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Q039FreeSystemVerilog

Find the unpaired fault ID

Difficulty
Easy
Topic
Streaming RTL
Language
SV
Interview prompt

Question

Consume a framed stream in which every fault ID appears exactly twice except one ID that appears once, then return the unpaired ID. Assume ID_W >= 1 and that the producer satisfies this frame-content promise; the unit need not validate multiplicities.

Candidate starting point

Implementation scaffold

module unpaired_fault_finder #(
  parameter int ID_W = 12
) (
  input  logic            clk,
  input  logic            rst_n,
  input  logic            fault_valid,
  input  logic            fault_last,
  input  logic [ID_W-1:0] fault_id,
  output logic            fault_ready,
  output logic            result_valid,
  input  logic            result_ready,
  output logic [ID_W-1:0] result_id
);
  logic [ID_W-1:0] xor_accum;

  assign fault_ready = !result_valid;

  always_ff @(posedge clk) begin : accumulate_and_publish
    // TODO: Implement accumulate_and_publish using the supplied state and interface.
  end
endmodule
Reviewed example

Trace one case

Input
accepted fault IDs=[5,2,5,2,7], last asserted with 7
Expected output
result_id=7

The two copies of 5 cancel, and the two copies of 2 cancel. Including the final accepted 7 leaves result_id=7, which remains valid until consumed. The same logic also supports an unpaired ID of zero.

What to cover

Requirements

  1. Change the accumulator only when fault_valid and fault_ready are both asserted.
  2. Include the accepted final item in the result when fault_last is asserted.
  3. Support an unpaired ID of zero.
  4. Hold result_id and result_valid stable during result backpressure.
Continue practicing
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