VHDL Syntax for Port, Mode, and Type
Signal Concurrency

Objectives

We have examined some simple VHDL entities and design entry procedures. Now let's take a closer look at some of the rules and options available to us when we create a design in VHDL.

1. VHDL Valid Names

A valid name for a port, signal, variable, entity name, architecture body, or similar object consists of a letter followed by any number of letters or numbers, without space. A valid name is also called a named identifier. VHDL is not case sensitive. However, an underscore may be used within a name, but may not begin or end the name. Two consecutive underscores are not permitted. A hyphenated name does not work.

Here are some examples of valid and invalid names: 

	Valid names:	

	decode8 	just_in_time 	array_4

	Invalid names:

	8decide		(begins with a digit)

	my design	(space inside a name)

	signal		(reserved word)

	your_words?	(special character ? not allowed)

	_what_4		(begins with underscore)

	in__time	(two consecutive underscores)
Here is a list of some reserved words for your reference: 

	abs	access	after 	alias		all	
	and	array	assert	attribute	architecture

	begin	block	body	buffer		bus

	case	constant	component	configuration

	downto	disconnect		

	else	elsif	end	entity		exit

	file	for		function

	group	generic		generate	guarded

	if	impure	in	inertial	inout	is

	label	loop	literal	linkage		library

	mod	map

	nand	new	next	nor	not	null

	of 	on	open	or	others	out

	port	pure	process	package	postponed	procedure

	range	record	rem	ror	rol	return	reject
	report	register

	select	sla	sll	sra	srl	signal	subtype
	severity	shared

	then	to	type	transport

	use	until	units	unaffected	variable

	wait	when	while	with	xnor	xor

2. Assignment and Comments

The operator <= is called an "assignment operator", which is a compound symbol, consisting of the "less than" and the "equal" sign. It indicates that whatever is on the right side of the statement is assigned to the left side of the statement. 
	Example:	y <= a and (not b);

Two consecutive hyphens are used to lead out the inline documentation/comment. A comment is explanatory text that is ignored by VHDL compiler. 

	Example:	-- This is a comment in VHDL.

3. Modes

The mode of a port defines its direction of data flow. Generally, the following four modes are used: IN, OUT, INOUT, and BUFFER. A port of mode IN only allows data to flow from an input pin to the CPLD logic. CPLD stands for Complex Programmable Logic Device in this context. A port of mode OUT allows data to flow from the CPLD logic to the output port. A port of mode INOUT is bidirectional; it will allow data to flow in both directions.
BUFFER is a special case of OUT, which allows the port value to be "updated" or "modified" by the CPLD logic and to be fed back into the CPLD logic to be used by another function.

4. Type, Array, and Range

A type in VHDL is a property applied to port, signal, or variable that determines what values that object can have.
Some of the most common types we will use in VHDL are BIT, STD_LOGIC, and INTEGER.

BIT and BIT_VECTOR


	The BIT type is native to VHDL and defined in the standard library of VHDL.
	BIT can have only two values:'0' and '1'.  The values are placed in 
	single quotes because VHDL treats them like ASCII characters.
If we want to assign a multiple-bit number to a series of related input and output ports, we can use the type BIT_VECTOR.
A BIT_VECTOR is defined as a one-dimensional array of elements, each of type BIT.
The range of the array is indicated by listing its upper and lower bounds. 

	For Example, define a 4-bit vector d:

	d:	IN BIT_VECTOR (3 downto 0);
Each of the elements of this vector can be individually addressed by indicating its position by a number in parentheses. 

	For the definition with range indication (3 downto 0), elements are:
	
	d(3), d(2), d(1), d(0);  d(3) holds the most significant bit.
	
	For the definition with range indication (0 to 3), elements are:
	
	d(0), d(1), d(2), d(3);  d(0) holds the most significant bit.

