Traffic Light Controller

Traffic Light Controller for a junction of three roads.

Assumptions for Traffic Light Controller:

  1. T intersection as shown in Figure below.
  2. Default to green on main road.
  3. Sensor enables green for cross street.
  4. Delay switching for right- turn- on- red from cross street.
  5. Programmable delays.

State machine level :

  1. Stay in thru G state until sensor is activated.
  2. Wait in pause state to see if sensor deactivates. (right- turn- on- red).
  3. Then proceed through sequence, waiting in each state for specified time delay.
  4. In each state, provide appropriate control signals for lights.
  5. 5 states, so at least 3 flip-flop.
Figure (b) : State diagram for three roads TLC Detailed state diagram and state table :
Table P. 4.11.1 : State table Current state S2S1S0 Inputs sd1d2d3d4 Outputs TEN tGtytrxGxyxr Next state ns2 ns1 ns0 0 0 0 X X X X 0 1 0 0 0 0 1 0 0 0 0 0 0 1 X X X X 0 1 0 0 0 0 1 0 0 1 0 0 1 0 X X X X 0 1 0 0 0 0 1 0 0 0 0 0 1 1 0 X X X 1 1 0 0 0 0 1 0 0 1 0 0 1 1 1 X X X 0 1 0 0 0 0 1 0 1 0 0 1 0 X X 0 X X 1 0 1 0 0 0 1 0 1 0 0 1 0 X X 1 X X 1 0 0 1 1 0 0 0 1 1 0 1 1 X X X 0 X 0 0 0 1 1 0 0 0 1 1 0 1 1 X X X 1 X 0 0 0 1 1 0 0 1 0 0 1 0 0 X X X X 0 1 0 0 1 0 1 0 1 0 0 1 0 0 X X X X 1 0 0 0 1 0 1 0 0 0 0 VHDL code for Traffic Light Controller :


library ieee; 
	use ieee.std_logic_1164.all;
	use ieee.std_logic_unsigned.all;
	entity trafficv is
	port (reset, sensor, clk: in std_logic;
	d1, d2, d3, d4: in std_ logic_vector( 3 downto 0);
	tg, ty, tr, xg, xy, xr: out std_logic);
	end trafficv;
	architecture trafficv_arch of trafficv is
	type state_type is (thrug, pause, thruy, thrur, crossy);
	signal state: state_type;
	signal timer: std_logic_vector( 3 downto 0);
	signal ten: std_logic;
	begin
	timer_process: process
	begin
	if ten = '0' then 
	timer <= "0000";
	else 
	timer <= timer + "0001";
	end if;
	wait until clk = '1';
	end process;
	state_machine_process: process
	begin
	if reset = '1' then 
	state <= thrug; ten <= '0';
	else
	if state = thrug and sensor = '0' then
	state <= thrug; ten <= '0';
	elsif state = thrug and sensor = '1' then
	state <= pause; ten <= '0';
	elsif state = pause and sensor = '0' then
	state <= thrug; ten <= '0';
	elsif state = pause and sensor = '1' and timer /= d1 then
	state <= pause; ten <= '1';
	elsif state = pause and sensor = '1' and timer = d1 then
	state <= thruy; ten <= '0';
	elsif state = thruy and timer /= d2 then
	state <= thruy; ten <= '1';
	elsif state = thruy and timer = d2 then
	state <= thrur; ten <= '0';
	elsif state = thrur and timer /= d3 then
	state <= thrur; ten <= '1';
	elsif state = thrur and timer = d3 then
	state <= crossy; ten <= '0';
	elsif state = crossy and timer /= d4 then
	state <= crossy; ten <= '1';
	elsif state = crossy and timer = d4 then
	state <= thrug; ten <= '0';
	end if;
	end if;
	wait until clk = '1';
	end process;
	tg <= '1' when (state = thrug) or (state = pause) else '0';
	ty <= '1' when (state = thruy) else '0';
	tr <= '1' when (state = thrur) else '0';
	xg <= '1' when (state = thrur) else '0';
	xy <= '1' when (state = crossy) else '0';
	xr <= '1' when (state = thrug) or (state = pause) or (state = thruy) else '0';
	end trafficv_ arch;

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