always_comb, always_ff, and always_latch

Choose the right procedural block and avoid multiple drivers, unintended latches, and sensitivity mistakes.

State what the block represents

Verilog uses always for several kinds of logic. SystemVerilog provides three specialized forms:

• always_comb for combinational logic;
• always_ff for registers and flip-flops;
• always_latch for an intentional latch.

These blocks describe intent and enable extra checks. A variable written in one of them should not also be written elsewhere.

Combinational logic

always_comb automatically builds its sensitivity from the signals that are read, including signals read by a called function. The block also runs once at the beginning of simulation.

module alu4 (
  input  logic [3:0] i_a,
  input  logic [3:0] i_b,
  input  logic [1:0] i_op,
  output logic [3:0] o_y
);
 
  always_comb begin
    o_y = '0;
    case (i_op)
      2'b00: o_y = i_a + i_b;
      2'b01: o_y = i_a - i_b;
      2'b10: o_y = i_a & i_b;
      2'b11: o_y = i_a | i_b;
    endcase
  end
 
endmodule

The default value ensures an assignment on every path. Without it, an incomplete if or case can request storage of the previous value and infer a latch.

Blocking assignments = fit combinational calculations because statements in the block can build ordered intermediate results.

Sequential logic

always_ff accepts a clock event and, depending on the chosen style, an asynchronous reset event.

always_ff @(posedge i_clk or negedge i_rst_n) begin
  if (!i_rst_n)
    o_q <= '0;
  else if (i_enable)
    o_q <= i_d;
end

Nonblocking assignments <= model registers updating together after the edge. No else branch is needed when i_enable is low because a flip-flop naturally holds its value.

Use a latch only on purpose

always_latch states that level-sensitive storage is intentional.

always_latch begin
  if (i_gate)
    o_q <= i_d;
end

Latches need careful timing analysis and are rarely needed in a typical FPGA flow. If an always_comb block infers a latch, look for a missing assignment instead of simply changing the keyword to always_latch.

Key points

• A specialized block lets tools check intent more closely.
• always_comb removes the need for a handwritten sensitivity list.
• Every combinational output needs a value on every path.
• Use nonblocking assignments in always_ff.
• Use always_latch only when the architecture really calls for a latch.

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