18Implementation

Add the yellow phase

Problem

Intended behavior. TrafficLight cycles red, green, yellow, and back to red. It moves one step on each clock edge where tick is high and holds its state otherwise. Reset starts it in red. The light output carries the state as a Light value, numbered in declaration order: red 0, green 1, yellow 2. stop is 1 in red and in yellow, and 0 only in green.

Current behavior. The starter declares only two states, so green returns straight to red, stop is tied to red alone, and Light is one bit wide.

Task. Add the yellow state and route the machine through it. Keep the TrafficLight name and its ports.

Read the starter

Start with object Light. Count its values and work out the width Chisel chooses for Light(). Then read the switch block: each is branch says where one state goes on a tick. Last, read how stop is computed from state.

Predict before editing

The public run prints the encoding line and then eight cycles with tick high on even cycles. For the finished design, predict:

the width on the ENCODING line, and which numbers the three names receive;
the state in every cycle from 0 to 7. Remember that a tick seen in one cycle shows up as the new state in the next;
the cycles in which stop is 0.

Run the starter first and note where its trace leaves green too early.

Make the change

Three places describe the yellow phase: the enum declaration, the transitions in switch, and the stop condition. Change each so that the code states the intended behavior directly. Compare states only with Light values, never with raw numbers.

Run and explain

Run the finished design and compare every line with your prediction. Explain why yellow first appears one line after the tick that ends green, and why the width on the ENCODING line changed without any edit to the port declarations.

Completion check

Light has three values numbered 0, 1, 2 in the order red, green, yellow, and is two bits wide.
Every state holds while tick is low, for any number of cycles.
Each tick advances exactly one step: red to green, green to yellow, yellow to red.
stop follows the state in every cycle, not only in the printed ones.
Reset returns the machine to red from any state.

Progressive hints

Hint 1

A new state is a new Value in the enum. Its number comes from its position in the declaration, so the order in which you list the names matters.

Hint 2

Green must no longer go to red. Yellow needs its own is branch; without one, no assignment happens in yellow and the register would hold there forever.

Hint 3

isOneOf tests a value against several enum values at once and returns a Bool.

Concept

A ChiselEnum gives each state a name, a number, and a type. Adding a value can change the width of every port and register of that type, which Chisel recalculates for you. Because comparisons accept only values of the same enum, the transitions and stop keep meaning the same thing even when the numbering changes.

Extension experiment

Imagine a fourth state, flashing yellow, declared last. Predict the width of Light and whether the numbers of the existing three states change. Then imagine declaring it first instead, and say which printed numbers would move.