Sensitivity List

The sensitivity list tells the always block when to execute. It is written inside parentheses after the @ symbol.

The sensitivity list is a set of signals or events that trigger the always block. When any signal in the list changes, the block executes.

Syntax:

always @(sensitivity_list) begin
  // Code runs when signals in the list change
end

Types of Sensitivity List

Option 1: All Signals (*)

The safest and most common for combinational logic.

always @(*) begin
  out = a & b;   // Runs when a or b changes
end

The * automatically includes all signals read in the block.

Option 2: Specific Signals

always @(a or b) begin
  out = a & b;   // Runs when a or b changes
end

If you forget a signal, the output does not update in simulation when that signal changes — so the simulation no longer matches the synthesized hardware.

Option 3: Edge Trigger (posedge)

always @(posedge clk) begin
  q <= d;        // Runs on rising edge of clock
end

Use posedge for rising edge, negedge for falling edge.

Option 4: Multiple Edges

always @(posedge clk or posedge reset) begin
  if (reset)
    q <= 0;
  else
    q <= d;
end

Runs on clock edge or reset edge.

Common Mistakes