SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

Resynchronising serial byte receiver

codex gpt-5.5 ✗ 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.
SubtypeWrong Approach
EvidenceTest output shows 196 'busy' signal mismatches with pattern 'got=1 exp=0' across multiple time points (busy16 and busy8 variants). Agent's implementation (trajectory lines 105-123) keeps `busy=0` throughout ST_START state and only asserts `busy=1` after transitioning to ST_DATA. Instruction requires `busy` to be raised when start is CONFIRMED at BITLEN/2, making it observable from the next clock onward. Agent's FSM structure registers `busy <= 1'b0` in ST_START (line 106) and `busy <= 1'b1` only in ST_DATA (line 126), violating the timing requirement.
Root causeThe agent implemented a four-state FSM (IDLE/START/DATA/STOP) but incorrectly timed the `busy` signal assertion. The agent kept `busy` low during the start qualification period (ST_START state) and only raised it after confirming and entering the data-sampling state (ST_DATA). The specification requires `busy` to transition high on or immediately after the confirmed start, not later when data sampling begins.
RecommendationN/A - task is fine. The instruction clearly specifies that `busy` must be high 'from the confirmed start through the data and closing-bit sampling' and that it 'becomes observable the clock after the confirmed start centre'. The agent needed to assert `busy` in the transition from ST_START to ST_DATA (or in the clock that confirms the start), not defer it. This is a standard hard RTL design problem requiring careful attention to state machine timing.
Trajectory
Tool-by-tool agent trajectory
18 tool calls · 2 tool types · 33 steps
<permissions instructions> Filesystem sandboxing defines which files can be read or written. `sandbox_mode` is `danger-full-access`: No filesystem sandboxing - all commands are permitted. Network access is enabled. Approval policy is currently never. Do not provide the `sandbox_permissions` for any reason, commands will be rejected. </permissions instructions><apps_instructions> ## Apps (Connectors) Apps (Connectors) can be explicitly triggered in user messages in the format `[$app-name](app://{connector_id})`. Apps can also be implicitly triggered as long as the context suggests usage of available apps. An app is equivalent to a set of MCP tools within the `codex_apps` MCP. An installed app's MCP tools are either provided to you already, or can be lazy-loaded through the `tool_search` tool. If `tool_search` is available, the apps that are searchable by `tools_search` will be listed by it. Do not additionally call list_mcp_resources or list_mcp_resource_templates for apps. </apps_instructions><skills_instructions> ## Skills A skill is a set of instructions provided through a `SKILL.md` source. Below is the list of skills that can be used. Each entry includes a name, description, and source locator. `file` locators are on the host filesystem, `environment resource` locators are owned by an execution environment, `orchestrator resource` locators are opaque non-filesystem resources, and `custom resource` locators use their provider's access mechanism. ### Available skills - imagegen: Generate or edit raster images when the task benefits from AI-created bitmap visuals such as photos, illustrations, textures, sprites, mockups, or transparent-background cutouts. Use when Codex should create a brand-new image, transform an existing image, or derive visual variants from references, and the output should be a bitmap asset rather than repo-native code or vector. Do not use when the task is better handled by editing existing SVG/vector/code-native assets, extending an established icon or logo system, or building the visual directly in HTML/CSS/canvas. (file: /tmp/codex-home/skills/.system/imagegen/SKILL.md) - openai-docs: Use when the user asks how to build with OpenAI products or APIs, asks about Codex itself or choosing Codex surfaces, needs up-to-date official documentation with citations, help choosing the latest model for a use case, or model upgrade and prompt-upgrade guidance; use OpenAI docs MCP tools for non-Codex docs questions, use the Codex manual helper first for broad Codex self-knowledge, and restrict fallback browsing to official OpenAI domains. (file: /tmp/codex-home/skills/.system/openai-docs/SKILL.md) - plugin-creator: Create and scaffold plugin directories for Codex with a required `.codex-plugin/plugin.json`, optional plugin folders/files, valid manifest defaults, and personal-marketplace entries by default. Use when Codex needs to create a new personal plugin, add optional plugin structure, generate or update marketplace entries for plugin ordering and availability metadata, or update an existing local plugin during development with the CLI-driven cachebuster and reinstall flow. (file: /tmp/codex-home/skills/.system/plugin-creator/SKILL.md) - skill-creator: Guide for creating effective skills. This skill should be used when users want to create a new skill (or update an existing skill) that extends Codex's capabilities with specialized knowledge, workflows, or tool integrations. (file: /tmp/codex-home/skills/.system/skill-creator/SKILL.md) - skill-installer: Install Codex skills into $CODEX_HOME/skills from a curated list or a [code-host] repo path. Use when a user asks to list installable skills, install a curated skill, or install a skill from another repo (including private repos). (file: /tmp/codex-home/skills/.system/skill-installer/SKILL.md) ### How to use skills - Discovery: The list above is the skills available in this session (name + description + source locator). `file` entries live on the host filesystem, `environment resource` entries are owned by their execution environment, `orchestrator resource` entries must be accessed through `skills.list` and `skills.read`, and `custom resource` entries use their provider's access mechanism. - Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description shown above, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned. - Missing/blocked: If a named skill isn't in the list or its source can't be read, say so briefly and continue with the best fallback. - How to use a skill (progressive disclosure): 1) After deciding to use a skill, the main agent must read its `SKILL.md` completely before taking task actions. For a `file` entry, open the listed path. For an `environment resource`, use the filesystem of the owning environment. For an `orchestrator resource`, call `skills.list` with `{"authority":{"kind":"orchestrator"}}`, select the matching package, and pass its `main_resource` to `skills.read`. If a read is truncated or paginated, continue until EOF. 2) When `SKILL.md` references another resource, use the same access mechanism. Resolve relative paths against a filesystem-backed skill directory. For orchestrator skills, pass the exact referenced resource identifier with the same authority and package to `skills.read`; do not treat `skill://` identifiers as filesystem paths. 3) If `SKILL.md` points to extra folders such as `references/`, use its routing instructions to identify the resources required for the task. The main agent must read each required instruction or reference file itself before acting on it. Do not delegate reading, summarizing, or interpreting skill instructions to a subagent. Subagents may still perform task work when the selected skill allows it. 4) For filesystem-backed skills, prefer running or patching provided scripts instead of retyping large code blocks. For orchestrator skills, use `skills.read` and the available tools; do not invent a local path. 5) Reuse provided assets or templates through the same source access mechanism instead of recreating them. - Coordination and sequencing: - If multiple skills apply, choose the minimal set that covers the request and state the order you'll use them. - Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why. - Context hygiene: - Progressive disclosure applies to selecting relevant files, not partially reading a selected instruction file. Do not load unrelated references, scripts, or assets. - Avoid deep reference-chasing: prefer opening only files directly linked from `SKILL.md` unless you're blocked. - When variants exist (frameworks, providers, domains), pick only the relevant reference file(s) and note that choice. - Safety and fallback: If a skill can't be applied cleanly (missing files, unclear instructions), state the issue, pick the next-best approach, and continue. </skills_instructions>
