SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

serial-receiver-framed

claude-code claude-opus-4-8 ✗ failed GOOD_FAILURE ↑ View task
Solved from the instruction alone, tests/ and solution/ were withheld from the agent's workspace and restored only for grading.
Reward = tests/test.sh exit code (0 → resolved); the classification below is post-hoc and cannot change it.
Classification , post-hoc; cannot change the reward
GOOD_FAILUREHonest miss, the agent ran correctly but couldn't solve it. Expected for a hard task; the task is sound.
SubtypeImplementation Bugs
EvidenceThe verifier reports `TEST reference_equivalence: FAIL (10 mismatches in 155243 samples)` after `dut_compiles: PASS`. The agent wrote a full UART receiver but used timing such as `if (clk_cnt == half)` for midpoint sampling, while the reference/testbench samples on `tick == half_bit_div - 16'd1`; the instruction explicitly requires midpoint sampling and exact commit/error timing such as `A good frame commits on the edge completing the last stop bit`.
Root causeThe task is well specified and the tests check behavioral equivalence for requirements stated in the prompt. The agent produced a near-complete implementation but had subtle cycle/timing mismatches, which is normal failure behavior for a hard RTL task.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
5 tool calls · 2 tool types · 12 steps
Write /app/dut.sv with synthesizable TopModule(clk, reset, in, baud_div_i[15:0], frame_fmt_i[2:0], clear_err_i, data_ack_i, out_byte[7:0], done, parity_err_o, frame_err_o, break_o, ovf_o, byte_count_o[7:0]). Synchronous active-high reset clears state/outputs. Serial idles high; only an idle high-to-low transition starts a frame. Latch baud and format on that edge; frame baud 0 means 1, and later changes are ignored. Each bit is 16*latched_baud clocks; sample start/data/parity/stop at the midpoint. A high start midpoint aborts with no flags. LSB-first formats: 000 8N1, 001 7N1 with out_byte[7]=0, 010 8E1, 011 8O1, 100 8N2; others are 000. Even parity makes XOR(data, parity)=0; odd makes it 1. parity_err_o asserts on the failing parity midpoint; frame_err_o asserts on any low stop midpoint. Error frames still consume remaining stop slots, return to idle only after the final stop slot, and do not update byte, done, or count. A good frame commits on the edge completing the last stop bit: byte, level done, and saturated count update then. Later data_ack_i clears done; ack on commit does not clear new done. A good commit while old done is high and ack is low sets sticky ovf_o and replaces the byte. Sticky error flags clear with clear_err_i; same-cycle error or overflow wins. break_o uses live nonzero baud, asserts after in is low for 11*16*baud_div_i clocks, and deasserts one clock after high. Forbidden: simulator/file/print tasks, preprocessor, DPI, force/release, plusargs, memory/dump calls, hidden grader names, and testbench hierarchy.
I'll design a UART receiver FSM implementing the spec carefully. Let me write the file.
write · /app/dut.sv
module TopModule(
    input              clk,
    input              reset,
    input              in,
    input      [15:0]  baud_div_i,
    input      [2:0]   frame_fmt_i,
    input              clear_err_i,
    input              data_ack_i,
    output reg [7:0]   out_byte,
    output reg         done,
    output reg         parity_err_o,
    output reg         frame_err_o,
    output reg         break_o,
    output reg         ovf_o,
    output reg [7:0]   byte_count_o
);

    // FSM states
    localparam [2:0] IDLE  = 3'd0,
                     START = 3'd1,
                     DATA  = 3'd2,
                     PAR   = 3'd3,
                     STOP  = 3'd4;

    reg [2:0]  state;
    reg [19:0] clk_cnt;    // clocks within current bit
    reg [3:0]  data_idx;   // data bit index
    reg [1:0]  stop_idx;   // stop bit index
    reg [7:0]  rx_shift;   // received data (LSB first)
    reg        par_acc;    // running XOR of data bits
    reg        frame_bad;  // frame has parity or framing error
    reg        in_d;       // previous serial value (edge detect)

    reg [15:0] baud_l;     // latched baud
    reg [2:0]  fmt_l;      // latched (normalized) format

    reg [31:0] low_cnt;    // consecutive-low counter for break

    // ---- normalized live inputs ----
    wire [2:0]  fmt_norm  = (frame_fmt_i > 3'b100) ? 3'b000 : frame_fmt_i;
    wire [15:0] baud_norm = (baud_div_i == 16'd0) ? 16'd1 : baud_div_i;
    wire [15:0] live_baud = (baud_div_i == 16'd0) ? 16'd1 : baud_div_i;
    wire [31:0] brk_thresh = live_baud * 32'd176; // 11*16*baud

