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March 03, 2025 01:16
asic
digital design
fpga
hardware description language
vhdl
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VHDL Cheatsheet

A quick reference guide to VHDL (VHSIC Hardware Description Language), covering syntax, data types, operators, and common constructs used in digital circuit design.

Basic Syntax and Structure

Entity Declaration

The entity declaration defines the interface of a design.

entity entity_name is
  port (
    port_name : mode data_type;
    ...
  );
end entity entity_name;

Example:

entity AND2 is
  port (
    A : in  std_logic;
    B : in  std_logic;
    Y : out std_logic
  );
end entity AND2;

Architecture Body

The architecture body implements the behavior of the entity.

architecture architecture_name of entity_name is
  -- Declarations (signals, components, etc.)
begin
  -- Concurrent statements
end architecture architecture_name;

Example:

architecture Behavioral of AND2 is
begin
  Y <= A and B;
end architecture Behavioral;

Libraries and Packages

VHDL uses libraries and packages for predefined types and functions.

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
  • ieee.std_logic_1164: Standard logic types (std_logic, std_logic_vector).
  • ieee.numeric_std: Arithmetic operations on std_logic_vector.

Data Types and Operators

Standard Data Types

std_logic

Represents a single bit with nine possible values (‘U’, ‘X’, ‘0’, ‘1’, ‘Z’, ‘W’, ‘L’, ‘H’, ‘-’).

std_logic_vector

An array of std_logic elements. std_logic_vector(7 downto 0).

integer

Represents signed integer values. Range is implementation-dependent.

boolean

Represents boolean values (TRUE or FALSE).

real

Represents floating-point values.

time

Represents time values with a specified unit (e.g., 10 ns).

Operators

Logical Operators

and, or, nand, nor, xor, xnor, not

Relational Operators

=, /=, <, <=, >, >=

Arithmetic Operators

+, -, *, /, mod, rem, abs, **

Shift Operators

sll, srl, sla, sra, rol, ror

Concatenation Operator

& (e.g., A & B)

Concurrent and Sequential Statements

Concurrent Statements

Concurrent statements execute in parallel.

  • Signal Assignment:
signal_name <= expression;
  • Conditional Signal Assignment:
signal_name <= expression1 when condition else expression2;
  • Selected Signal Assignment:
with selector select
  signal_name <= expression1 when choice1,
                 expression2 when choice2,
                 ...
                 expressionN when others;
  • Process:
 
process (sensitivity_list)
begin
  -- Sequential statements
end process;

Sequential Statements

Sequential statements execute in order within a process.

  • if-then-else:
if condition then
  -- statements
elsif condition then
  -- statements
else
  -- statements
end if;
  • case:
case expression is
  when choice1 =>
    -- statements
  when choice2 =>
    -- statements
  when others =>
    -- statements
end case;
  • for loop:
for loop_variable in range loop
  -- statements
end loop;
  • while loop:
while condition loop
  -- statements
end loop;
  • variable assignment:
variable_name := expression;

Advanced Concepts

Components

Components are instances of entities used within an architecture.

  • Component Declaration:
component component_name is
  port (
    port_name : mode data_type;
    ...
  );
end component;
  • Component Instantiation:
instance_name : component_name
  port map (
    port_name => signal_name,
    ...
  );

Functions and Procedures

Function

A function returns a value and cannot have side effects.

function function_name (parameter_list) return return_type is
  -- Declarations
begin
  -- Statements
  return return_value;
end function;

Procedure

A procedure does not return a value directly and can have side effects.

procedure procedure_name (parameter_list) is
  -- Declarations
begin
  -- Statements
end procedure;

Generics

Generics provide a way to parameterize entities and components.

entity entity_name is
  generic (
    generic_name : data_type := default_value;
    ...
  );
  port (
    ...
  );
end entity;

Example:

entity Adder is
  generic (
    WIDTH : integer := 8
  );
  port (
    A : in  std_logic_vector(WIDTH-1 downto 0);
    B : in  std_logic_vector(WIDTH-1 downto 0);
    Y : out std_logic_vector(WIDTH-1 downto 0)
  );
end entity Adder;