Traffic Light Controller with Pedestrian Request

Medium · Counters & Timers · Finite State Machines

Design the controller for a single traffic light. The light cycles RED → GREEN → YELLOW → RED → ..., each color lasting a fixed number of clock cycles. A pedestrian push-button can cut the GREEN phase short, but never below a minimum green time.

Parameters

Parameter	Default	Meaning
G_CYCLES	5	Normal length of GREEN, in clock cycles.
Y_CYCLES	2	Length of YELLOW.
R_CYCLES	4	Length of RED.
MIN_GREEN	2	Minimum length of GREEN when a pedestrian request shortens it.

You may assume 1 <= MIN_GREEN <= G_CYCLES and all lengths are between 1 and 255. Your design must work for any such values: the hidden tests also instantiate it with other legal values, including the smallest and the largest. The examples below use the defaults.

Ports

Port	Dir	Width	Meaning
clk	input	1	Clock.
rst	input	1	Synchronous, active-high reset (takes effect only at a rising edge of clk).
ped_req	input	1	Pedestrian request (level), sampled at each rising edge.
red, yellow, green	output	1 each	The lamps. Exactly one is 1 in every cycle (one-hot).

The outputs depend only on the controller's state registers (Moore outputs, no combinational path from ped_req): they change only right after a rising edge of clk.

Behavior

Define the age of the current phase as the number of clock cycles the current color has been shown, including the current cycle (the first cycle of a phase has age 1).

At each rising edge with rst = 1: the light becomes RED with age 1 (the cycle right after the reset edge is the first RED cycle). While rst is held, the light stays RED with age 1.

At each rising edge with rst = 0, given the current color and age k:

Color	Moves to the next color at this edge when...	Next color
RED	k == R_CYCLES	GREEN
GREEN	k == G_CYCLES, or ped_req == 1 && k >= MIN_GREEN	YELLOW
YELLOW	k == Y_CYCLES	RED

Otherwise the color stays and the age increases by one. So without pedestrian requests, RED lasts exactly R_CYCLES cycles, GREEN G_CYCLES, YELLOW Y_CYCLES.

Pedestrian request details:

• ped_req only matters at an edge where the light is GREEN. It is ignored during RED and

YELLOW.

• It is not remembered: a request that is sampled while GREEN is younger than

MIN_GREEN has no effect (unless ped_req is still 1 at a later edge).

• A shortened GREEN is followed by the normal YELLOW (the light never skips YELLOW).

Example (default parameters)

Cycle 0 is the cycle in which rst = 1; the reset edge closes it. Cycle n is the n-th cycle after the reset edge.

Scenario	RED	GREEN	YELLOW	RED	GREEN
No requests	1–4	5–9	10–11	12–15	16–
ped_req = 1 during cycle 6 only (GREEN age 2 = MIN_GREEN)	1–4	5–6	7–8	9–12	13–
ped_req = 1 during cycle 5 only (GREEN age 1 < MIN_GREEN): ignored	1–4	5–9	10–11	12–15	16–
ped_req = 1 held from cycle 1 onwards	1–4	5–6	7–8	9–12	13–14 ...

Sign up free to solve Traffic Light Controller with Pedestrian Request in your browser.

More Counters & Timers problems

• Switch Debouncer
• Decade (Mod-10) Counter
• UART Receiver (8N1) with Glitch Filter and Framing Error
• UART Transmitter (8N1)
• 4-bit Up Counter