthesis tools can be used to create a gate-level netlistRTL (register transfer level) is a combination of dataflow and behavioral modeling

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-7

Syllabus

ObjectivesDataflow modeling

IntroductionContinuous assignmentExpressionsDelays

OperandsOperators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-8

Basic Statements

Continuous assignmentsVariations

Net declaration assignmentsImplicit net declarations

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-9

Continuous Assignments

Syntaxassign [delay] net_lvalue = expression;assign [delay] net1_lvalue = expr1,

[delay] net2_lvalue = expr2, ...,

[delay] netn_lvalue = exprn;

assign #5 {c_out, sum[3:0]} = a[3:0] + b[3:0] + c_in;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-10

Net Declaration Assignments

Only one net declaration assignment per net

wire out; // regular continuous assignment assign out = in1 & in2;

wire out = in1 & in2; // net declaration assignment

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-11

Implicit Net Declarations

A feature of Verilog HDL

wire in1, in2; assign out = in1 & in2;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-12

Syllabus

ObjectivesDataflow modeling

IntroductionContinuous assignmentExpressionsDelays

OperandsOperators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-13

Expressions

expression = operators + operandsOperatorsOperands

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-14

Operators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-15

Operands

constantsparametersnets variables (reg, integer, time, real, realtime)bit-select part-selarray elementfunction calls

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-16

Syllabus

ObjectivesDataflow modeling

IntroductionContinuous assignmentExpressionsDelays

OperandsOperators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-17

Three Ways of Specifying Delays

Regular assignment delayNet declaration assignment delayNet declaration delay

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-18

Regular Assignment Delays

The inertial delay model is defaulted

wire in1, in2, out; assign #10 out = in1 & in2;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-19

Net Declaration Assignment Delays

To specify both the delay and assignment on the netThe inertial delay model is defaulted

// net declaration assignment delaywire #10 out = in1 & in2;

// regular assignment delaywire out;assign #10 out = in1 & in2;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-20

Net Declaration Delays

Associate a delay value with a net declaration

// net delayswire #10 out; assign out = in1 & in2;

// regular assignment delaywire out;assign #10 out = in1 & in2;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-21

Syllabus

ObjectivesDataflow modelingOperands

ConstantsData types

Operators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-22

Types of Constants

integerreal string

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-23

Integer Constants

Simple decimal form

-12312345

Base format notation16’habcd2009 4’sb1001 // -7

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-24

Real Constants

Decimal notation 1.5 //.3 // illegal1294.872 //

Scientific notation15E1232E-626.176_45_e-12

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-25

String Constants

A sequence of characters enclosed by double quotes ("")Can only be specified in a single lineEach character is represented as an 8-bit ASCII code

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-26

Syllabus

ObjectivesDataflow modelingOperands

ConstantsData types

Operators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-27

Data Types

NetsAny hardware connection points

VariablesAny data storage elements

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-28

Types of Variable Data Types

reginteger time realrealtime

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-29

The reg Variable

Hold a value between assignmentsMay be used to model hardware registersExamples

reg a, b; reg [7:0] data_a; reg [0:7] data_b; reg signed [7:0] d;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-30

The integer Variable

Contains integer valuesHas at least 32 bitsIs treated as a signed reg variable with the LSB being bit 0

integer i, j; integer data[7:0];

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-31

The time Variable

Used for storing and manipulating simulation timeUsed in conjunction with the $time system taskOnly unsigned value and at least 64 bits, with the LSB being bit 0

time events; time current_time;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-32

The real and realtime Variables

Cannot use range declaration Defaulted to zero (0.0)

real events; realtime current_time;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-33

Vectors

A vector = multiple bit width[high:low] or [low:high]The leftmost bit is the MSBinclude nets and reg data types

A scalar = 1-bit vector

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-34

Bit-Select and Part-Select

integer and time are allowedreal and realtime are not allowed Constant part select: data_bus[3:0], bus[3]Variable part select:

[<starting_bit>+:width]: data_bus[8+:8] [<starting_bit>-:width]: data_bus[15-:8]

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-35

Array and Memory Elements

All net and variable data types are allowed An array element can be a scalar or a vector

wire a[3:0]; reg d[7:0]; wire [7:0] x[3:0]; reg [31:0] y[15:0]; integer states [3:0]; time current[5:0];

