VHDL is a hardware description language, it can be described the behavior of electronic circuits and systems.
We developed ieee.1164 library.
Using VHDL language description of the circuit can be synthesis and simulation.
VHDL code three basic components: a library (Library) statement, the entity (Entity) and the structure (architecture).
Library (library) Disclaimer: lists all the files needed to use the current design, such as ieee, std, work and so on.
Entity (entity): defines the input and output pins of the circuit.
Structure (architecture): contains the code for the experiment described circuit functions.
1, library declaration
Library (library) the establishment and use of beneficial reuse of design and code-sharing, and can make the code more clear structure.
2, the type library
There are three commonly used libraries in VHDL design: ieee library, std libraries and library work.
Std_logic_1164.all a standard ieee ieee officially recognized in the library. ieee library contains many packages set.
such as:
std_logic_1164: defines std_logic (8 values) and std_ulogic (9 value) logic system.
std_logic_arith: defines the signal (signed) and unsigned (unsigned) and associated data type arithmetic operations and comparison operations. It contains
Many data type conversion functions, data type conversion functions this can be achieved.
Common data type conversion function comprises conv_integer (p), conv_unsigned (p, b), conv_signed (p, b) and conv_std_logic_vector (p, b).
std_logic_signed: Internal contains functions that can std_LOGIC_vector types of data are signed image data for the same
Arithmetic operation.
std_logic_unsigned: Internal contains functions that can std_logic_vector datatype as the same data type unsigned operation.
std library
std library is standard VHDL design the repository environment, including content type and data input / output text and the like. std library storing package set standard and textio.
library work
work library is currently working library, all of the current design codes are stored in library work, library work does not require the use of any statement.