How to Avoid Race Condition in Verilog: Causes and Fixes

To avoid race conditions in Verilog, use non-blocking assignments (<=) inside always @(posedge clock) blocks for sequential logic and avoid mixing blocking (=) and non-blocking assignments improperly. This ensures signals update predictably on clock edges, preventing simultaneous conflicting updates.

Why This Happens

A race condition in Verilog happens when multiple assignments to the same signal occur in the same simulation time step, causing unpredictable results. This often occurs when blocking assignments (=) are used in sequential logic, making the order of execution affect the final value.

verilog

module race_condition_example(input clk, input rst, input d, output reg q);
  always @(posedge clk or posedge rst) begin
    if (rst) begin
      q = 0;  // Blocking assignment causes race
    end else begin
      q = d;  // Blocking assignment causes race
    end
  end
endmodule

Output

Simulation may show unpredictable or incorrect values for q due to race conditions.

The Fix

Use non-blocking assignments (<=) inside always @(posedge clk or posedge rst) blocks for sequential logic. This schedules updates to happen after all right-hand side evaluations, avoiding race conditions and ensuring stable, predictable signal updates.

verilog

module fixed_example(input clk, input rst, input d, output reg q);
  always @(posedge clk or posedge rst) begin
    if (rst) begin
      q <= 0;  // Non-blocking assignment fixes race
    end else begin
      q <= d;  // Non-blocking assignment fixes race
    end
  end
endmodule

Output

Signal q updates correctly on clock edges without unpredictable behavior.

Prevention

To prevent race conditions in Verilog:

• Use non-blocking assignments (<=) for sequential logic inside clocked always blocks.
• Use blocking assignments (=) only for combinational logic.
• Avoid mixing blocking and non-blocking assignments on the same signal.
• Keep signal assignments consistent and clear in your design.
• Use linting tools that warn about potential race conditions.

Related Errors

Other common errors related to race conditions include:

• Glitches: Temporary incorrect outputs due to improper combinational logic.
• Multiple drivers: Assigning the same signal in multiple places causing conflicts.
• Incorrect sensitivity lists: Missing signals in always @(*) blocks causing simulation mismatches.

Fixes usually involve proper use of assignments, correct sensitivity lists, and clear signal ownership.

Key Takeaways

Always use non-blocking assignments (<=) for sequential logic to avoid race conditions.

Use blocking assignments (=) only for combinational logic.

Never mix blocking and non-blocking assignments on the same signal.

Keep clocked always blocks clean and consistent.

Use linting tools to catch potential race conditions early.