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-36

MemoryMemory

model ROM, RAM, or register filesReference

a whole word ora portion of a word of memory

reg [7:0] mema [7:0]; reg [7:0] memb [3:0][3:0]; wire sum [7:0][3:0];

mema[4][3] mema[5][7:4] memb[3][1][1:0] sum[5][0]

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-37

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-38

Bitwise Operators

A bit-by-bit operationA z is treated as x The shorter operand is zero-extended

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-39

An Example --- A 4-to-1 MUX

// using basic and, or , not logic operatorsassign out = (~s1 & ~s0 & i0) |

(~s1 & s0 & i1) |(s1 & ~s0 & i2) |(s1 & s0 & i3) ;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-40

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operators Reduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-41

Arithmetic Operators

The result is x if any operand bit has a value xNegative numbers are represented as 2’s complement

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-42

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-43

Concatenation/Replication Operators

Concatenation operatory = {a, b[0], c[1]};Replication operatory = {a, {4{b[0]}}, c[1]};

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-44

An Example --- A 4-Bit Full Adder

// specify the function of a 4-bit adderassign {c_out, sum} = x + y + c_in;

sum_1[4:0]

+ sum[3:0]

[3:0]

[4:0][3:0] [3:0][3:0]

c_out[4]

sum[3:0][3:0]

c_in

y[3:0] [3:0]

x[3:0] [3:0]

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-45

A 4-Bit Two’s Complement Adder

// specify the function of a two's complement adderassign t = y ^ {4{c_in}}; assign {c_out, sum} = x + t + c_in;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-46

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-47

Reduction Operators

Perform only on one vector operandCarry out a bit-wise operationYield a 1-bit resultWork bit by bit from right to left

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-48

A 9-Bit Parity Generator

// dataflow modeling using reduction operatorassign ep = ^x; // even parity generatorassign op = ~ep; // odd parity generator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-49

An All-Bit-Zero/One Detector

// dataflow modelingassign zero = ~(|x); // all-bit zero assign one = &x; // all-bit one

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-50

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-51

Logical Operators

Always evaluate to a 1-bit value, 0, 1, or xThe result is x (a false condition) if any operand bit is x or z

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-52

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-53

Relational Operators

The result is 1 if the expression is true and 0 if the expression is false

Return x if any operand bit is x or z

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-54

Equality Operators

Compare the two operands bit by bitThe shorter operand is zero-extendedReturn 1 if the expression is true and 0 if the expression is false

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-55

Equality Operators

The operators (==, !=) yield an x if any operand bit is x or z

The operators (===, !==) yield a 1 if the two operands exactly match 0 if the two operands not exactly match

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-56

A 4-B Magnitude Comparator

// dataflow modeling using relation operatorsassign Oaeqb = (a == b) && (Iaeqb == 1); // =assign Oagtb = (a > b) || ((a == b)&& (Iagtb == 1)); // >assign Oaltb = (a < b) || ((a == b)&& (Ialtb == 1)); // <

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-57

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-58

Shift Operators

Logical shift operatorsArithmetic shift operators

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-59

An Example of Shift Operators

input signed [3:0] x;output [3:0] y;output signed [3:0] z;

assign y = x >> 1; assign z = x >>> 1;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-60

Syllabus

ObjectivesDataflow modelingOperandsOperators

Bitwise operatorsArithmetic operatorsConcatenation/replication operatorsReduction operatorsLogical operatorsRelational/equality operatorsShift operatorsConditional operator

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-61

The Conditional Operator

Usage: condition_expr ? true_expr: false_expr;If condition_expr = x or z: the result = true_expr & false_expr (0 and 0 gets 0, 1 and 1 gets 1, others gets x )

For example

assign out = selection ? in_1: in_0;

Chapter 3: Dataflow Modeling

Digital System Designs and Practices Using Verilog HDL and FPGAs @ 2008~2010, John Wiley 3-62

An Example --- A 4-to-1 MUX

// using conditional operator (?:)assign out = s1 ? ( s0 ? i3 : i2) : (s0 ? i1 : i0) ;