ECE C03 Lecture 121 Lecture 12 Introduction to VHDL Hai Zhou ECE 303 Advanced Digital Design Spring 2002.

ECE C03 Lecture 12 1

Lecture 12Introduction to VHDL

Hai Zhou

ECE 303

Advanced Digital Design

Spring 2002


Outline

• VHDL Language Basics• Interface• Architecture Body• Process• Signal Assignment and Delay Models• Various Sequential Statements• READING: Dewey 11.2, 11.3, 11.4, 11.5. 11.6,

15.1, 15.2, 18.2, 18.3, 18.4, 18.5


Modeling Digital Systems

• Digital system: Any digital circuit that processes or stores information in digital form: gates to functional units

• Model represents only relevant information and abstracts away irrelevant detail

• Model needed to:– Develop and specify requirements

– Communicate understanding of a system to a user

– Allow testing of design through simulation

– Allow formal verification

– Allow automated synthesis


What is VHDL• (Very High Speed Integrated Circuits) VHSIC Hardware

Description Language• Used for two things

– (1) Used to model digital systems, designs can then be SIMULATED

• A simulator runs a VHDL description computing the outputs of a modeled system

– (2) Used as a language to enter designs into CAD tools, designs can then be SYNTHESIZED

• VHDL also provides a blackboard for designing digital systems• An initial design is progressively expanded and refined

• Another popular hardware language is Verilog


Relationship between VHDL and hardware

Model

VHDL description

Simulator

Simulated / actual outputs

Hardware


Example of VHDL DescriptionVHDL Model of a 2input exclusive OR gate

entity XOR2_OP is

-- input output ports

port

(A, B : in BIT;

Z : out BIT);

-- Body

architecture EX_DISJUNCTION of XOR_OP2 is

begin

Z <= A xor B;

end EX_DISJUNCTION;


VHDL Entity Definitions• A VHDL entity consists of two parts:

– interface denoted by keyword “entity”– body denoted by keyword “architecture”

• Interface describes aspects visible outside

• Body describes how black box operates inside

• FORMAT:

entity identifier is

port (name: in / out / inout BIT/type);

end identifier;

-- lines beginning with two dashes are comments


VHDL Architecture Body• Architecture body describes how entity operates

• Allows for different implementations

• Can have behavioral or structural or mixed representations

FORMAT

architecture EX_DISJUNCTION of XOR_OP2 is

begin

Z <= A xor B;

end EX_DISJUNCTION;


Architecture Body• Body is divided into two parts

– Declarative part– Statement part

architecture EX_DISJUNTION of XOR_OP2 is

-- declarative part

-- objects must be declared before they are used

begin

-- statement part

Z <= A xor B;

end EX_DISJUNCTION;


Data Types in VHDL

• The type of a data object defines the set of values that object can assume and set of operations on those values – VHDL is a strongly typed language

• Four classes of objects– constants

– variables

– signals

– files


Constant Declaration

• The value of a constant cannot be changed• FORMAT:

constant identifier {, } : subtype [ := expression]• EXAMPLES:

constant number_of_bytes : integer := 4;

constant prop_delay : time := 3nsec;

constant e : real := 2.2172;


Variable Declaration

• The value of a variable can be changed• FORMAT

variable identifier {, ..} subtype [ := expression]• EXAMPLES

variable index: integer := 0;

variable sum, average, largest : real;

variable start, finish : time : = 0 nsec;


Variable Assignment Statement• Once a variable is declared, its value can be modified by

an assignment statement• FORMAT: [ label : ] name := expression;• EXAMPLES: program_counter := 0; index := index + 1;• Variable assignment different from signal assignment

– A variable assignment immediately overviews variable with new value

– A signal assignment schedules new value at later time


Scalar Types

• Variable can only assign values of nominated type• Default types

– “integer” , “real”, “character,” “boolean”, “bit”

• User defined types– FORMAT: type small_int is range 0 to 255;

• Enumerated type:– FORMAT: type logiclevel is (unknown, low, driven, high);


Sub Types

• A type defines a set of values• We can define a sub-type as a restricted set of

values from a base type– FORMAT

subtype identifier is name range simple_expression to/downto simple_expression

– EXAMPLE

subtype small_int is integer range -128 to 127;

subtype bit_index is integer range 31 downto 0;


Attributes of Types• A type defines a set of values and set of applicable operations• A predefined set of attributes are used to give information

about the values included in the type• T’left = first (leftmost) value in T• T’right = last (righmost) value in T• T’value(s) = the value in T that is represented by s• EXAMPLES:

type set_index_range is range 21 downto 11;set_index_range’left = 21set_index_range’right = 11set_index_range’value(“20”) = 20


Expressions and Operators

Operator Operation Operand Types** Exponentiation Integer, realAbs Absoluet value numeric*, / , mod, rem Mul, div Integer, realAnd, nand, or, nor, xor, xnor, not Logical ops Bit, boolean, or 1-D arraySll, srl, sla,sra Shift left/right 1-D array of bit/boolean+, - Add, subtract Integer, real=, /=, <, <=, >, >= Equal, greater Scalar


VHDL Modeling Concepts

• Semantics (meaning) is heavily based on SIMULATION

• A design is described as a set of interconnected modules

• A module could be another design (component) or could be described as a sequential program (process)


A general VHDL design

I1I2

O1IO1

Entity … is…End entity;

component

process 1 process 2

concurrentassignment

concurrentassignment

I1

I2

O1

IO1

s1

s2

s3 s4

s5

s6

s7

s8 s9

architecture … of … is...begin…end;


