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// Capstone: debug the hang. A requester sends two requests to a device over a
// valid/ready interface. A power controller puts the device to sleep with the
// step 11 handshake when the device reports no activity for two cycles, and wakes
// it when the device reports activity again. The first request is served; the
// second never is. Find out why from the waveform before reading the solution.
// The checker reports whether the hang reproduced.
module tb;
  logic clk = 0;
  always #5 clk = ~clk;
  initial begin $dumpfile("wave.vcd"); $dumpvars(0, tb); end

  int cyc = 0;
  always @(posedge clk) cyc <= cyc + 1;

  // Requester: request 1 in cycle 1, request 2 in cycle 8; holds vld until accepted
  logic vld = 0;
  logic rdy;
  always @(posedge clk) begin
    if ((cyc == 1 || cyc == 8) && !vld) vld <= 1;
    else if (vld && rdy) vld <= 0;
  end

  // Device: busy for 2 cycles per request; cannot accept while asleep or busy
  logic busy = 0, slp_acc = 0, slp_req = 0, active;
  logic [1:0] work = 0;
  always_comb begin
    rdy    = !slp_acc && !busy;
    active = busy;                                 // activity reported to the controller
  end
  always @(posedge clk) begin
    if (vld && rdy) begin busy <= 1; work <= 1; end
    else if (busy) begin if (work == 0) busy <= 0; else work <= work - 1; end
    // sleep handshake (device side): accept when idle and nothing is arriving
    if (slp_req && !slp_acc && !busy && !vld) slp_acc <= 1;
    else if (!slp_req) slp_acc <= 0;
  end

  // Power controller: sleep after 2 idle cycles, wake on activity
  logic [1:0] idle = 0;
  always @(posedge clk) begin
    idle <= active ? 0 : (idle == 3 ? 3 : idle + 1);
    if (!slp_req && idle >= 2) slp_req <= 1;
    else if (slp_req && slp_acc && active) slp_req <= 0;
  end

  int served = 0, stuck = 0;
  always @(posedge clk) begin
    if (vld && rdy) served++;
    if (vld && !rdy) stuck++;
  end
  initial begin
    #300;
    if (served == 1 && stuck > 15 && slp_acc)
      $display("PASS: hang reproduced: request 2 has waited %0d cycles while the device sleeps", stuck);
    else $display("FAIL: served %0d stuck %0d asleep %0d", served, stuck, slp_acc);
    $finish;
  end
endmodule