Recursion in VHDL

Our example demonstrates recursive functions in VHDL. Here’s a classic example of factorial calculation.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity Recursion is
    Port ( n : in INTEGER;
           result : out INTEGER);
end Recursion;

architecture Behavioral of Recursion is
    function fact(n : INTEGER) return INTEGER is
    begin
        if n = 0 then
            return 1;
        else
            return n * fact(n - 1);
        end if;
    end function;

    function fib(n : INTEGER) return INTEGER is
    begin
        if n < 2 then
            return n;
        else
            return fib(n - 1) + fib(n - 2);
        end if;
    end function;

begin
    process(n)
    begin
        result <= fact(n);
        report "Factorial of " & integer'image(n) & " is " & integer'image(fact(n));
        report "Fibonacci of " & integer'image(n) & " is " & integer'image(fib(n));
    end process;
end Behavioral;

This fact function calls itself until it reaches the base case of fact(0).

In VHDL, we define recursive functions within the architecture. The fact function calculates the factorial of a given number.

We also define a fib function to calculate Fibonacci numbers recursively. In VHDL, we don’t have closures, so we define it as a regular function.

The main process in the architecture calculates both the factorial and Fibonacci number for the input n and reports the results.

To simulate this design, you would need to create a testbench that instantiates the Recursion entity and provides different values for n. The results would be visible in the simulation output.

Note that while recursion is possible in VHDL, it’s generally not recommended for hardware design due to its potential for creating complex and inefficient circuits. Iterative solutions are usually preferred in hardware implementations.