960 IEEE Open-Silicon 2026: Adaptive Traffic Light Controller with Emergency Override :: Quicker, easier and cheaper to make your own chip!

Adaptive Traffic Light Controller with Emergency Override

How it works

The design implements a 7-state Moore FSM controlling a two-road intersection (N/S and E/W). Each road has three possible signal states — RED, YELLOW, GREEN — encoded as a 2-bit value on the output pins.

Normal operation

NS_GREEN ──(timer)──► NS_YELLOW ──(timer)──► EW_GREEN ──(timer)──► EW_YELLOW ──(timer)──┐
    ▲                                                                                      │
    └──────────────────────────────────────────────────────────────────────────────────────┘

Each phase runs for a configurable number of clock cycles (default: green=30, yellow=5). The yellow phase acts as a safety guard — it is never skipped even under emergency preemption, unless the design is in a yellow phase when the emergency fires (in which case the remaining yellow cycles drain before EMRG_HOLD is entered).

Adaptive green extension

If a vehicle sensor (sensor_ns or sensor_ew) remains asserted when fewer than MIN_REMAINING cycles are left in the active green phase, and no emergency is pending, the timer is extended by EXTEND_STEP cycles. This repeats up to a hard ceiling of MAX_GREEN cycles, preventing indefinite green monopolisation.

Emergency override

When emrg_ns or emrg_ew is asserted, the pending flag is latched. At the end of the current phase the FSM transitions:

any state ──(emrg_pending)──► EMRG_HOLD (all RED, 3 cycles)
                                    │
                      ┌─────────────┴─────────────┐
                      ▼                           ▼
                 EMRG_NS (N/S green)        EMRG_EW (E/W green)
                      │                           │
                      └──────────► NS_GREEN / EW_GREEN (resume normal)

The EMRG_HOLD state guarantees an all-red gap before the emergency vehicle receives its green, preventing conflicting movements from vehicles already in the intersection. If both axes assert simultaneously, N/S receives priority; E/W is served on the next EMRG_HOLD cycle.

How to test

With the TT demo board

`ui_in` bit	Switch	Action
0	SW0	Toggle N/S vehicle sensor
1	SW1	Toggle E/W vehicle sensor
2	SW2	Assert N/S emergency
3	SW3	Assert E/W emergency

Reading the outputs:

`uo_out` bits	Meaning
[1:0] = 01	N/S GREEN
[1:0] = 10	N/S YELLOW
[1:0] = 00	N/S RED
[3:2] = 01	E/W GREEN
[3:2] = 10	E/W YELLOW
[3:2] = 00	E/W RED
[4] = 1	Emergency active
[7:5]	FSM state (0–6)

LED mapping suggestion: Wire uo_out[1:0] to a green/yellow LED pair for N/S and uo_out[3:2] to a green/yellow LED pair for E/W. Red is implied when both LEDs are off.

Timing at 10 kHz clock (default demo board speed)

Parameter	Cycles	Real time
GREEN_TIME	30	3 ms
YELLOW_TIME	5	0.5 ms
EMRG_TIME	20	2 ms

For realistic intersection timing at 10 kHz, set GREEN_TIME = 300_000 (30 s) in the top-level parameters and resynthesize.

Automated tests

Run with cocotb:

cd test
make

Six test cases are included covering: reset state, normal rotation, emergency preemption, simultaneous dual-axis emergency, adaptive extension, and all-red guarantee during EMRG_HOLD.

Safety properties

This design was written with verifiable safety invariants in mind:

Mutual exclusion: N/S and E/W can never simultaneously be GREEN (enforced by Moore output table — only one axis is ever assigned SIG_GREEN per state).
All-red gap: EMRG_HOLD guarantees at least HOLD_TIME cycles of all-red before any emergency green, preventing intersection clearing violations.
No starvation: The adaptive extension has a hard ceiling (MAX_GREEN), and emergency requests are latched and always eventually serviced.
Reset safety: On rst_n assertion, all outputs default to RED via the sequential reset path.

These invariants can be formally verified with tools such as SymbiYosys / Yosys smtbmc.

#	Input	Output
0	sensor_ns: vehicle sensor on N/S road (active high)	sig_ns[0]: N/S GREEN (high = green lit)
1	sensor_ew: vehicle sensor on E/W road (active high)	sig_ns[1]: N/S YELLOW (high = yellow lit; both low = red)
2	emrg_ns: emergency vehicle on N/S axis (active high)	sig_ew[0]: E/W GREEN
3	emrg_ew: emergency vehicle on E/W axis (active high)	sig_ew[1]: E/W YELLOW
4		emrg_active: high during any emergency FSM state
5		state[0]: FSM state bit 0 (for 7-segment debug)
6		state[1]: FSM state bit 1
7		state[2]: FSM state bit 2

960 IEEE Open-Silicon 2026: Adaptive Traffic Light Controller with Emergency Override