Counters

Counters are digital circuits that increment or decrement by 1, often used in conjunction with a button that we press to change the value of our counter.

HDL implementation

Say we want an active-low parallel load loadn, and an asynchronous active-low reset resetn.

module count8 (clock, loadn, resetn, Q, D);
	input clock, resetn, loadn;
	input [7:0] D;
	//..
	always@(posedge clock, negedge resetn)
	begin
		if (resetn == 1'b0)
			Q <= 'b0;
		else if (loadn == 1'b0)
			Q <= D;
		else
			Q <= Q + 1'b1;
	end
endmodule

Under the hood

Under the hood, counters are implemented with T flip-flops. We may add additional logic to only change the value if a signal is passed through (i.e., a button is pressed). Consider this 3-bit up-counter schematic: Here’s a sample timing diagram:¹

See also

• Rate divider, which uses counters

Footnotes

1. From Prof Korst’s lecture notes. ↩