<environment_context> <cwd>/work</cwd> <shell>bash</shell> <current_date>2026-06-20</current_date> <timezone>Etc/UTC</timezone> <filesystem><workspace_roots><root>/work</root></workspace_roots><permission_profile type="disabled"><file_system type="unrestricted" /></permission_profile></filesystem> </environment_context>
# Resynchronising serial byte receiver ## Context We have a single wire coming in from another board. It sits high when nothing is happening, and a byte arrives as a burst: the line drops low for one bit time to announce a byte is starting, then eight data bits go by one bit time each, then the line goes back high for at least one bit time to close the frame. There is no separate clock on the wire, so `srx` has to recover the timing itself from the falling edge and a fixed, known bit period. The wire is noisy. Two things make this fiddly, and they share the same counter and state, so they cannot be bolted on independently: - a brief dip on the line is not a real byte and must not start anything; and - once a byte ends badly, the receiver must not go chasing the next falling edge until the line has actually settled back to idle. The package and the wrapper are in place; only `rtl/srx.sv` is a stub. Implement it. ## Interface ``` input clk // single clock input rst_n // asynchronous, active low input rx // serial input; idles high input take // downstream-consumed strobe output [7:0] data // received byte output valid // good-byte strobe output frame_err // framing-error strobe output busy // receiving a frame ``` `BITLEN` is a parameter: the bit period, in `clk` cycles. It is even and at least 8. Your design must work for any such `BITLEN` (the checker exercises more than one value); do not hard-code a specific period. `BITLEN/2` is the offset from a falling edge to the centre of that bit. ## What the receiver does The receiver hunts for the falling edge that leaves idle, confirms it is a real start, samples the eight data bits and the closing bit at their centres, and then either delivers the byte or flags a framing error , and the way it recovers after a framing error is the part that ties back to how it qualifies a start. ### Starting a byte (the part that bites) A falling edge on `rx` is only a *candidate*. The start is **confirmed** only if `rx` is **still low at the centre of that first bit** , that is, `BITLEN/2` clocks after the falling edge. If `rx` has returned high by then, the dip was a glitch: ignore it completely, stay idle, and never raise `busy`. A confirmed start raises `busy`; an unconfirmed one never does. This re-check at the bit centre is deliberate and is the only thing that distinguishes a real start from line noise , a design that begins framing on the first low it sees, or after a fixed wait with no second look, is wrong. ### Sampling the data After a confirmed start, sample the eight data bits at their **centres**, one `BITLEN` apart: the first data bit is sampled `BITLEN` clocks after the start centre, the next `BITLEN` after that, and so on. The bits arrive **least significant first** , the first sampled bit is bit 0 of the byte, the eighth is bit 7. ### Finishing a byte One `BITLEN` after the eighth data bit, sample the closing bit at its centre: - if it is **1**, the frame is good: assert `valid` for exactly one clock and present the assembled byte on `data`; - if it is **0**, the frame is bad: assert `frame_err` for exactly one clock and do **not** assert `valid` and do **not** change `data`. `busy` is high from the cycle the start is confirmed up to and including the sampling that produces the result, and drops the cycle after. ### Recovering after a framing error After a `frame_err`, the receiver must **not** begin a new byte until `rx` has been **continuously high for a full `BITLEN`** , i.e. the line has genuinely re-idled. A falling edge that occurs before that has elapsed is ignored (no `busy`, no new frame); any low seen during this window restarts the wait. A **good** frame does not impose this wait , after a `valid`, the very next falling edge is a candidate start as usual. ## Conventions (pin these down) - **All four outputs are registered.** `data`, `valid`, `frame_err` and `busy` take their new values on a rising `clk` edge and are observable from just after that edge until the next edge. There is no combinational path from `rx` to any output within the same cycle. - **`valid` and `frame_err` are one-cycle pulses**, never levels: each is high for exactly one clock when its event occurs and low otherwise. **`busy` is a level**, high for the whole reception. - **`data` is a level-held bus.** It holds the most recently delivered good byte and changes only when the next good frame overwrites it. It is **not** cleared or altered by `take`, and it does **not** change on a framing error. `take` is an input the receiver accepts but that does not affect `data`, `valid`, `frame_err` or `busy`. - **Every state change commits on the rising edge of `clk`.** - **`rst_n` is asynchronous and active low.** While it is low, `data`, `valid`, `frame_err` and `busy` are all forced to 0 immediately on the falling edge of `rst_n` (not waiting for a clock), and the receiver returns to idle. Reset wins over everything. ## One worked timing (start qualification + first data sample) Let the falling edge that leaves idle be sampled on clock `F` (the first clock where `rx` reads 0). With `BITLEN = 8` (so the centre offset `BITLEN/2 = 4`): ``` clock F F+1 F+2 F+3 F+4 ... F+12 rx (sampled) 0 0 0 0 0(*) d0(**) busy 0 0 0 0 1 1 event -- -- -- -- confirm start sample data bit 0 ``` (*) `F+4` is the centre of the start bit (`F + BITLEN/2`). `rx` must still be 0 here for the start to confirm; `busy` rises on this clock. If `rx` were 1 here, nothing would have happened and `busy` would have stayed 0 throughout. (**) `F+12` is `F+4 + BITLEN`: the centre of the first data bit, sampled as bit 0. Subsequent data bits follow every `BITLEN` clocks; the closing bit is sampled one `BITLEN` after data bit 7. ## Submission Write your implementation into `rtl/srx.sv`, keeping the module name, ports and parameters exactly as given. It must elaborate under Icarus Verilog 11 (`iverilog -g2012`); note that on that tool a bit- or part-select of an *expression* (a sum, a shift, a function result) is a syntax error, so do any such bit work inside a `function automatic` on a named value, or index only named signals. The grader compiles your receiver with a held-back checker and compares `data`, `valid`, `frame_err` and `busy` cycle by cycle against a reference over directed and random serial stimulus.
I’ll inspect the repository structure and the existing wrapper/package first, then I’ll patch only `rtl/srx.sv` and run the available Verilog checks.
shell
rg --files
output
Chunk ID: 6b6b9f
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 17
Output:
srx_core/rtl/srx_top.sv
srx_core/rtl/srx_pkg.sv
srx_core/rtl/srx.sv
shell
git status --short
output
Chunk ID: 02d6e3
Wall time: 0.0000 seconds
Process exited with code 127
Original token count: 11
Output:
/bin/bash: line 1: git: command not found
The workspace is small: the receiver lives under `srx_core/rtl`. `git` is not installed in this environment, so I can’t use it to distinguish pre-existing changes, but I’ll keep the edit scoped to the stub.
shell
sed -n '1,220p' rtl/srx.sv
output
Chunk ID: ce07fc
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 854
Output:
//==============================================================================
// srx.sv
//
// Asynchronous serial byte receiver for a line that idles high.
//
// ---------------------------------------------------------------------------
//  >>> IMPLEMENTATION REQUIRED <<<
//
//  The module ports, parameters and a set of state registers are provided. The
//  frame FSM, the bit-period counter, the sampling datapath and the outputs are
//  left as a TODO and currently drive constant zeros, so the receiver produces
//  no useful result and the verifier FAILS.
//
//  Implement the behaviour described in instruction.md. Do NOT change the
//  module name, the port list, or the parameter list.
//
//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
//  function-call result) is a syntax error under `iverilog -g2012`. Index only
//  named signals, and do any bit work that needs an intermediate value inside a
//  `function automatic` using named locals.
// ---------------------------------------------------------------------------
//==============================================================================
`timescale 1ns/1ps

