Actuated Traffic Light Controller (Verilog)

FSM-based controller for a 4-way intersection: NS is the main road, EW is the side street. It reacts to vehicle sensors and pedestrian buttons instead of running a fixed timer loop.

What it does

• NS rests in green when nobody is waiting on EW and nobody pressed a ped button.
• Min / max green: green is held for at least T_GREEN_MIN cycles. After that it gaps out (drops to yellow) as soon as the sensor on that road goes quiet, or is cut off at T_GREEN_MAX if traffic never lets up.
• Calls are latched, so a 1-cycle pulse on a car sensor or ped button is not lost. A latch clears when its phase is served.
• Phases get skipped when there is nothing to serve: no ped call means no walk phase, no EW demand means no EW green/yellow.
• Sync active-low reset (rst_n) puts it back in NS green and clears the latches.

Inputs are expected to be synchronised and debounced outside this block.

States

S_NS_G -> S_NS_Y -> S_EW_W -> S_EW_G -> S_EW_Y -> S_NS_W -> S_NS_G

State	NS	EW	Walk	Leaves when
S_NS_G	G	R	-	after min green, if someone is waiting on EW or a ped called, and NS gapped out or hit max
S_NS_Y	Y	R	-	T_YELLOW; next is S_EW_W if EW ped, else S_EW_G if EW car, else S_NS_W if NS ped, else back to S_NS_G
S_EW_W	R	R	EW	T_RED; next is S_EW_G if EW car, else S_NS_W if NS ped, else S_NS_G
S_EW_G	R	G	-	after min green, EW gapped out or hit max
S_EW_Y	R	Y	-	T_YELLOW; next is S_NS_W if NS ped, else S_NS_G
S_NS_W	R	R	NS	T_RED, always back to S_NS_G

Lights are 3-bit {R,Y,G}: 001 green, 010 yellow, 100 red.

Parameters

Name	Default	Notes
BITS	16	timer width
T_GREEN_MIN	6	must be >= 1
T_GREEN_MAX	T_GREEN (15)	must be >= T_GREEN_MIN
T_GREEN	15	old name, just feeds T_GREEN_MAX
T_YELLOW	4	must be >= 1
T_RED	6	ped walk time, must be >= 1

Bad values are caught at time 0 in simulation and the run stops.

Ports (top)

Port	Dir	Description
clk, rst_n	in	clock, sync active-low reset
car_sensor_ns, car_sensor_ew	in	vehicle presence
ped_call_ns, ped_call_ew	in	ped buttons
ns_lights[2:0], ew_lights[2:0]	out	{R,Y,G}
walk_ns, walk_ew	out	walk lamps

Layout

rtl/Trafficlightcontroller.v   counter, fsm, top
tb/testbench.v                 self-checking tb

Simulating

iverilog -Wall -g2005-sv -o sim.vvp rtl/Trafficlightcontroller.v tb/testbench.v
vvp sim.vvp
gtkwave simulation.vcd     # optional

Expected tail of the output: 275 checks, 0 errors then PASSED.

Testbench

Always-on checks (every clock after reset): never both greens, no non-red on both roads, all cars red during any walk, never both walks, lights always one-hot.

Directed scenarios:

#	What
1	NS rests in green with no demand
2	NS gap-out
3	EW walk skipped without a ped call
4	EW gap-out at min green
5	NS walk skipped without a ped call
6	dense traffic, both greens run to max
7	EW ped call (1-cycle pulse) served
8	NS ped call (1-cycle pulse) served
9	reset mid-cycle
10	EW car pulse latched
11	both ped buttons at once
12	NS car pulse latched during EW green
13	NS ped only: EW phases skipped
14	EW ped only: EW green/yellow skipped
15	stuck ped button doesn't lock out NS
16	T_GREEN override reaches T_GREEN_MAX