VHDL Simulatorstart

stop

Initt = 0

more event

get earliestevent

delta delay

updatesignals

advancetime

execute triggeredprocesses

during process execution,new events may be added


Process Statements• FORMAT

PROCESS_LABEL: process-- declarative part declares functions, procedures, types, constants,

variables, etcbegin-- Statement partsequential statement;sequential statement;wait statement; -- eg. Wait for 1 ms; or wait on ALARM_A;sequential statement;…wait statement;end process;

Flow of

control


Sequential Statements

• Sequential statements of various types are executed in sequence within each VHDL process

• Variable statementvariable := expression;

• Signal Assignment• If statement• Case statement• Loop statement• Wait statement


Variable and Sequential Signal Assignment

• Variable assignment– new values take effect immediately after execution

variable LOGIC_A, LOGIC_B : BIT;

LOGIC_A := ‘1’;

LOGIC_B := LOGIC_A;

• Signal assignment– new values take effect after some delay (delta if not specified)

signal LOGIC_A : BIT;

LOGIC_A <= ‘0’;

LOGIC_A <= ‘0’ after 1 sec;

LOGIC_A <= ‘0’ after 1 sec, ‘1’ after 3.5 sec;


Signal Declaration and Assignment• Signal declaration: describes internal signal signal identifier {…} : subtype [ := expression]• EXAMPLE: signal and_a, and_b : bit;

• Signal Assignment name <= value_expression [ after time_expression];• EXAMPLE y <= not or_a_b after 5 ns;

• This specifies that signal y is to take on a new value at a time 5 ns later statement execution.

• Difference from variable assignment:– which only assigns some values to a variable


Concepts of Delays and Timing

• The time dimension in the signal assignment refers to simulation time in a discrete event simulation

• There is a simulation time clock• When a signal assignment is executed, the delay

specified is added to current simulation time to determine when new value is applied to signal– Schedules a transaction for the signal at that time

input

output


Specifying Technology Information• One predefined physical type in VHDL: TIME• Units: fs (10** -15 seconds), ps (1000 fs), ns, us, ms, sec,

min ( 60 sec), hr (60 min)• User-defined physical types

type CAPACITANCE is range 0 to INTEGER’HIGHunitsfF; -- FemtofaradspF = 1000 fF; -- PicofaradsnF = 1000 pF; -- Nanofaradsend units

type VOLTAGE is range 0 to 2 ** 32 -1unitsuV; -- Microvolt;mV = 1000 uV;V = 1000 mV;end units;


Specifying Delays

• Inertial Delay Model– reflects physical inertia of physical systems

– glitches of very small duration not reflected in outputs

• SIG_OUT <= not SIG_IN after 7 nsec --implicit• SIG_OUT <= inertial ( not SIG_IN after 7 nsec )• Logic gates exhibit lowpass filtering

SIG_IN

SIG_OUT2ns

9 ns 19 ns

3 ns10ns


Transport Delays

• Under this model, ALL input signal changes are reflected at the output

• SIG_OUT <= transport not SIG_IN after 7 ns;

SIG_IN

SIG_OUT2ns

9 ns 19 ns

3 ns10ns

30 ns


If Statement• FORMAT if boolean_expression then {sequential statement} elsif boolean_expression then {sequential statement} else {sequential statement} endif;• EXAMPLE if sel=0 then result <= input_0; -- executed if sel = 0 else result <= input_1; -- executed if sel = 1

endif;


Case Statement• EXAMPLE of an ALU operation:case func is when pass1 => result := operand1; when pass2 => result := operand2; when add => result := operand1 + operand2; when subtract => result := operand1 - operand2; end case;


Loop Statements

While condition loop

{sequential statements}

end loop;

for identifier in range loop

{sequential statements}

end loop;

while index > 0 loop index := index -1;end loop;

for count in 0 to 127 loop count_out <= count;wait for 5 ns;end loop;

for i in 1 to 10 loop count := count + 1;end loop;


Wait Statement

• A wait statement specifies how a process responds to changes in signal values.

wait on signal_name

wait until boolean_expression

wait for time_expression• Example on right shows process sensitivity list

EXAMPLE: SAME AS:half_add: process isbegin sum <= a xor b after T_pd; carry <= a and b after T_pd; wait on a, b;end process;

half_add: process (a,b) isbegin sum <= a xor b after T_pd; carry <= a and b after T_pd;end process;


Example of Architecture Body(AND_OR_INVERT)

architecture primitive of and_or_inv is

signal and_a, and_b, or_a_b : bit;

begin

and_gate_a : process (a1,a2) is

begin

and_a <= a1 and a2;

end process and_gate_a;

and_gate_b : process (b1,b2) is

begin

and_b <= b1 and b2;

end process and_gate_b;

or_gate: process (and_a, and_b) is

begin

or_a_b <= and_a or and_b;

end process or_gate;

inv : process (or_a_b) is

begin

y <= not or_a_b;

end process inv;

end architecture primitive;a1a2

b1b2

y


Process Declaration of Clock Generator

Clock_gen: process (clk) is

begin

if clk = ‘0’ then

clk <= ‘1’ after T_pw, ‘0’ after 2*T_pw;

endif;

end process clock_gen;T_pw

2*T_pw


Process Generator for Multiplexer

a

b

sel

z

mux: process (a, b, sel) is

begin

case sel is

when ‘0’ =>

z <= a after prop_delay;

when ‘1’ =>

z <= b after prop_delay;

end process mux;


Summary

• VHDL Language Basics

• Interface

• Architecture Body

• Process

• Signal Assignment and Delay Models

• Various Sequential Statements

• NEXT LECTURE: VHDL Structural Description

• READING: Dewey 12.1, 12.2, 12.3, 12.4, 13.1, 13.2, 13.3. 13.4, 13.6, 13.7. 13.8

ECE C03 Lecture 121 Lecture 12 Introduction to VHDL Hai Zhou ECE 303 Advanced Digital Design Spring 2002.

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