	VHDL always makes vector assignments from left to right.
	The contents of those vectors can be chosen by selecting 
	how the elements are numbered.
Constant values assigned to BIT_VECTORs are written in double quotes, because the VHDL compiler treats them
like strings, or array of characters. Look at the following example: 
	
	If a port d is defined by 

		d:      IN BIT_VECTOR (3 downto 0);

	then the following groups of the statements are equivalent.

		d(3) <= '0';		d <= "0101";
		d(2) <= '1';
		d(1) <= '0';
		d(0) <= '1';

	If a port d is defined by 

		d:      IN BIT_VECTOR (0 to 3);

	then the following groups of the statements are equivalent.

		d(0) <= '1';
		d(1) <= '0';
		d(2) <= '1';
		d(3) <= '0';		d <= "1010";

STD_LOGIC and STD_LOGIC_VECTOR

The STD_LOGIC (standard logic) type, also called IEEE Std.1164 Multi-Valued Logic, has been defined to give a broader range of output values than just '0' and '1'. Any port, signal, or variable of type STD_LOGIC or STD_LOGIC_VECTOR can have any of the following values: 
 
	'U'	-- Uninitialized
	'X'	-- Forcing Unknown
	'0'	-- Forcing 0
	'1'	-- Forcing 1
	'Z'	-- High Impedance
	'W'	-- Weak Unknown
	'L'	-- Weak 0
	'H'	-- Weak 1
	'-'	-- Don't care
To use STD_LOGIC in a VHDL file, you must include the following reference to the VHDL library called IEEE and the std_logic_1164 package before the entity declaration: 
library IEEE;
use IEEE.std_logic_1164.all;
The std_logic_1164 package contains the type definitions of the STD_LOGIC types.

Example:

Consider a 4-bit AND array, that is, an array of four 2-bit AND gates.
Now let's see how to use VHDL file to describe it. 

library IEEE;
use IEEE.std_logic_1164.all;


entity Four_and_array is 

 port(
		a0	: in	std_logic;
		a1	: in	std_logic;
		a2	: in	std_logic;
		a3	: in	std_logic;
		b0	: in	std_logic;
		b1	: in	std_logic;
		b2	: in	std_logic;
		b3	: in	std_logic;
		c0	: out	std_logic;
		c1	: out	std_logic;
		c2	: out	std_logic;
		c3	: out	std_logic
	);

end Four_and_array;


architecture arch1 of Four_and_array is

begin

  -- Your VHDL code defining the model goes here
  c0 <= a0 and b0;
  c1 <= a1 and b1;
  c2 <= a2 and b2;
  c3 <= a3 and b3;
end arch1;
Now rewrite the VHDL file for the 4-bit AND array using STD_LOGIC_VECTOR types. 

library IEEE;
use IEEE.std_logic_1164.all;


entity Four_and_vec is 

 port(
		a	: in	std_logic_vector(0 to 3);
		b	: in	std_logic_vector(0 to 3);
		c	: out	std_logic_vector(0 to 3)
	);

end Four_and_vec;


architecture arch1 of Four_and_vec is

begin

  -- Your VHDL code defining the model goes here
	c <= a and b;
end arch1;

5. Signal Concurrency

Within VHDL, signals are assigned values by using signal assignment statements. These statements specify a new value of a signal and the time at which the signal is to acquire this value. Multiple signal assignment statements are executed concurrently in simulated time and are referred to as concurrent signal assignment statements (CSAs).

The statements for the Boolean expressions are examples of concurrent signal assignment statements. The idea behind signal concurrency is that all concurrent statements are evaluated at the same time. (Concurrent means "simultaneous".)

In the other programming languages such as BASIC, c, or C++, the statements are executed in sequential order. However, for a hardware description language, signal concurrency does make sense since its purpose is to synthesize hardware rather than run a sequence of program statements.

Let's use half adder as an example,
Two signal bits a and b are added to produce the sum s and carry c. The circuit is defined by the two equations: s = a xor b and c = a and b. Suppose both inputs are 1, then carry is 1 and the sum bit is 0. Further suppose that input b is changed to a 0. Now the carry is 0 and the sum is 1.

Which changed first, sum or carry? Since both outputs depend on the same two inputs, both outputs ideally change at the same time. That is how the hardware behaves. In VHDL, we would write the following two statements: 

	s <= a xor b;
	c <= a and b;

6. Lab Assignments

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