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@RickKimball RickKimball/nco.vhd
Last active Oct 19, 2017

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NCO - Arbitrary Frequency Generator VHDL
-------------------------------------------------------------------------
-- nco.vhd - Numerically Controlled Oscillator
--
-- Author: Rick Kimball rick@kimballsoftware.com
-- Date: 10/19/2017
-- Version: 3
--
-- vim: set ts=3 sw=3 expandtab
--
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
-------------------------------------------------------------------------
--
--
entity nco is
generic(
clk_freq : integer := 100000000; -- input frequency
freq : integer := 100000 -- desired output frequency
);
port(
reset : in std_logic; -- active low
clk_in : in std_logic; -- input clock of clk_freq
clk_out : out std_logic -- output square wave of freq
);
end nco;
-------------------------------------------------------------------------
--
--
architecture behavior of nco is
constant increment : integer := (freq*2)-clk_freq;
signal accumulator : integer range increment to (freq*2):= 0;
signal phase_change : std_logic := '0';
begin
process(clk_in, reset)
begin
if (reset = '0') then
accumulator <= 0;
phase_change <= '0';
elsif rising_edge(clk_in) then
if ( accumulator + increment >= 0 ) then
accumulator <= accumulator + increment;
phase_change <= not phase_change;
clk_out <= phase_change;
else
accumulator <= accumulator + increment + clk_freq;
end if;
end if;
end process;
end behavior;
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