module srx #(
    parameter BITLEN = 16,
    parameter DW     = 8
) (
    input  logic               clk,
    input  logic               rst_n,    // asynchronous, active low
    input  logic               rx,       // serial input, idles high
    input  logic               take,     // downstream-consumed strobe
    output logic [DW-1:0]      data,      // received byte (level held)
    output logic               valid,     // 1-cycle good-frame strobe
    output logic               frame_err, // 1-cycle framing-error strobe
    output logic               busy        // high from confirmed start to result
);

  // ---------------------------------------------------------------------------
  // State registers (provided). You may add more if your design needs them.
  // ---------------------------------------------------------------------------
  logic [1:0]      state;      // frame FSM state
  integer          cnt;        // bit-period down-counter
  integer          bit_idx;    // which bit is being sampled
  logic [DW-1:0]   shifter;    // assembling byte
  integer          hi_run;     // line-idle (high-run) counter

  // ===========================================================================
  // TODO: IMPLEMENT THE RECEIVER.
  //   * qualify a start, sample the data bits, drive valid/frame_err/data/busy
  //     and run the post-error resync window per the contract in instruction.md;
  //   * asynchronous active-low reset clears all state and outputs.
  //
  // The placeholder below keeps the module compilable but INCORRECT.
  // ===========================================================================
  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      state     <= 2'd0;
      cnt       <= 0;
      bit_idx   <= 0;
      shifter   <= {DW{1'b0}};
      hi_run    <= 0;
    end else begin
      // TODO: replace with the real FSM + datapath.
      state   <= state;
      cnt     <= cnt;
      bit_idx <= bit_idx;
      shifter <= shifter;
      hi_run  <= hi_run;
    end
  end

  // TODO: replace these placeholder outputs with the real ones.
  assign data      = {DW{1'b0}};
  assign valid     = 1'b0;
  assign frame_err = 1'b0;
  assign busy      = 1'b0;

endmodule : srx
shell
sed -n '1,220p' rtl/srx_top.sv
output
Chunk ID: bf85e7
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 312
Output:
//==============================================================================
// srx_top.sv
//
// Thin integration wrapper. The verification environment drives this wrapper so
// the instantiation wiring is exercised alongside the receiver itself. It
// instantiates srx with the project-default geometry from srx_pkg.
//
// DO NOT change this file.
//==============================================================================
`timescale 1ns/1ps
`include "srx_pkg.sv"

module srx_top (
    input  logic                       clk,
    input  logic                       rst_n,
    input  logic                       rx,
    input  logic                       take,
    output logic [srx_pkg::DW-1:0]     data,
    output logic                       valid,
    output logic                       frame_err,
    output logic                       busy
);

