VHDL之package

　　

Pacakge

　　Frequently used pieces of VHDL code are usually written in the form of COMPONENTS, FUNCTIONS, or PROCEDURES. Such codes are then placed inside a PACKAGE and compiled into the destination LIBRARY.

1　　Syntax

　　Besides COMPONENTS, FUNCTIONS, and PROCEDURES, it can also contain TYPE and CONSTANT definitions, among others. Its syntax is presented below.

PACKAGE package_name IS
    (declarations)
END package_name;

[PACKAGE BODY package_name IS
    (FUNCTION and PROCEDURE descriptions)
END package_name;]

2　　Simple Package

　　It shows a PACKAGE called my_package. It contains only TYPE and CONSTANT declarations, so a PACKAGE BODY is not necessary.

 ------------------------------------------------
 LIBRARY ieee;
 USE ieee.std_logic_1164.all;
 ------------------------------------------------
 PACKAGE my_package IS
 TYPE state IS (st1, st2, st3, st4);
 TYPE color IS (red, green, blue);
 CONSTANT vec: STD_LOGIC_VECTOR( DOWNTO ) := "";
 END my_package;
 ------------------------------------------------

　　The PACKAGE above can now be compiled, becoming then part of our work LIBRARY (or any other). To make use of it in a VHDL code, we have to add a new USE clause to the main code (USE work.my_package.all), as shown below.

 ------------------------------------
 LIBRARY ieee;
 USE ieee.std_logic_1164.all;
 USE work.my_package.all;
 ------------------------------------
 ENTITY...
 ...
 ARCHITECTURE...
 ...
 ------------------------------------

3　　Package in ASIC

　　In ASIC design, use ieee.std_logic_1164, and ieee.numeric_std, NEVER use ieee.std_logic_arith

　　numeric_std defines numeric types and arithmetic functions for use with synthesis tools.

　　　-　two numeric types are defined: UNSIGNED, SIGNED (represents a SIGNED number in vector form)

　　　-　base element type is type STD_LOGIC. The leftmost bit is treated as the most significant bit.

　　　-　signed vectors are represented in two's complement form.

　　　-　contains overloaded arithmetic operators on the SIGNED and UNSIGNED types.

　　　-　contains useful type conversions functions.