    // ---- derived config from latched format ----
    reg [3:0] ndata;
    reg       par_en;
    reg       par_odd;
    reg [1:0] nstop;
    always @* begin
        case (fmt_l)
            3'b000: begin ndata = 4'd8; par_en = 1'b0; par_odd = 1'b0; nstop = 2'd1; end
            3'b001: begin ndata = 4'd7; par_en = 1'b0; par_odd = 1'b0; nstop = 2'd1; end
            3'b010: begin ndata = 4'd8; par_en = 1'b1; par_odd = 1'b0; nstop = 2'd1; end
            3'b011: begin ndata = 4'd8; par_en = 1'b1; par_odd = 1'b1; nstop = 2'd1; end
            3'b100: begin ndata = 4'd8; par_en = 1'b0; par_odd = 1'b0; nstop = 2'd2; end
            default:begin ndata = 4'd8; par_en = 1'b0; par_odd = 1'b0; nstop = 2'd1; end
        endcase
    end

    // bit length and midpoint (16*baud, 8*baud)
    wire [19:0] bit_len = {baud_l, 4'b0000};
    wire [19:0] half    = {1'b0, baud_l, 3'b000};

    // per-cycle event flags (combinational temporaries)
    reg ev_parity, ev_frame, ev_commit;

    always @(posedge clk) begin
        if (reset) begin
            state        <= IDLE;
            clk_cnt      <= 20'd0;
            data_idx     <= 4'd0;
            stop_idx     <= 2'd0;
            rx_shift     <= 8'd0;
            par_acc      <= 1'b0;
            frame_bad    <= 1'b0;
            in_d         <= 1'b1;
            baud_l       <= 16'd1;
            fmt_l        <= 3'b000;
            low_cnt      <= 32'd0;
            out_byte     <= 8'd0;
            done         <= 1'b0;
            parity_err_o <= 1'b0;
            frame_err_o  <= 1'b0;
            break_o      <= 1'b0;
            ovf_o        <= 1'b0;
            byte_count_o <= 8'd0;
        end else begin
            // default event flags
            ev_parity = 1'b0;
            ev_frame  = 1'b0;
            ev_commit = 1'b0;

            in_d <= in;

            // ---------------- break detection (live baud) ----------------
            if (in) begin
                low_cnt <= 32'd0;
                break_o <= 1'b0;
            end else begin
                if (low_cnt != brk_thresh)
                    low_cnt <= low_cnt + 32'd1;
                if (low_cnt + 32'd1 >= brk_thresh)
                    break_o <= 1'b1;
            end

            // ---------------- receive FSM ----------------
            case (state)
                IDLE: begin
                    if (in_d && !in) begin
                        baud_l    <= baud_norm;
                        fmt_l     <= fmt_norm;
                        clk_cnt   <= 20'd0;
                        data_idx  <= 4'd0;
                        stop_idx  <= 2'd0;
                        rx_shift  <= 8'd0;
                        par_acc   <= 1'b0;
                        frame_bad <= 1'b0;
                        state     <= START;
                    end
                end

                START: begin
                    if (clk_cnt == half) begin
                        if (in)
                            state <= IDLE;   // high start midpoint: abort, no flags
                    end
                    if (clk_cnt == bit_len - 20'd1) begin
                        clk_cnt  <= 20'd0;
                        data_idx <= 4'd0;
                        state    <= DATA;
                    end else begin
                        clk_cnt <= clk_cnt + 20'd1;
                    end
                end

                DATA: begin
                    if (clk_cnt == half) begin
                        rx_shift[data_idx] <= in;
                        par_acc <= par_acc ^ in;
                    end
                    if (clk_cnt == bit_len - 20'd1) begin
                        clk_cnt <= 20'd0;
                        if (data_idx == ndata - 4'd1) begin
                            if (par_en) begin
                                state <= PAR;
                            end else begin
                                state    <= STOP;
                                stop_idx <= 2'd0;
                            end
                        end else begin
                            data_idx <= data_idx + 4'd1;
                        end
                    end else begin
                        clk_cnt <= clk_cnt + 20'd1;
                    end
                end

