Run-Length Bit Stuffer

Verilog problems / Sequential Design

What you must build

Flag when a stuff bit must be inserted after five consecutive 1s in a serial stream — the core rule behind CAN bus bit stuffing.

Engineers use “Run-Length Bit Stuffer” 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 medium kata: you will need sequential logic or a small FSM. Decide what is registered versus combinational before you type. Reset polarity and clock edge must match the spec; the judge will fail you on the first mismatched cycle.

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  bit_in,
  output reg stuff_needed
);

  // Your code here — count consecutive 1s; pulse stuff_needed and reset the count after the 5th.

endmodule

Why this shows up in interviews

Run-Length Bit Stuffer sits under Sequential Design (sequential, protocol). Concept: Protocols like CAN insert a complementary bit after every 5 identical bits in a row so receivers never lose bit-sync during long runs. Track a running count of consecutive 1s; the moment it would hit 5 ( run==4 and the current bit is also 1), raise stuff_needed for one cycle and reset the run — the inserted 0 breaks the streak, so counting must restart from zero rather than continuing.

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.

FAQ