  // Bare instantiation: the receiver carries its own default geometry, so a
  // submission needs no parameter override to elaborate correctly.
  srx u_srx (
      .clk       (clk),
      .rst_n     (rst_n),
      .rx        (rx),
      .take      (take),
      .data      (data),
      .valid     (valid),
      .frame_err (frame_err),
      .busy      (busy)
  );

endmodule : srx_top
shell
sed -n '1,220p' rtl/srx_pkg.sv
output
Chunk ID: e7b8f1
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 203
Output:
//==============================================================================
// srx_pkg.sv
//
// Shared geometry for the asynchronous serial byte receiver. The block carries
// one bit-period counter, a small frame FSM, an LSB-first shift register and a
// resync high-run counter. Parameters are declared without an explicit data
// type for broad simulator compatibility (Icarus Verilog 11); they default to
// integer constants, which is sufficient here.
//
// DO NOT change this file.
//==============================================================================
`ifndef SRX_PKG_SV
`define SRX_PKG_SV

package srx_pkg;

  localparam BITLEN = 16;   // bit period in clk cycles (>= 8, even)
  localparam DW     = 8;    // data word width (one serial byte)

endpackage : srx_pkg

`endif // SRX_PKG_SV
shell
find . -maxdepth 3 -type f -print
output
Chunk ID: 6532b6
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 12
Output:
./rtl/srx.sv
./rtl/srx_pkg.sv
./rtl/srx_top.sv
I’ve got the timing shape now. I’m going to add a small FSM: idle, candidate-start qualification, data sampling, stop-bit sampling, and post-error high-run resync. The counters will be loaded so the observable events land on the exact cycles described in the prompt.
I’m about to replace the placeholder assignments with registered outputs and move the whole behavior into the clocked block. `take` will remain intentionally unused because the receiver contract says it has no effect.
apply_patch
*** Begin Patch
*** Update File: /work/srx_core/rtl/srx.sv
@@
   logic [DW-1:0]   shifter;    // assembling byte
   integer          hi_run;     // line-idle (high-run) counter
+  logic            rx_q;       // previous sampled rx value
+
+  localparam [1:0] ST_IDLE   = 2'd0;
+  localparam [1:0] ST_START  = 2'd1;
+  localparam [1:0] ST_DATA   = 2'd2;
+  localparam [1:0] ST_STOP   = 2'd3;
+
+  logic            resync;     // post-framing-error idle qualification
 
   // ===========================================================================
-  // TODO: IMPLEMENT THE RECEIVER.
-  //   * qualify a start, sample the data bits, drive valid/frame_err/data/busy
-  //     and run the post-error resync window per the contract in instruction.md;
-  //   * asynchronous active-low reset clears all state and outputs.
-  //
-  // The placeholder below keeps the module compilable but INCORRECT.
+  // Receiver FSM.
   // ===========================================================================
   always_ff @(posedge clk or negedge rst_n) begin
     if (!rst_n) begin
       state     <= 2'd0;
       cnt       <= 0;
       bit_idx   <= 0;
       shifter   <= {DW{1'b0}};
       hi_run    <= 0;
+      rx_q      <= 1'b1;
+      resync    <= 1'b0;
+      data      <= {DW{1'b0}};
+      valid     <= 1'b0;
+      frame_err <= 1'b0;
+      busy      <= 1'b0;
     end else begin
-      // TODO: replace with the real FSM + datapath.
-      state   <= state;
-      cnt     <= cnt;
-      bit_idx <= bit_idx;
-      shifter <= shifter;
-      hi_run  <= hi_run;
+      rx_q      <= rx;
+      valid     <= 1'b0;
+      frame_err <= 1'b0;
+
+      if (resync) begin
+        busy <= 1'b0;
+
+        if (rx) begin
+          if (hi_run == BITLEN - 1) begin
+            hi_run <= 0;
+            resync <= 1'b0;
+            state  <= ST_IDLE;
+          end else begin
+            hi_run <= hi_run + 1;
+          end
+        end else begin
+          hi_run <= 0;
+        end
+      end else begin
+        case (state)
+          ST_IDLE: begin
+            busy    <= 1'b0;
+            cnt     <= 0;
+            bit_idx <= 0;
+            hi_run  <= 0;
+
+            if (rx_q && !rx) begin
+              state <= ST_START;
+              cnt   <= (BITLEN / 2) - 1;
+            end
+          end
+
+          ST_START: begin
+            busy <= 1'b0;
+
+            if (cnt == 0) begin
+              if (!rx) begin
+                state   <= ST_DATA;
+                cnt     <= BITLEN - 1;
+                bit_idx <= 0;
+                shifter <= {DW{1'b0}};
+                busy    <= 1'b1;
+              end else begin
+                state   <= ST_IDLE;
+                cnt     <= 0;
+                bit_idx <= 0;
+              end
+            end else begin
+              cnt <= cnt - 1;
+            end
+          end
+
+          ST_DATA: begin
+            busy <= 1'b1;
+
+            if (cnt == 0) begin
+              shifter[bit_idx] <= rx;
+              cnt              <= BITLEN - 1;
+
+              if (bit_idx == DW - 1) begin
+                state   <= ST_STOP;
+                bit_idx <= 0;
+              end else begin
+                bit_idx <= bit_idx + 1;
+              end
+            end else begin
+              cnt <= cnt - 1;
+            end
+          end
+
+          ST_STOP: begin
+            busy <= 1'b1;
+
+            if (cnt == 0) begin
+              state <= ST_IDLE;
+              cnt   <= 0;
+
+              if (rx) begin
+                data  <= shifter;
+                valid <= 1'b1;
+              end else begin
+                frame_err <= 1'b1;
+                resync    <= 1'b1;
+                hi_run    <= 0;
+              end
+            end else begin
+              cnt <= cnt - 1;
+            end
+          end
+
+          default: begin
+            state   <= ST_IDLE;
+            cnt     <= 0;
+            bit_idx <= 0;
+            hi_run  <= 0;
+            resync  <= 1'b0;
+            busy    <= 1'b0;
+          end
+        endcase
+      end
     end
   end
-
-  // TODO: replace these placeholder outputs with the real ones.
-  assign data      = {DW{1'b0}};
-  assign valid     = 1'b0;
-  assign frame_err = 1'b0;
-  assign busy      = 1'b0;
 
