-- file "distance.vhd"
---------------------------------------------------------------------
-- calculates the distance (in micrometres) of the wall given the
-- number of pulses from a 12.5 MHz clock
-- written by Shaun Luong, Clifton Yeung, Jon Paul Kansky
-- and Patrick Asiedu-Ampem, University of Alberta
-- November 1998
----------------------------------------------------------------------

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all; 
use ieee.std_logic_unsigned.all;

entity distance is
	generic
	(
		distance_width: positive:= 38;
		pulse_width: positive:= 19
	);
	port
	(
		clock, reset: in std_logic;
		pulse_count: in std_logic_vector(pulse_width-1 downto 0);
		distance: out std_logic_vector(distance_width-1 downto 0);
		done: out std_logic
	);
end distance;

architecture structural of distance is
	
	constant velocity: positive:= 7;	-- change this constant for different clock speeds
	constant max_dist: positive:= 3000000; -- max. distance = 3 m
		
begin
	
	process(reset, clock)

		variable duration: std_logic_vector(pulse_width-1 downto 0);
		variable dist_max: std_logic_vector(distance_width-1 downto 0);
		variable dist: std_logic_vector(distance_width-1 downto 0);
		
	begin

		-- convert the integer 'velocity' to a binary vector 'duration'
		duration := conv_std_logic_vector(velocity, pulse_width);
		-- convert the integer 'max_dist' to a binary vector 'dist_max'
		dist_max := conv_std_logic_vector(max_dist, distance_width);
		
		if reset = '1' then		-- active high
			done <= '0';
		elsif rising_edge(clock) then
			dist := pulse_count*duration;
			if dist > dist_max then		-- set limit on distance
				dist := dist_max;
			end if;
			done <= '1';				-- finished calculating
		end if;
		distance <= dist;

	end process;
	
end structural;