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|
-------------------------------------------------------------------------------
--
-- Synthesizable model of TI's SN76489AN.
--
-- $Id: sn76489_tone.vhd,v 1.5 2006/02/27 20:30:10 arnim Exp $
--
-- Tone Generator
--
-------------------------------------------------------------------------------
--
-- Copyright (c) 2005, 2006, Arnim Laeuger (arnim.laeuger@gmx.net)
--
-- All rights reserved
--
-- Redistribution and use in source and synthezised forms, with or without
-- modification, are permitted provided that the following conditions are met:
--
-- Redistributions of source code must retain the above copyright notice,
-- this list of conditions and the following disclaimer.
--
-- Redistributions in synthesized form must reproduce the above copyright
-- notice, this list of conditions and the following disclaimer in the
-- documentation and/or other materials provided with the distribution.
--
-- Neither the name of the author nor the names of other contributors may
-- be used to endorse or promote products derived from this software without
-- specific prior written permission.
--
-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-- POSSIBILITY OF SUCH DAMAGE.
--
-- Please report bugs to the author, but before you do so, please
-- make sure that this is not a derivative work and that
-- you have the latest version of this file.
--
-------------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity sn76489_tone is
port (
clock_i : in std_logic;
clk_en_i : in boolean;
res_n_i : in std_logic;
we_i : in boolean;
d_i : in std_logic_vector(0 to 7);
r2_i : in std_logic;
ff_o : out std_logic;
tone_o : out signed(0 to 7)
);
end sn76489_tone;
use work.sn76489_comp_pack.sn76489_attenuator;
architecture rtl of sn76489_tone is
signal f_q : std_logic_vector(0 to 9);
signal a_q : std_logic_vector(0 to 3);
signal freq_cnt_q : unsigned(0 to 9);
signal freq_ff_q : std_logic;
signal freq_s : signed(0 to 1);
function all_zero(a : in std_logic_vector) return boolean is
variable result_v : boolean;
begin
result_v := true;
for idx in a'low to a'high loop
if a(idx) /= '0' then
result_v := false;
end if;
end loop;
return result_v;
end;
begin
-----------------------------------------------------------------------------
-- Process cpu_regs
--
-- Purpose:
-- Implements the registers writable by the CPU.
--
cpu_regs: process (clock_i, res_n_i)
begin
if res_n_i = '0' then
f_q <= (others => '0');
a_q <= (others => '1');
elsif clock_i'event and clock_i = '1' then
if clk_en_i and we_i then
if r2_i = '0' then
-- access to frequency register
if d_i(0) = '0' then
f_q(0 to 5) <= d_i(2 to 7);
else
f_q(6 to 9) <= d_i(4 to 7);
end if;
else
-- access to attenuator register
-- both access types can write to the attenuator register!
a_q <= d_i(4 to 7);
end if;
end if;
end if;
end process cpu_regs;
--
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-- Process freq_gen
--
-- Purpose:
-- Implements the frequency generation components.
--
freq_gen: process (clock_i, res_n_i)
begin
if res_n_i = '0' then
freq_cnt_q <= (others => '0');
freq_ff_q <= '0';
elsif clock_i'event and clock_i = '1' then
if clk_en_i then
if freq_cnt_q = 0 then
-- update counter from frequency register
freq_cnt_q <= unsigned(f_q);
-- and toggle the frequency flip-flop if enabled
if not all_zero(f_q) then
freq_ff_q <= not freq_ff_q;
else
-- if frequency setting is 0, then keep flip-flop at +1
freq_ff_q <= '1';
end if;
else
-- decrement frequency counter
freq_cnt_q <= freq_cnt_q - 1;
end if;
end if;
end if;
end process freq_gen;
--
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-- Map frequency flip-flop to signed value for attenuator.
-----------------------------------------------------------------------------
freq_s <= to_signed(+1, 2)
when freq_ff_q = '1' else
to_signed(-1, 2);
-----------------------------------------------------------------------------
-- The attenuator itself
-----------------------------------------------------------------------------
attenuator_b : sn76489_attenuator
port map (
attenuation_i => a_q,
factor_i => freq_s,
product_o => tone_o
);
-----------------------------------------------------------------------------
-- Output mapping
-----------------------------------------------------------------------------
ff_o <= freq_ff_q;
end rtl;
|
