Counters

Counters are registers that advance through a numeric sequence — free-running, gated, loadable, or up/down.

Basic binary counter

always_ff @(posedge clk or negedge rst_n) begin
  if (!rst_n)      cnt <= '0;
  else if (clear)  cnt <= '0;
  else if (enable) cnt <= cnt + 1'b1;
end

Decide wrap behavior explicitly:

else if (enable) cnt <= (cnt == MAX) ? '0 : cnt + 1'b1;

Useful variants

Terminal count

Generate a single-cycle (or level) flag:

assign tc = enable && (cnt == MAX);

If tc must be clean for async consumers, register it.

Width & overflow

• Size cnt for MAX ($clog2(MAX+1)).
• cnt + 1 on an all-ones vector wraps in two’s complement — intentional for free-running timers, bugs for “stop at MAX” designs.

Test ideas

• Reset then count N cycles; check value.
• Enable low → hold.
• Load mid-count.
• Wrap boundary (MAX → 0).