An n-bit binary counter can be constructed using a modified n-bit register where the data inputs for the register come from a decrementer (subtractor) for a down counter. Starting with a value stored in a register, to get to the next down count sequence, we simply have to substract a one to it.
A 4-bit binary down counter with asynchronous preset :
The behavioral VHDL code for the 4-bit binary down counter is shown in Fig. 2.13.2. The statement USE IEEE.STD_LOGIC_UNSIGNED.ALL is needed in order to perform subtractions on STD_LOGIC_VECTORs.
use ieee.std_logic_unsigned.all; -- need this to subtract std_logic_vectors
entity counter is
port (clock: in std_logic;
preset: in std_logic;
count: in std_logic;
q : out std_logic_vector(3 downto 0));
architecture behav of counter is
signal value: std_logic_vector(3 downto 0);
process (clock, clear)
if preset = '1' then
value <= (others => '1'); -- 4-bit vector of 1, same as "1111"
elsif (clock'event and clock='1') then
if count = '1' then
value <= value - 1;
q <= value;
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