VHDL: Behavioral Counter
This example implements a behavioral counter with load, clear, and up/down features. It has not been optimized for a particular device architecture, so performance may vary. Altera recommends using the lpm_counter function to implement a counter (see VHDL: Down Counter). This example is provided to show counter implementation that does not require the LPM.
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counters.vhd
ENTITY counters IS
PORT(
d : IN INTEGER RANGE 0 TO 255;
clk : IN BIT;
clear : IN BIT;
load : IN BIT;
up_down : IN BIT;
qd : OUT INTEGER RANGE 0 TO 255);
END counters;
ARCHITECTURE a OF counters IS
BEGIN
-- An up/down counter
PROCESS (clk)
VARIABLE cnt : INTEGER RANGE 0 TO 255;
VARIABLE direction : INTEGER;
BEGIN
IF (up_down = '1') THEN --Generate up/down counter
direction := 1;
ELSE
direction := -1;
END IF;
IF (clk'EVENT AND clk = '1') THEN
IF (load = '1') THEN --Generate loadable
cnt := d; --counter. Take these
ELSE --lines out to increase performance.
cnt := cnt + direction;
END IF;
--The following lines will produce a synchronous
--clear on the counter
IF (clear = '0') THEN
cnt := 0;
END IF;
END IF;
qd <= cnt; --Generate outputs
END PROCESS;
END a;
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