SPI Full-Duplex Shift Register

Verilog problems / Sequential Design

What you must build

Build the core shift register behind SPI: transmit a byte MSB-first while simultaneously receiving one, both directions moving on the same clock edge.

Engineers use “SPI Full-Duplex Shift Register” 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  load,
  input  mosi,
  input  [7:0] tx_data,
  output miso,
  output reg [7:0] shreg
);

  // Your code here — on load, shreg<=tx_data; otherwise shift left, shreg<={shreg[6:0],mosi}. miso=shreg[7].

Why this shows up in interviews

SPI Full-Duplex Shift Register sits under Sequential Design (sequential, protocol). Concept: SPI's shift register does two things on every clock edge at once: the top bit shifts out on miso , and a new bit shifts in from mosi at the bottom — shreg <= {shreg[6:0], mosi} , with miso reading shreg[7] combinationally. A load pulse parallel-loads a fresh byte to transmit. Shifting the wrong direction ( {mosi, shreg[7:1]} ) swaps which end transmits first and which end receives — a mirror-image bug that still compiles and still shifts, just not the way SPI actually works.

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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