 endmodule : srx
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /work/srx_core/rtl/srx.sv
The implementation is in place. Next I’m compiling it with Icarus in SystemVerilog mode, then I’ll run a small local timing test because the held-back checker is not in the repo.
shell
iverilog -g2012 -Wall -o /tmp/srx_compile.vvp rtl/srx_pkg.sv rtl/srx.sv rtl/srx_top.sv
output
Chunk ID: a7e15d
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 72
Output:
rtl/srx_top.sv:12: Include file srx_pkg.sv not found
warning: Some design elements have no explicit time unit and/or
       : time precision. This may cause confusing timing results.
       : Affected design elements are:
       :   -- package srx_pkg declared here: rtl/srx_pkg.sv:15
shell
sed -n '1,260p' rtl/srx.sv
output
Chunk ID: df6ea1
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 1393
Output:
//==============================================================================
// srx.sv
//
// Asynchronous serial byte receiver for a line that idles high.
//
// ---------------------------------------------------------------------------
//  >>> IMPLEMENTATION REQUIRED <<<
//
//  The module ports, parameters and a set of state registers are provided. The
//  frame FSM, the bit-period counter, the sampling datapath and the outputs are
//  left as a TODO and currently drive constant zeros, so the receiver produces
//  no useful result and the verifier FAILS.
//
//  Implement the behaviour described in instruction.md. Do NOT change the
//  module name, the port list, or the parameter list.
//
//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
//  function-call result) is a syntax error under `iverilog -g2012`. Index only
//  named signals, and do any bit work that needs an intermediate value inside a
//  `function automatic` using named locals.
// ---------------------------------------------------------------------------
//==============================================================================
`timescale 1ns/1ps

module srx #(
    parameter BITLEN = 16,
    parameter DW     = 8
) (
    input  logic               clk,
    input  logic               rst_n,    // asynchronous, active low
    input  logic               rx,       // serial input, idles high
    input  logic               take,     // downstream-consumed strobe
    output logic [DW-1:0]      data,      // received byte (level held)
    output logic               valid,     // 1-cycle good-frame strobe
    output logic               frame_err, // 1-cycle framing-error strobe
    output logic               busy        // high from confirmed start to result
);

  // ---------------------------------------------------------------------------
  // State registers (provided). You may add more if your design needs them.
  // ---------------------------------------------------------------------------
  logic [1:0]      state;      // frame FSM state
  integer          cnt;        // bit-period down-counter
  integer          bit_idx;    // which bit is being sampled
  logic [DW-1:0]   shifter;    // assembling byte
  integer          hi_run;     // line-idle (high-run) counter
  logic            rx_q;       // previous sampled rx value

  localparam [1:0] ST_IDLE   = 2'd0;
  localparam [1:0] ST_START  = 2'd1;
  localparam [1:0] ST_DATA   = 2'd2;
  localparam [1:0] ST_STOP   = 2'd3;

  logic            resync;     // post-framing-error idle qualification

  // ===========================================================================
  // Receiver FSM.
  // ===========================================================================
  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      state     <= 2'd0;
      cnt       <= 0;
      bit_idx   <= 0;
      shifter   <= {DW{1'b0}};
      hi_run    <= 0;
      rx_q      <= 1'b1;
      resync    <= 1'b0;
      data      <= {DW{1'b0}};
      valid     <= 1'b0;
      frame_err <= 1'b0;
      busy      <= 1'b0;
    end else begin
      rx_q      <= rx;
      valid     <= 1'b0;
      frame_err <= 1'b0;

      if (resync) begin
        busy <= 1'b0;

        if (rx) begin
          if (hi_run == BITLEN - 1) begin
            hi_run <= 0;
            resync <= 1'b0;
            state  <= ST_IDLE;
          end else begin
            hi_run <= hi_run + 1;
          end
        end else begin
          hi_run <= 0;
        end
      end else begin
        case (state)
          ST_IDLE: begin
            busy    <= 1'b0;
            cnt     <= 0;
            bit_idx <= 0;
            hi_run  <= 0;

            if (rx_q && !rx) begin
              state <= ST_START;
              cnt   <= (BITLEN / 2) - 1;
            end
          end

          ST_START: begin
            busy <= 1'b0;

            if (cnt == 0) begin
              if (!rx) begin
                state   <= ST_DATA;
                cnt     <= BITLEN - 1;
                bit_idx <= 0;
                shifter <= {DW{1'b0}};
                busy    <= 1'b1;
              end else begin
                state   <= ST_IDLE;
                cnt     <= 0;
                bit_idx <= 0;
              end
            end else begin
              cnt <= cnt - 1;
            end
          end

          ST_DATA: begin
            busy <= 1'b1;

            if (cnt == 0) begin
              shifter[bit_idx] <= rx;
              cnt              <= BITLEN - 1;

              if (bit_idx == DW - 1) begin
                state   <= ST_STOP;
                bit_idx <= 0;
              end else begin
                bit_idx <= bit_idx + 1;
              end
            end else begin
              cnt <= cnt - 1;
            end
          end

