One-Hot Traffic-Light FSM
Implement a three-state one-hot finite-state machine.
Task
Cycle RED -> GREEN -> YELLOW -> RED whenever advance is high, otherwise hold the current state.
Examples
- reset -> RED
- RED + advance -> GREEN
- GREEN + advance -> YELLOW
Constraints
- State output is 3-bit one-hot: RED=001, GREEN=010, YELLOW=100.
- Synchronous active-high reset.
What this problem is testing
A strong answer translates the requirement into synthesizable state and combinational logic, states reset and update semantics precisely, and handles boundary conditions rather than only the nominal case.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Check reset or initialization before normal operation.
- Exercise the nominal sequence cycle by cycle.
- Test boundaries such as zero, maximum count, wraparound, full/empty, or simultaneous controls when they apply.
- Prove the design holds state when an enable or qualifying condition is absent.
Common mistakes
- Incomplete combinational assignments that infer a latch.
- Blocking/non-blocking assignments that do not match combinational versus sequential intent.
- Parameter-width and boundary errors.
- Passing the happy path while missing reset, overflow, simultaneous operations, or illegal requests.
Interview follow-up questions
- How would you parameterize this design without introducing width or boundary bugs?
- What changes if reset timing or polarity changes?
Concepts to review
fsm, one-hot, sequential
Open One-Hot Traffic-Light FSM in the HDLForge interactive editor.
Related problems
- 2:1 Multiplexer — Easy
- Parameterized Counter — Easy
- Priority Encoder — Medium
- Synchronous FIFO — Hard
- Shift Register — Easy
Explore the RTL Design Practice & Interview Preparation hub.