                PAR: begin
                    if (clk_cnt == half) begin
                        if ((par_acc ^ in) != par_odd) begin
                            ev_parity = 1'b1;
                            frame_bad <= 1'b1;
                        end
                    end
                    if (clk_cnt == bit_len - 20'd1) begin
                        clk_cnt  <= 20'd0;
                        stop_idx <= 2'd0;
                        state    <= STOP;
                    end else begin
                        clk_cnt <= clk_cnt + 20'd1;
                    end
                end

                STOP: begin
                    if (clk_cnt == half) begin
                        if (!in) begin
                            ev_frame  = 1'b1;
                            frame_bad <= 1'b1;
                        end
                    end
                    if (clk_cnt == bit_len - 20'd1) begin
                        clk_cnt <= 20'd0;
                        if (stop_idx == nstop - 2'd1) begin
                            if (!frame_bad)
                                ev_commit = 1'b1;
                            state <= IDLE;
                        end else begin
                            stop_idx <= stop_idx + 2'd1;
                        end
                    end else begin
                        clk_cnt <= clk_cnt + 20'd1;
                    end
                end

                default: state <= IDLE;
            endcase

            // ---------------- sticky error flags (set wins over clear) ----------------
            if (ev_parity)        parity_err_o <= 1'b1;
            else if (clear_err_i) parity_err_o <= 1'b0;

            if (ev_frame)         frame_err_o <= 1'b1;
            else if (clear_err_i) frame_err_o <= 1'b0;

            // ---------------- commit / done / overflow / count ----------------
            if (ev_commit) begin
                out_byte     <= rx_shift;
                done         <= 1'b1;
                byte_count_o <= (byte_count_o == 8'hFF) ? 8'hFF : byte_count_o + 8'd1;
                if (done && !data_ack_i)   ovf_o <= 1'b1;  // overflow (set wins)
                else if (clear_err_i)      ovf_o <= 1'b0;
            end else begin
                if (data_ack_i) done <= 1'b0;
                if (clear_err_i) ovf_o <= 1'b0;
            end
        end
    end

endmodule
Let me verify it compiles cleanly.

Compile check with iverilog

shell
(iverilog -g2012 -o /tmp/dut.out dut.sv 2>&1 && echo "COMPILE OK") || echo "iverilog not available or error"
output
COMPILE OK

[stdout]
COMPILE OK
Compiles. Let me run a quick self-check simulation (basic 8N1 frame) to validate behavior.

Simulate a basic 8N1 frame

shell
cat > /tmp/tb.sv <<'EOF'
module tb;
  reg clk=0, reset=1, in=1, clear_err_i=0, data_ack_i=0;
  reg [15:0] baud_div_i=1;
  reg [2:0] frame_fmt_i=3'b000;
  wire [7:0] out_byte, byte_count_o;
  wire done, parity_err_o, frame_err_o, break_o, ovf_o;
  TopModule d(clk,reset,in,baud_div_i,frame_fmt_i,clear_err_i,data_ack_i,
    out_byte,done,parity_err_o,frame_err_o,break_o,ovf_o,byte_count_o);
  always #5 clk=~clk;
  integer i,b;
  task send_bit(input v); begin in=v; repeat(16) @(posedge clk); end endtask
  reg [7:0] data;
  initial begin
    repeat(3) @(posedge clk); reset=0; @(posedge clk);
    data=8'h5A;
    // start
    send_bit(0);
    for(i=0;i<8;i=i+1) send_bit(data[i]);
    // 8N1 no parity, stop high
    send_bit(1);
    repeat(20) @(posedge clk);
    $display("out_byte=%h done=%b count=%d ferr=%b perr=%b", out_byte, done, byte_count_o, frame_err_o, parity_err_o);
    if(out_byte==8'h5A && done && byte_count_o==1) $display("PASS 8N1");
    else $display("FAIL 8N1");
    // ack clears done
    data_ack_i=1; @(posedge clk); data_ack_i=0; @(posedge clk);
    $display("after ack done=%b", done);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tb.out /tmp/dut_and_tb 2>/dev/null; iverilog -g2012 -o /tmp/tb.out dut.sv /tmp/tb.sv && vvp /tmp/tb.out
output
out_byte=5a done=1 count=  1 ferr=0 perr=0
PASS 8N1
after ack done=1
/tmp/tb.sv:28: $finish called at 1855 (1s)

[stdout]
out_byte=5a done=1 count=  1 ferr=0 perr=0
PASS 8N1
after ack done=1
/tmp/tb.sv:28: $finish called at 1855 (1s)
8N1 works. Let me investigate the ack-clears-done result more carefully with cleaner timing.

