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Q056FreeSystemVerilog

Validate nested protocol delimiters

Difficulty
Hard
Topic
Streaming RTL
Language
SV
Interview prompt

Question

Consume a framed stream of two delimiter kinds: token=00 is OPEN_A, 01 is CLOSE_A, 10 is OPEN_B, and 11 is CLOSE_B. Report balanced nesting and the first error. Use error_code 0=NONE, 1=MISMATCH, 2=UNDERFLOW, 3=OVERFLOW, and 4=UNFINISHED_OPEN.

Candidate starting point

Implementation scaffold

module delimiter_checker (
  input  logic       clk,
  input  logic       rst_n,
  input  logic       token_valid,
  input  logic       token_last,
  input  logic [1:0] token,
  output logic       token_ready,
  input  logic       result_ready,
  output logic       result_valid,
  output logic       balanced,
  output logic [2:0] error_code
);
  localparam logic [2:0] ERR_NONE      = 3'd0;
  localparam logic [2:0] ERR_MISMATCH  = 3'd1;
  localparam logic [2:0] ERR_UNDERFLOW = 3'd2;
  localparam logic [2:0] ERR_OVERFLOW  = 3'd3;
  localparam logic [2:0] ERR_OPEN      = 3'd4;

  logic stack_q [8];
  logic stack_next [8];
  logic [3:0] depth_q, depth_next;
  logic [2:0] error_q, error_next, final_error;

  assign token_ready = !result_valid;

  always_comb begin : decode_token
    // TODO: Implement decode_token using the supplied state and interface.
  end

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

Trace one case

Input
Accepted tokens are OPEN_A, OPEN_B, CLOSE_B, CLOSE_A, with token_last on CLOSE_A. Insert one cycle with token_valid=0 between CLOSE_B and CLOSE_A.
Expected output
At the final accepted token, balanced=1, error_code=0, and result_valid=1. These fields remain stable while result_ready=0.

The valid-low gap leaves the stack at depth one. The final CLOSE_A is included before the result is classified, so the updated stack is empty. This reference does not apply input backpressure inside a frame; it blocks input while holding a completed result.

What to cover

Requirements

  1. Assume each frame contains one through 16 accepted tokens and ends with token_last. Enforce a nesting depth of at most eight.
  2. Advance parser state only when token_valid && token_ready. Keep consuming after the first error, preserving its error code.
  3. A close pops the most recent open if present and reports a type mismatch when needed. Opening at depth eight reports overflow without pushing; closing at depth zero reports underflow. At the final token, report unfinished-open only if no earlier error exists and the updated stack is nonempty.
  4. Hold result_valid, balanced, and error_code until result_ready accepts them. Block the next frame while a result is pending; producer valid-low gaps within a frame do not advance the parser.
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