Chip Coverage

Silicon Hub

Blocking vs. Non-blocking Assignments

Blocking vs. Non-blocking Assignments in SystemVerilog

Blocking Assignment (=)

  • Immediate Execution: In blocking assignments, the right-hand side of the assignment is evaluated immediately, and the result is assigned to the left-hand side variable.
  • Sequential Execution: Assignments are executed sequentially in the order they appear in the code.
  • Example:
a = b; <span class="hljs-comment">// Value of 'b' is assigned to 'a' immediately</span>
c = a + <span class="hljs-number">1</span>; <span class="hljs-comment">// Value of 'a' (after the previous assignment) is used to calculate 'c'</span>

Non-blocking Assignment (<=)

  • Scheduled Execution: Non-blocking assignments are scheduled to occur at the end of the current time step. The actual assignment happens after all non-blocking assignments within the same time step are evaluated.
  • Concurrent Execution: Multiple non-blocking assignments within the same always block are scheduled concurrently.
  • Example:
a <= b; <span class="hljs-comment">// Schedule the assignment of 'b' to 'a' for the end of the time step</span>
c <= a + <span class="hljs-number">1</span>; <span class="hljs-comment">// Schedule the assignment of 'a+1' to 'c' for the end of the time step</span>

Key Differences and Implications:

Feature Blocking Assignment (=) Non-blocking Assignment (<=)
Execution Immediate Scheduled
Order Sequential Concurrent
Timing Can introduce unintended delays Essential for modeling sequential logic
Use Cases Combinational logic, procedural blocks with sequential execution Sequential logic, register updates, modeling clocked circuits

Illustrative Example:

<span class="hljs-keyword">always</span> @ (<span class="hljs-keyword">posedge</span> clk) <span class="hljs-keyword">begin</span>
  a = b; <span class="hljs-comment">// Blocking assignment</span>
  c <= a + <span class="hljs-number">1</span>; <span class="hljs-comment">// Non-blocking assignment</span>
<span class="hljs-keyword">end</span>

In this example:

  1. At the positive edge of clk, b is assigned to a immediately.
  2. The value of a (after the blocking assignment) is used to calculate c.
  3. The assignment of a and the calculation of c are scheduled to happen at the end of the current time step.

Important Notes:

  • Non-blocking assignments are crucial for modeling sequential logic and updating registers at the rising or falling edge of the clock.
  • Blocking assignments are typically used within combinational logic or for immediate assignments within a procedural block.
  • Using blocking assignments within an always block with a sensitivity list that includes the signals being assigned can lead to unexpected behavior.