          ST_STOP: begin
            busy <= 1'b1;

            if (cnt == 0) begin
              state <= ST_IDLE;
              cnt   <= 0;

              if (rx) begin
                data  <= shifter;
                valid <= 1'b1;
              end else begin
                frame_err <= 1'b1;
                resync    <= 1'b1;
                hi_run    <= 0;
              end
            end else begin
              cnt <= cnt - 1;
            end
          end

          default: begin
            state   <= ST_IDLE;
            cnt     <= 0;
            bit_idx <= 0;
            hi_run  <= 0;
            resync  <= 1'b0;
            busy    <= 1'b0;
          end
        endcase
      end
    end
  end

endmodule : srx
The first compile command found an include-path issue in the wrapper invocation, not in `srx.sv` itself. I’m rerunning with the RTL include directory exactly as the wrapper expects.
shell
iverilog -g2012 -Wall -I rtl -o /tmp/srx_compile.vvp rtl/srx.sv rtl/srx_top.sv
output
Chunk ID: c9b999
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 34
Output:
rtl/srx_pkg.sv:15: warning: timescale for srx_pkg inherited from another file.
rtl/srx_top.sv:10: ...: The inherited timescale is here.
The compile succeeds with the correct include path. I’m adding a temporary self-checking testbench outside the repo to exercise the edge cases: rejected start glitch, LSB-first byte delivery, framing error, and the post-error high-run gate.
apply_patch
*** Begin Patch
*** Add File: /tmp/srx_tb.sv
+`timescale 1ns/1ps
+
+module tb;
+  localparam BITLEN = 8;
+  localparam DW = 8;
+
+  logic clk = 1'b0;
+  logic rst_n = 1'b0;
+  logic rx = 1'b1;
+  logic take = 1'b0;
+  logic [DW-1:0] data;
+  logic valid;
+  logic frame_err;
+  logic busy;
+  integer valid_count;
+  integer err_count;
+  integer busy_count;
+
+  always #5 clk = ~clk;
+
+  srx #(.BITLEN(BITLEN), .DW(DW)) dut (
+    .clk(clk),
+    .rst_n(rst_n),
+    .rx(rx),
+    .take(take),
+    .data(data),
+    .valid(valid),
+    .frame_err(frame_err),
+    .busy(busy)
+  );
+
+  task automatic tick;
+    begin
+      @(posedge clk);
+      #1;
+    end
+  endtask
+
+  task automatic drive_cycles(input logic value, input integer cycles);
+    integer i;
+    begin
+      for (i = 0; i < cycles; i = i + 1) begin
+        @(negedge clk);
+        rx = value;
+        tick();
+      end
+    end
+  endtask
+
+  task automatic idle_cycles(input integer cycles);
+    begin
+      drive_cycles(1'b1, cycles);
+    end
+  endtask
+
+  task automatic send_frame(input [7:0] value, input logic stop_value);
+    integer i;
+    begin
+      drive_cycles(1'b0, BITLEN);
+      for (i = 0; i < 8; i = i + 1) begin
+        drive_cycles(value[i], BITLEN);
+      end
+      drive_cycles(stop_value, BITLEN);
+    end
+  endtask
+
+  always @(posedge clk or negedge rst_n) begin
+    if (!rst_n) begin
+      valid_count <= 0;
+      err_count   <= 0;
+      busy_count  <= 0;
+    end else begin
+      if (valid) valid_count <= valid_count + 1;
+      if (frame_err) err_count <= err_count + 1;
+      if (busy) busy_count <= busy_count + 1;
+    end
+  end
+
+  initial begin
+    repeat (3) tick();
+    rst_n = 1'b1;
+    idle_cycles(3);
+
+    drive_cycles(1'b0, (BITLEN / 2) - 1);
+    drive_cycles(1'b1, BITLEN);
+    if (busy_count != 0 || valid_count != 0 || err_count != 0) begin
+      $fatal(1, "glitch started receiver");
+    end
+
+    send_frame(8'hA5, 1'b1);
+    idle_cycles(2);
+    if (valid_count != 1 || err_count != 0 || data != 8'hA5) begin
+      $fatal(1, "good frame failed: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
+    end
+
+    send_frame(8'h3c, 1'b0);
+    idle_cycles(2);
+    if (valid_count != 1 || err_count != 1 || data != 8'hA5) begin
+      $fatal(1, "bad frame failed: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
+    end
+
+    idle_cycles(BITLEN - 1);
+    send_frame(8'h55, 1'b1);
+    idle_cycles(2);
+    if (valid_count != 1 || err_count != 1 || data != 8'hA5) begin
+      $fatal(1, "resync accepted early start: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
+    end
+
+    idle_cycles(BITLEN);
+    send_frame(8'h5a, 1'b1);
+    idle_cycles(2);
+    if (valid_count != 2 || err_count != 1 || data != 8'h5a) begin
+      $fatal(1, "post-resync good frame failed: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
+    end
+
+    $display("PASS");
+    $finish;
+  end
+endmodule
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
A /tmp/srx_tb.sv
shell
iverilog -g2012 -Wall -I rtl -o /tmp/srx_tb.vvp rtl/srx.sv /tmp/srx_tb.sv && vvp /tmp/srx_tb.vvp
output
Chunk ID: ce06a6
Wall time: 0.0000 seconds
Process exited with code 1
Original token count: 28
Output:
FATAL: /tmp/srx_tb.sv:106: resync accepted early start: valid=2 err=1 data=55
       Time: 2696000 Scope: tb
The first local testbench exposed a test issue rather than an RTL issue: I had already given the receiver enough high cycles before the “early” post-error start. I’m tightening that check so the early falling edge really happens before the full idle window.
apply_patch
*** Begin Patch
*** Update File: /tmp/srx_tb.sv
@@
     send_frame(8'h3c, 1'b0);
-    idle_cycles(2);
     if (valid_count != 1 || err_count != 1 || data != 8'hA5) begin
       $fatal(1, "bad frame failed: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
     end
 
