VHDL Greater Than and Less Than?

Question

So in VHDL, how do you do either a greater than or a less than check?

Answer 1

VHDL Greater Than and Less Than

Quick Syntax

Greater Than Symbol: >

if input1 > input2 than
  output <= '1';
else
  output <= '0';
end if;

Greater Than or Equal To Symbol: >=

if input1 >= input2 than
  output <= '1';
else
  output <= '0';
end if;

Less Than Symbol: <

if input1 < input2 than
  output <= '1';
else
  output <= '0';
end if;

Less Than or Equal To Symbol: <=

if input1 <= input2 than
  output <= '1';
else
  output <= '0';
end if;

Purpose

Comparison operators like greater than and less than are commonly used in VHDL. The syntax is very basic and pretty easy to get the hang of, simply check out the examples above.

Keep in mind that you have to include a library, where the ieee.numeric_std is recommended on new designs. In order to use these operators, the items that you are comparing must be signed, unsigned, or integer. You can't compare a raw std_logic_vector without casting it as signed or unsigned first.

Here's how you would compare two std_logic_vectors:

if unsigned(input1) <= unsigned(input2) than
  output <= '1';
else
  output <= '0';
end if;

Answer 2

Example :

signal x : unsigned ( 5 downto 0 ) ;
signal y : unsigned ( 5 downto 0 ) ;
signal output : std_logic ;

greater_less : process ( x , y ) is
  begin
  if x > y then
    output <= '1' ;
  elsif x < y then
    output <= '0' ;
  else
    output <= 'Z' ;
  end if ;
end process greater_less ;

Answer 3

Greater than and less than are relational operators. In VHDL we have the option of using the following relational operators: '=' Equal, '/=' Not Equal, '< ' Less Than, '<=' Less Than or Equal To, '>' Greater Than, '>=' Greater Than or Equal To.

Like in all other programming languages, relational operators test the relation between two numbers. They can be used as a condition for 'if', 'when' and 'until' statements. An important thing to remember while using relational operators is that the two numbers being compared must be of the same 'type'. An integer cannot be compared to a std_logic_vector type value.

For example, consider signals named count1 and pulse1 used in the following code snippet:

if count1< 8 then
  pulse1 <= '0';
else
  pulse1 <=1;
end if;

In this code section, if the value of count1 is less than 8, then pulse1 is assigned a value of 0, otherwise, pulse1 is assigned a value of 1. This is an example of using the relational operator 'less than'. The value stored in 'count1' is compared with 8 and then the value of pulse1 is assigned as per the result.