An n-bit binary counter can be constructed using a modified n-bit register where the data inputs for the register come from an
incrementer (adder) for an up counter. Starting with a value stored in a
register, to get to the next up count sequence, we simply have to add a one
A 4-bit binary up counter with asynchronous clear :
The behavioral VHDL code for the 4-bit binary up counter is shown in Fig. 2.13.1. The statement USE IEEE.STD_LOGIC_UNSIGNED.ALL is needed in order to perform additions on STD_LOGIC_VECTORs.
use ieee.std_logic_unsigned.all; -- need this to add std_logic_vectors
entity counter is
port (clock: in std_logic;
clear: in std_logic;
count: in std_logic;
q : out std_logic_vector(3 downto 0)); -- no. of bits can be change in this line.
architecture behav of counter is
signal value: std_logic_vector(3 downto 0); -- no. of bits can be change in this line
process (clock, clear)
if clear = '1' then
value <= (others => '0'); -- 4-bit vector of 0, same as "0000"
elsif (clock'event and clock='1') then
if count = '1' then
value <= value + 1; -- + sign for up count
q <= value;