Test ack and overflow behavior

shell
cat > /tmp/tb2.sv <<'EOF'
module tb;
  reg clk=0, reset=1, in=1, clear_err_i=0, data_ack_i=0;
  reg [15:0] baud_div_i=1;
  reg [2:0] frame_fmt_i=3'b000;
  wire [7:0] out_byte, byte_count_o;
  wire done, parity_err_o, frame_err_o, break_o, ovf_o;
  TopModule d(clk,reset,in,baud_div_i,frame_fmt_i,clear_err_i,data_ack_i,
    out_byte,done,parity_err_o,frame_err_o,break_o,ovf_o,byte_count_o);
  always #5 clk=~clk;
  integer i;
  reg [7:0] data;
  task send_bit(input v); begin in=v; repeat(16) @(posedge clk); end endtask
  initial begin
    repeat(3) @(posedge clk); reset=0; @(posedge clk);
    data=8'h5A;
    send_bit(0);
    for(i=0;i<8;i=i+1) send_bit(data[i]);
    send_bit(1);
    repeat(5) @(posedge clk);
    $display("done=%b", done);
    @(negedge clk); data_ack_i=1;
    @(negedge clk); data_ack_i=0;
    @(negedge clk);
    $display("after ack done=%b", done);
    // overflow test: send two frames without ack
    data=8'hA5;
    send_bit(0); for(i=0;i<8;i=i+1) send_bit(data[i]); send_bit(1);
    repeat(3)@(posedge clk);
    $display("frame2 out=%h done=%b ovf=%b count=%d", out_byte, done, ovf_o, byte_count_o);
    // do not ack, third frame -> should have already had done high from frame2? ack was done earlier so done was 0 before frame2. So no ovf yet.
    // send third without ack
    data=8'h3C;
    send_bit(0); for(i=0;i<8;i=i+1) send_bit(data[i]); send_bit(1);
    repeat(3)@(posedge clk);
    $display("frame3 out=%h done=%b ovf=%b count=%d", out_byte, done, ovf_o, byte_count_o);
    clear_err_i=1; @(negedge clk); clear_err_i=0; @(negedge clk);
    $display("after clear ovf=%b", ovf_o);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tb2.out dut.sv /tmp/tb2.sv && vvp /tmp/tb2.out
output
done=1
after ack done=0
frame2 out=a5 done=1 ovf=0 count=  2
frame3 out=3c done=1 ovf=1 count=  3
after clear ovf=0
/tmp/tb2.sv:38: $finish called at 4980 (1s)

[stdout]
done=1
after ack done=0
frame2 out=a5 done=1 ovf=0 count=  2
frame3 out=3c done=1 ovf=1 count=  3
after clear ovf=0
/tmp/tb2.sv:38: $finish called at 4980 (1s)
Ack, overflow, and clear all work. Now let me test parity error, framing error, and break.

Test parity, framing errors, abort, break

shell
cat > /tmp/tb3.sv <<'EOF'
module tb;
  reg clk=0, reset=1, in=1, clear_err_i=0, data_ack_i=0;
  reg [15:0] baud_div_i=1;
  reg [2:0] frame_fmt_i=3'b010; // 8E1
  wire [7:0] out_byte, byte_count_o;
  wire done, parity_err_o, frame_err_o, break_o, ovf_o;
  TopModule d(clk,reset,in,baud_div_i,frame_fmt_i,clear_err_i,data_ack_i,
    out_byte,done,parity_err_o,frame_err_o,break_o,ovf_o,byte_count_o);
  always #5 clk=~clk;
  integer i;
  reg [7:0] data; reg par;
  task send_bit(input v); begin in=v; repeat(16) @(posedge clk); end endtask
  initial begin
    repeat(3) @(posedge clk); reset=0; @(posedge clk);
    // 8E1 good: data 8'hFF -> parity even = xor bits =0 -> parity bit 0
    data=8'hFF; par=^data; // even parity bit = xor(data) to make total 0
    send_bit(0); for(i=0;i<8;i=i+1) send_bit(data[i]); send_bit(par); send_bit(1);
    repeat(3)@(posedge clk);
    $display("8E1 good: out=%h done=%b perr=%b count=%d", out_byte,done,parity_err_o,byte_count_o);
    // bad parity: send wrong parity bit
    data=8'h0F; send_bit(0); for(i=0;i<8;i=i+1) send_bit(data[i]); send_bit(~(^data)); send_bit(1);
    repeat(3)@(posedge clk);
    $display("8E1 badpar: out=%h done=%b perr=%b count=%d (count should stay 1)", out_byte,done,parity_err_o,byte_count_o);
    clear_err_i=1;@(negedge clk);clear_err_i=0;@(negedge clk);
    $display("after clear perr=%b", parity_err_o);
    // framing error 8N1: stop low
    frame_fmt_i=3'b000;
    data=8'h55; send_bit(0); for(i=0;i<8;i=i+1) send_bit(data[i]); send_bit(0); // low stop
    send_bit(1); repeat(3)@(posedge clk);
    $display("frame err: ferr=%b count=%d (count still 1)", frame_err_o, byte_count_o);
    clear_err_i=1;@(negedge clk);clear_err_i=0;
    // abort: falling edge then high at midpoint
    in=0; repeat(4)@(posedge clk); in=1; repeat(20)@(posedge clk);
    $display("abort: perr=%b ferr=%b done unchanged", parity_err_o, frame_err_o);
    // break: hold low for > 11*16 = 176 cycles
    in=0; repeat(180)@(posedge clk);
    $display("break asserted=%b", break_o);
    in=1; @(posedge clk); @(posedge clk);
    $display("break after high=%b", break_o);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tb3.out dut.sv /tmp/tb3.sv && vvp /tmp/tb3.out
output
8E1 good: out=ff done=1 perr=0 count=  1
8E1 badpar: out=ff done=1 perr=1 count=  1 (count should stay 1)
after clear perr=0
frame err: ferr=1 count=  1 (count still 1)
abort: perr=0 ferr=0 done unchanged
break asserted=1
break after high=0
/tmp/tb3.sv:40: $finish called at 7475 (1s)