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/srx_tb.sv
shell
iverilog -g2012 -Wall -I rtl -o /tmp/srx_tb.vvp rtl/srx.sv /tmp/srx_tb.sv && vvp /tmp/srx_tb.vvp
output
Chunk ID: 72316d
Wall time: 0.0000 seconds
Process exited with code 1
Original token count: 28
Output:
FATAL: /tmp/srx_tb.sv:112: post-resync good frame failed: valid=1 err=2 data=a5
       Time: 3576000 Scope: tb
That stricter test also clarified an important point: a full bit-period of high anywhere after an error satisfies the specified re-idle condition. I’m changing the local check to use a plain premature low pulse before the high-run completes, then a clean idle period and a good frame.
apply_patch
*** Begin Patch
*** Update File: /tmp/srx_tb.sv
@@
     idle_cycles(BITLEN - 1);
-    send_frame(8'h55, 1'b1);
-    idle_cycles(2);
+    drive_cycles(1'b0, BITLEN);
     if (valid_count != 1 || err_count != 1 || data != 8'hA5) begin
-      $fatal(1, "resync accepted early start: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
+      $fatal(1, "resync reacted to early low: valid=%0d err=%0d data=%02h", valid_count, err_count, data);
     end
 
     idle_cycles(BITLEN);
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/srx_tb.sv
shell
iverilog -g2012 -Wall -I rtl -o /tmp/srx_tb.vvp rtl/srx.sv /tmp/srx_tb.sv && vvp /tmp/srx_tb.vvp
output
Chunk ID: fa89d5
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 2
Output:
PASS
The focused timing test now passes. I’m doing one more compile variant that includes the package file explicitly, since graders often invoke all RTL files at once and that can expose include-order or duplicate-definition problems.
shell
iverilog -g2012 -Wall -I rtl -o /tmp/srx_compile_all.vvp rtl/srx_pkg.sv rtl/srx.sv rtl/srx_top.sv
output
Chunk ID: da5160
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 58
Output:
warning: Some design elements have no explicit time unit and/or
       : time precision. This may cause confusing timing results.
       : Affected design elements are:
       :   -- package srx_pkg declared here: rtl/srx_pkg.sv:15
Both compile shapes pass; the only warning is the existing package timescale warning. I’m reading the final file with line numbers now so I can give you exact references.
shell
nl -ba rtl/srx.sv | sed -n '1,240p'
output
Chunk ID: 3fd6b4
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 1701
Output:
     1	//==============================================================================
     2	// srx.sv
     3	//
     4	// Asynchronous serial byte receiver for a line that idles high.
     5	//
     6	// ---------------------------------------------------------------------------
     7	//  >>> IMPLEMENTATION REQUIRED <<<
     8	//
     9	//  The module ports, parameters and a set of state registers are provided. The
    10	//  frame FSM, the bit-period counter, the sampling datapath and the outputs are
    11	//  left as a TODO and currently drive constant zeros, so the receiver produces
    12	//  no useful result and the verifier FAILS.
    13	//
    14	//  Implement the behaviour described in instruction.md. Do NOT change the
    15	//  module name, the port list, or the parameter list.
    16	//
    17	//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
    18	//  function-call result) is a syntax error under `iverilog -g2012`. Index only
    19	//  named signals, and do any bit work that needs an intermediate value inside a
    20	//  `function automatic` using named locals.
    21	// ---------------------------------------------------------------------------
    22	//==============================================================================
    23	`timescale 1ns/1ps
    24	
    25	module srx #(
    26	    parameter BITLEN = 16,
    27	    parameter DW     = 8
    28	) (
    29	    input  logic               clk,
    30	    input  logic               rst_n,    // asynchronous, active low
    31	    input  logic               rx,       // serial input, idles high
    32	    input  logic               take,     // downstream-consumed strobe
    33	    output logic [DW-1:0]      data,      // received byte (level held)
    34	    output logic               valid,     // 1-cycle good-frame strobe
    35	    output logic               frame_err, // 1-cycle framing-error strobe
    36	    output logic               busy        // high from confirmed start to result
    37	);
    38	
    39	  // ---------------------------------------------------------------------------
    40	  // State registers (provided). You may add more if your design needs them.
    41	  // ---------------------------------------------------------------------------
    42	  logic [1:0]      state;      // frame FSM state
    43	  integer          cnt;        // bit-period down-counter
    44	  integer          bit_idx;    // which bit is being sampled
    45	  logic [DW-1:0]   shifter;    // assembling byte
    46	  integer          hi_run;     // line-idle (high-run) counter
    47	  logic            rx_q;       // previous sampled rx value
    48	
    49	  localparam [1:0] ST_IDLE   = 2'd0;
    50	  localparam [1:0] ST_START  = 2'd1;
    51	  localparam [1:0] ST_DATA   = 2'd2;
    52	  localparam [1:0] ST_STOP   = 2'd3;
    53	
    54	  logic            resync;     // post-framing-error idle qualification
    55	
    56	  // ===========================================================================
    57	  // Receiver FSM.
    58	  // ===========================================================================
    59	  always_ff @(posedge clk or negedge rst_n) begin
    60	    if (!rst_n) begin
    61	      state     <= 2'd0;
    62	      cnt       <= 0;
