2-Floor Elevator Controller

Verilog problems / Sequential Design

What you must build

A simplified elevator FSM: sit idle until called, take one cycle to transit between floors, and only move when actually requested.

Engineers use “2-Floor Elevator Controller” as a building block in sequential design. Interviewers ask for the same ports and the same corner cases this judge covers. Completing it in the browser is the same skill as writing synthesizable RTL at work, minus the EDA license.

Port contract

The judge instantiates exactly these ports. Extra ports or a different module name fail to elaborate.

How to approach this kata

This is a hard kata. Sketch the state bits and the illegal overlaps (full/empty, wrap, simultaneous enable) on paper first. A design that “usually works” in your head will fail a directed corner in the hidden tests.

Hidden tests instantiate top_module, drive the ports, and compare every sample against a golden model. They do not grade coding style. They do grade X/Z, off-by-one counters, and ignoring enables. Sign in only when you want the run saved on the leaderboard — the specification below is public.

Starter shape

Copy this skeleton into the editor (or press Reset starter). Fill the body; do not rename the module.

module top_module(
  input  clk,
  input  rst,
  input  call1,
  input  call2,
  output at_floor2,
  output moving
);

  // Your code here — 4-state FSM: AT1, MOVING_UP, AT2, MOVING_DOWN.

endmodule

Why this shows up in interviews

2-Floor Elevator Controller sits under Sequential Design (sequential, fsm). Concept: A 4-state FSM — idle at floor 1, moving up, idle at floor 2, moving down. Each idle state only leaves when its matching call button is pressed; each moving state unconditionally arrives at the destination on the next cycle. The easy bug to make is auto-cycling through all four states regardless of whether anyone actually pressed a button.

A passing solution is synthesizable intent: no delays in the DUT, no initial blocks inside top_module, and no reference to testbench tasks. Use blocking assignments only in combinational always blocks; use non-blocking for registers clocked by clk.

Related problems

• D Flip-Flop with Asynchronous Reset — Standard D flip-flop with an active-low asynchronous reset. Reset clears q immediately, without waiting for a clock edge.
• 4-Bit Shift Register (SIPO) — Serial-in, parallel-out shift register. Each clock, shift left and load sin into the LSB. Sync active-high reset clears q.
• JK Flip-Flop — The flip-flop with no forbidden state: j=k=1 toggles instead of racing. Classic building block for counters.
• 4-Bit Up/Down Counter — A synchronous counter that increments or decrements each clock edge depending on a direction input, with a synchronous reset.

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