Relational operators in VHDL

Introduction

Relational operators help you compare two values to see how they relate, like if one is bigger or equal to the other.

Checking if a signal value is greater than a threshold to trigger an action.

Comparing two inputs to decide which one is larger.

Testing if two values are equal to enable a specific process.

Determining if a counter has reached a limit to reset it.

Verifying if a value is less than another to control flow.

Syntax

VHDL

signal_a < signal_b
signal_a <= signal_b
signal_a > signal_b
signal_a >= signal_b
signal_a <= signal_b
signal_a /= signal_b

Relational operators return a boolean result: TRUE or FALSE.

They can be used with numeric types and some other compatible types in VHDL.

Examples

Checks if input_signal is greater than 10.

VHDL

if input_signal > 10 then
  -- do something
end if;

Checks if count is less than or equal to max_count.

VHDL

if count <= max_count then
  -- continue counting
end if;

Checks if data_a is not equal to data_b.

VHDL

if data_a /= data_b then
  -- values are not equal
end if;

Sample Program

This program compares two integer inputs a and b. If a is greater than b, it sets result to '1'. Otherwise, it sets result to '0'.

VHDL

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity RelationalExample is
  port(
    a : in integer;
    b : in integer;
    result : out std_logic
  );
end RelationalExample;

architecture Behavioral of RelationalExample is
begin
  process(a, b)
  begin
    if a > b then
      result <= '1';  -- a is greater
    else
      result <= '0';  -- a is not greater
    end if;
  end process;
end Behavioral;

Important Notes

Relational operators work with many data types, but make sure the types are compatible.

Use parentheses to group comparisons if you combine multiple conditions.

Remember that VHDL is case-insensitive, but consistent style helps readability.

Summary

Relational operators compare two values and return TRUE or FALSE.

They include <, <=, >, >=, =, and /=.

Use them to control decisions and flow in your VHDL designs.