    63	      bit_idx   <= 0;
    64	      shifter   <= {DW{1'b0}};
    65	      hi_run    <= 0;
    66	      rx_q      <= 1'b1;
    67	      resync    <= 1'b0;
    68	      data      <= {DW{1'b0}};
    69	      valid     <= 1'b0;
    70	      frame_err <= 1'b0;
    71	      busy      <= 1'b0;
    72	    end else begin
    73	      rx_q      <= rx;
    74	      valid     <= 1'b0;
    75	      frame_err <= 1'b0;
    76	
    77	      if (resync) begin
    78	        busy <= 1'b0;
    79	
    80	        if (rx) begin
    81	          if (hi_run == BITLEN - 1) begin
    82	            hi_run <= 0;
    83	            resync <= 1'b0;
    84	            state  <= ST_IDLE;
    85	          end else begin
    86	            hi_run <= hi_run + 1;
    87	          end
    88	        end else begin
    89	          hi_run <= 0;
    90	        end
    91	      end else begin
    92	        case (state)
    93	          ST_IDLE: begin
    94	            busy    <= 1'b0;
    95	            cnt     <= 0;
    96	            bit_idx <= 0;
    97	            hi_run  <= 0;
    98	
    99	            if (rx_q && !rx) begin
   100	              state <= ST_START;
   101	              cnt   <= (BITLEN / 2) - 1;
   102	            end
   103	          end
   104	
   105	          ST_START: begin
   106	            busy <= 1'b0;
   107	
   108	            if (cnt == 0) begin
   109	              if (!rx) begin
   110	                state   <= ST_DATA;
   111	                cnt     <= BITLEN - 1;
   112	                bit_idx <= 0;
   113	                shifter <= {DW{1'b0}};
   114	                busy    <= 1'b1;
   115	              end else begin
   116	                state   <= ST_IDLE;
   117	                cnt     <= 0;
   118	                bit_idx <= 0;
   119	              end
   120	            end else begin
   121	              cnt <= cnt - 1;
   122	            end
   123	          end
   124	
   125	          ST_DATA: begin
   126	            busy <= 1'b1;
   127	
   128	            if (cnt == 0) begin
   129	              shifter[bit_idx] <= rx;
   130	              cnt              <= BITLEN - 1;
   131	
   132	              if (bit_idx == DW - 1) begin
   133	                state   <= ST_STOP;
   134	                bit_idx <= 0;
   135	              end else begin
   136	                bit_idx <= bit_idx + 1;
   137	              end
   138	            end else begin
   139	              cnt <= cnt - 1;
   140	            end
   141	          end
   142	
   143	          ST_STOP: begin
   144	            busy <= 1'b1;
   145	
   146	            if (cnt == 0) begin
   147	              state <= ST_IDLE;
   148	              cnt   <= 0;
   149	
   150	              if (rx) begin
   151	                data  <= shifter;
   152	                valid <= 1'b1;
   153	              end else begin
   154	                frame_err <= 1'b1;
   155	                resync    <= 1'b1;
   156	                hi_run    <= 0;
   157	              end
   158	            end else begin
   159	              cnt <= cnt - 1;
   160	            end
   161	          end
   162	
   163	          default: begin
   164	            state   <= ST_IDLE;
   165	            cnt     <= 0;
   166	            bit_idx <= 0;
   167	            hi_run  <= 0;
   168	            resync  <= 1'b0;
   169	            busy    <= 1'b0;
   170	          end
   171	        endcase
   172	      end
   173	    end
   174	  end
   175	
   176	endmodule : srx
Implemented `rtl/srx.sv`. Key points: - Start bit is only confirmed at `BITLEN/2`; glitches never raise `busy`. - Data bits are sampled LSB-first every `BITLEN`. - `valid`, `frame_err`, `busy`, and `data` are all registered. - `data` only updates on good frames. - Framing errors trigger the required full-`BITLEN` high resync window. Verified with: - `iverilog -g2012 -Wall -I rtl -o /tmp/srx_compile.vvp rtl/srx.sv rtl/srx_top.sv` - `iverilog -g2012 -Wall -I rtl -o /tmp/srx_compile_all.vvp rtl/srx_pkg.sv rtl/srx.sv rtl/srx_top.sv` - A temporary self-checking testbench covering glitch rejection, good frame, bad frame, and post-error resync. Only warning seen is the existing `srx_pkg.sv` timescale warning.
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.sh: compiling with iverilog...
----- iverilog output -----
---------------------------
test.sh: running simulation with vvp...
----- simulation output -----
ERROR busy16 mismatch @t=1606000 got=1 exp=0
ERROR busy16 mismatch @t=3476000 got=1 exp=0
ERROR busy16 mismatch @t=5346000 got=1 exp=0
ERROR busy16 mismatch @t=7216000 got=1 exp=0
ERROR busy16 mismatch @t=10426000 got=1 exp=0
ERROR busy16 mismatch @t=12626000 got=1 exp=0
ERROR busy16 mismatch @t=14496000 got=1 exp=0
ERROR busy16 mismatch @t=18196000 got=1 exp=0
ERROR busy16 mismatch @t=20066000 got=1 exp=0
ERROR busy16 mismatch @t=21686000 got=1 exp=0
ERROR busy16 mismatch @t=23556000 got=1 exp=0
ERROR busy16 mismatch @t=25876000 got=1 exp=0
ERROR busy8 mismatch @t=27266000 got=1 exp=0
ERROR busy16 mismatch @t=28896000 got=1 exp=0
ERROR busy8 mismatch @t=29986000 got=1 exp=0
ERROR busy16 mismatch @t=31656000 got=1 exp=0
ERROR busy8 mismatch @t=32596000 got=1 exp=0
ERROR busy8 mismatch @t=33746000 got=1 exp=0
ERROR busy16 mismatch @t=35406000 got=1 exp=0
ERROR busy8 mismatch @t=36336000 got=1 exp=0
INFO samples=56542 dmiss=0 vmiss=0 femiss=0 bymiss=196
SAMPLES=56542
TEST FAILED total_err=196 samples=56542
-----------------------------
test.sh: FAIL - failure marker present in simulation output

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_22fc534995d6467e. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_22fc534995d6467e · verifier authoritative; classifier explanatory.