VHDL lecture 2.ppt

VERY HIGH SPEED INTEGRATED CIRCUIT
HARDWARE DESCRIPTION LANGUAGE
LECTURE - II
Presented By :
Parag Parandkar
Assistant Professor & Head,
ECE Department,
Oriental University, Indore, M.P. , India

ACKNOWLEDGEMENT
The Presenter would like to thank and
acknowledge the power point presentation
slides of VHDL – A Comprehensive tutorial by
anonymous.

CONTENTS
 Subprogram
 Function and Procedure
 Package
Package declaration and Package
Body
 Use Clause
 Analysis Rule for Units
 Objects and Data Types
 Scalar Type
 Literal

SUBPROGRAMS
Subprograms are of two types :
- functions and procedures
A subprogram consists of a sequence of
declarations and statements which can be repeated
from different locations in VHDL descriptions
subprograms can be overloaded
functions can be used for operator overloading
procedures can assign values to its parameter
objects while functions can not
A subprogram can be separated into its
subprogram declaration and subprogram body

SUBPROGRAMS (CONTD.)
Full form of subprogram declaration is
 subprogram-specification;
Two forms of subprogram - specification
procedure identifier interface_list
[pure | impure] function identifier interface_list
return type_mark
Full form of subprogram body is
subprogram-specification is
 declarations
begin
 statements
end identifier;

FUNCTIONS
 Intended to be used strictly for computing values and not
for changing value of any objects associated with the
function’s formal parameters
 All parameters must be of mode in and class signal or
constant or File.
 If no mode is specified, the parameter is interpreted as
having mode in. If no class is specified parameters are
interpreted as being of class constant. Parameter of type
FILE has no mode.
 Examples of function declaration
Object class of parameter is implicit
function Mod_256 (X : Integer) return Byte;
Object class of parameter is explicit
function Mod_256(constant X : in Integer) return Byte;

EXAMPLE
Function declaration
function Min (X, Y : Integer) return Integer;
-- Function Specification
function Min (X, Y : Integer) return Integer is
begin
if (X < Y) then
return X;
else
return Y;
end if;
end Min;

PROCEDURES
 Procedures are allowed to change the values of the
objects associated with its formal parameters
 Parameters of procedures may of mode in, out or inout
 If no mode is specified the parameter is interpreted as
having mode in. If no class is specified parameters of
mode in are interpreted as being of class constant and
parameters of mode out or inout are interpreted as being
of class variable. Parameter of type FILE has no mode.
 Examples of procedure declaration
Object class of parameter is implicit
procedure Mod_256 (X : inout Integer);
Object class of parameter is explicit
procedure Mod_256(variable X : inout Integer);

EXAMPLE
Procedure declaration
--- X is of class variable
Procedure ModTwo (X : inout Integer);
-- Procedure Specification
Procedure ModTwo (X : inout Integer) is
begin
case X is
When 0 | 1 => null;
When others X := X mod 2;
end case;
end ModTwo;

PACKAGES
Allows data types, subprograms, object
declarations (signal, constants, shared variables and
files), component declarations etc. to be shared
by multiple design units.
package identifier is
 declarations
end [package] [identifier];
 Example :
package logic is
 type Three_level_logic is (‘0’, ‘1’, ‘z’);
 function invert (Input : Three_level_logic) return
Three_level_logic;
end logic;

PACKAGE BODY
Package declarations and bodies are separately
described
Package declarations contain public and visible
declarations
Items declared inside package body is not visible
outside the package body
Package body has the same name as the
corresponding package declaration
package body identifier is
 declarations
end [package body] [identifier];

PACKAGE BODY (CONTD.)
 Example of a package body for package logic
package body logic is
-- subprogram body of function invert
function invert (Input: Three_level_logic) return
Three_level_logic is
begin
case Input is
when ‘0’ => return ‘1’;
when ‘1’ => return ‘0’;
when ‘Z’ => return ‘Z’;
end invert;
end logic;

USE CLAUSE
 Use clause preceding an unit makes all the elements of a
package or a particular element of a package visible to
the unit
 An architecture body inherits the use clauses of its
entity. So if those use clauses are sufficient for the
descriptions inside the architecture, then no explicit use
clause is necessary for the architecture
 Simple examples :
Makes all items of package std_logic_1164 in library
ieee visible
use ieee.std_logic_1164.all;
Makes Three_level_logic in package Logic in library
work visible
use work.Logic.Three_level_logic;

USE CLAUSE (CONTD.)
library my_lib;
use my_lib.Logic.Three_level_logic;
use my_lib.Logic.Invert;
entity Inverter is
port (X : in Three_level_logic;
Y : out Three_level_logic);
end inverter;

ANALYSIS RULES FOR UNITS
 Units can be separately analyzed provided following rules are
obeyed
 a secondary unit can be analyzed only after its primary unit is
analyzed
 a library unit that references another primrary unit can be
analyzed only after the referred unit has been analyzed
 an entity, architecture, configuration referenced in a
configuration declaration must be analyzed before the
configuration declaration is analyzed
 A library clause makes library visible and an use clause makes
the units inside the library visible to other units
library Basic_Library;
Use Basic_Library.Logic;
Use Logic.all;

OBJECTS AND DATA TYPES
 Something that can hold a value is an object (e.g signal)
 In VHDL, every object has a type, the type determining
the kind of value the object can hold
 VHDL is strongly typed language
 The type of every object and expression can be
determined statically. i.e the types are determined prior
to simulation.
 Three basic VHDL data types are
integer types
floating point types
enumerated types
 Data types can be user defined

DATA TYPES
Data type
Scalar Type Composite TypeAccess Type File Type
Integer
Float
Physical
Enumeratio
n
Record
Array
Integer and enumeration types are called discrete types
Integer, Real and Physical types are called numeric types

DATA TYPES(CONTD.)
 Scalar Type
Most atomic
Can be ordered along a single scale
Integer types
type Byte is range -128 to 127;
type Bit_pos is range 7 downto 0;
Floating types
type fraction_type is range 100.1 to 200.1;
Enumerated Types
consists of enumeration literal
a literal is either an identifier or a character literal
type Three_Level_Logic is (‘0’, ‘1’, ‘z’);
type Color_set is (RED, GREEN, BLUE);

DATA TYPES : SCALAR TYPE
(CONTD.)
Physical Type
Values of physical type represent measurement of some physical
quantity such as time, distance etc.
Any value of a physical type is an integral multiple of the
primary unit of measurement for the type
Example
 type Time is range -(2**31 -1) to (2**31 -1)
 units
 fs ; --- primary unit
 ps = 1000 fs; --- secondary unit
 ns = 1000 ps; --- secondary unit
 end units;

LITERAL
 These are symbols whose value is immediately evident
from the symbol
 Six Literal Types
integer, floating, characters, strings,
bit_string and physical literal;
 Examples
 2 19878 16#D2# 8#720#
2#1000100
 1.9 65971.3333 8#43.6#e+4 43.6E-4
 “ABC()%”
 B”1100” X”Ff” O”70”
 15 ft 10 ohm 2.3 sec

VHDL lecture 2.ppt

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