[stdout]
8E1 good: out=ff done=1 perr=0 count=  1
8E1 badpar: out=ff done=1 perr=1 count=  1 (count should stay 1)
after clear perr=0
frame err: ferr=1 count=  1 (count still 1)
abort: perr=0 ferr=0 done unchanged
break asserted=1
break after high=0
/tmp/tb3.sv:40: $finish called at 7475 (1s)
All behaviors validate correctly. `/app/dut.sv` is complete and synthesizable. ## Summary I implemented `TopModule` as a UART receiver FSM. Verified behaviors: - **8N1 frame** → correct byte, `done`, count=1 ✓ - **Ack** clears `done` ✓ - **Overflow** → second unacked commit sets `ovf_o`, replaces byte, keeps counting ✓ - **8E1 parity** , good frame commits; bad parity sets `parity_err_o`, no commit (count unchanged) ✓ - **Framing error** (low stop) sets `frame_err_o`, no commit ✓ - **`clear_err_i`** clears sticky flags ✓ - **Abort** (high start midpoint) → no flags ✓ - **Break** asserts after 11·16·baud low clocks, deasserts one clock after high ✓ ### Design notes - **Sampling:** `bit_len = 16·baud_l`, midpoint `half = 8·baud_l`; a within-bit counter samples start/data/parity/stop at the midpoint and advances at bit end. - **Latching:** baud (0→1) and normalized format latched on the idle high→low edge (`in_d && !in`); live changes ignored mid-frame. Formats decode to data width / parity / stop-count; unknown → `000`. - **Parity:** running XOR `par_acc` of data bits; error when `(par_acc ^ parity_bit) != par_odd`. - **Error frames** set `frame_bad`, consume all remaining stop slots, return to IDLE without touching byte/done/count. - **Good commit** at the last-stop-bit completion edge: updates `out_byte`, sets level `done`, saturating `byte_count_o`. Commit's `done<=1` beats same-cycle ack; commit while `done && !ack` sets `ovf_o`. - **Sticky flags** (`parity_err_o`, `frame_err_o`, `ovf_o`): set-wins-over-clear so a same-cycle error/overflow beats `clear_err_i`. - **Break** runs independently on live nonzero baud (`11·16·baud` threshold), deasserting one clock after `in` goes high. - No forbidden constructs (no system/file/print tasks, preprocessor, DPI, force/release, plusargs, or dump calls); passes `iverilog -g2012`.
Final diff , what the agent changed

No file changes, the agent left the workspace untouched.

Verifier output , the proof behind the reward
tests/test.sh
TEST dut_present: PASS (/app/dut.sv exists)
TEST dut_forbidden: PASS (no forbidden constructs)
TEST harness_prepare: PASS (harness names nonce-renamed)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 10 in 155243 samples)
TEST reference_equivalence: FAIL (10 mismatches in 155243 samples)

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_557adaa1d351412c. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_557adaa1d351412c · verifier authoritative; classifier explanatory.