VERILOG2122: “always_ff” block should not contain any blocking assignments for non-local variables

Summary

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Reliability	Security	Maintainability	Code attribute	Effort to fix	Tags
HIGH	-	-	Intentionality | Logical	20min	synthesis, vscode, simulation, linter

Description

always_ff block should not contain any blocking assignments for non-local variables.

Noncompliant code example

always_ff @(posedge clk) begin
  y = 0; // Noncompliant: Blocking assignment
  z <= 1; // Compliant: Non-blocking assignment
  int k;
  k = 42; // Compliant: Blocking assignment on local variable
end

Compliant solution

always_ff @(posedge clk) begin
  y = 0; // Compliant: Non-blocking assignment
  z <= 1; // Compliant: Non-blocking assignment
  int k;
  k = 42; // Compliant: Blocking assignment on local variable
end

Articles & blog posts

• Nonblocking Assignments in Verilog Synthesis, Coding Styles That Kill! - SNUG-2000 - Clifford E. Cummings

Parameters

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