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Q079FreeSystemVerilog

Build a reloadable countdown timer

Difficulty
Medium
Topic
RTL Design
Language
SV
Interview prompt

Question

Design a parameterized countdown timer with load, load_value, busy, and a one-cycle expired pulse. Add a BFM task that starts a nonzero delay and waits for expiry. W is a positive integer. The environment resets the timer, initializes load=0, and serializes calls to the BFM task.

Candidate starting point

Implementation scaffold

module countdown_timer #(
  parameter int unsigned W = 16
) (
  input  logic         clk,
  input  logic         rst_n,
  input  logic         load,
  input  logic [W-1:0] load_value,
  output logic         busy,
  output logic         expired
);
  logic [W-1:0] count_q;

  always_ff @(posedge clk or negedge rst_n) begin : timer_state
    // TODO: Implement timer_state using the supplied state and interface.
  end
endmodule

interface timer_if #(parameter int unsigned W = 16) (input logic clk);
  logic load, busy, expired;
  logic [W-1:0] load_value;
  clocking cb @(posedge clk);
    default input #1step output #0;
    output load, load_value;
    input  busy, expired;
  endclocking
  modport TB (clocking cb);
endinterface

class timer_bfm #(int unsigned W = 16);
  virtual timer_if #(W).TB vif;
  function new(virtual timer_if #(W).TB vif); this.vif = vif; endfunction

  task start_and_wait(input logic [W-1:0] value);
    // TODO: implement this body.
  endtask
endclass
Reviewed example

Trace one case

Input
load value 3, observe three countdown edges; later load value 0
Expected output
busy during the first two countdown edges and expired pulses on the third; zero load pulses expired immediately and remains idle

A nonzero N expires after exactly N active edges, while load priority and the zero special case each produce one-cycle pulses.

What to cover

Requirements

  1. Loading has priority over counting.
  2. A value N expires after N active countdown edges.
  3. expired is a pulse, not a permanent idle indicator.
  4. A zero load expires immediately and leaves the timer idle.
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