1 VHDL units
VHDL code is composed of at least 3 fundamental sections:
1) LIBRARY declarations: Contains a list of all libraries to be used in the design. For example: ieee, std, work, etc.
2) ENTITY: Specifies the I/O pins of the circuit.
3) ARCHITECTURE: Contains the VHDL code proper, which describes how the circuit should behave (function).
2 Library declarations
A LIBRARY is a collection of commonly used pieces of code. Placing such pieces inside a library allows them to be reused or shared by other designs.
To declare a LIBRARY(that is, to make it visble to the design) two lines of code are needed:
1 library library_name; 2 use library_name.package_name.package_parts;
Three packages from three different libraries, are usually needed in design:
- ieee.std_logic_1164 (from the ieee library)
- standard (from the std library)
- work (work library)
1 library IEEE; 2 use ieee.std_logic_1164.all 3 4 library std; 5 use std.standard.all 6 7 library work; 8 use work.all;
The libraries std and work are made visible by default, so there is no need to declare them; only the ieee library must be explicitly written when the STD_LOGIC(or STD_ULOGIC) data type is employed in design.