Verilog代码求助：16位输入首比特未识别及阻塞非阻塞赋值混用问题

Hey there! Let's break down and fix the two issues you're facing with your Verilog FSM for pattern matching.

1. Fixing the First Data Bit Not Being Processed #

The first bit getting ignored usually boils down to one of a few common misconfigurations:

• Counter Initialization: If your index i starts at 1 instead of 0, you’re skipping the first bit entirely. Double-check your reset logic—make sure i is set to 0 when reset, not 1.
• FSM Initial State: Maybe your FSM’s idle state isn’t set to start sampling the first bit immediately. For example, if you’re waiting for a trigger signal before starting pattern matching, ensure the first input bit triggers the transition out of the idle state.
• Input Sampling Delay: If you’re shifting bits into a register, confirm the first bit is loaded on the first clock edge. A stray flip-flop in the input path could delay sampling by one cycle, causing the first bit to be missed.

Quick debug tip: Add a testbench that feeds only the first bit of your target pattern and check if the FSM reacts to it. You can also add a debug signal to track when the first bit is received.

2. Resolving Mixed Blocking/Non-Blocking Assignments on i #

Mixing blocking (=) and non-blocking (<=) assignments on the same variable is a classic Verilog pitfall—it creates race conditions that make your code’s behavior unpredictable. Here’s how to fix it properly:

Why This Breaks Things #

Blocking assignments take effect immediately in the current time step, while non-blocking assignments schedule updates for the next clock cycle. When you mix them on i, the value of i can flip-flop between old and new values within a single cycle, leading to wrong counts or state transitions.

The Correct Fix #

If i is a sequential counter (updated on clock edges), all assignments to i must use non-blocking operators. For example, if your current code looks like this:

always @(posedge clk) begin
    if (reset) begin
        i = 0; // Blocking assignment here
    end else begin
        i <= i + 1; // Non-blocking here
    end
end

Change the blocking reset assignment to non-blocking:

always @(posedge clk) begin
    if (reset) begin
        i <= 0; // Now non-blocking
    end else begin
        i <= i + 1;
    end
end

If you had combinational logic using i (like state transitions), move that to a separate combinational always @(*) block where blocking assignments are safe:

// Sequential counter update (all non-blocking)
always @(posedge clk) begin
    if (reset)
        i <= 0;
    else if (sampling_enabled)
        i <= i + 1;
end

// Combinational state logic (blocking is okay here)
always @(*) begin
    next_state = current_state; // Blocking assignment
    case(i)
        0: if (input_bit == pattern[0]) next_state = STATE_1;
        1: if (input_bit == pattern[1]) next_state = STATE_2;
        // ... rest of your pattern checks
    endcase
end

Why Removing One Type Broke Your Code #

If you previously mixed assignments, switching to all blocking might have caused i to increment multiple times in one cycle (since blocking assignments take effect right away). Switching to all non-blocking without adjusting combinational logic might have meant your state transitions were using the old value of i instead of the updated one. Separating sequential and combinational logic into distinct blocks fixes this conflict.

Quick Final Tips #

• Add Debug Signals: Output current_state and i to your testbench so you can see exactly what’s happening on each clock edge.
• Testbench Coverage: Write a test that feeds the first bit of your target pattern and verifies the FSM transitions correctly. This will confirm the first bit issue is resolved.

内容的提问来源于stack exchange，提问作者sandeep thagaram