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88 lines
3.3 KiB
VHDL
88 lines
3.3 KiB
VHDL
-------------------------------------------------------------------------------
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--
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-- Synthesizable model of TI's SN76489AN.
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--
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-- $Id: sn76489_attenuator.vhd,v 1.7 2006/02/27 20:30:10 arnim Exp $
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--
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-- Attenuator Module
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--
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-------------------------------------------------------------------------------
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--
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-- Copyright (c) 2005, 2006, Arnim Laeuger (arnim.laeuger@gmx.net)
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--
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-- All rights reserved
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--
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-- Redistribution and use in source and synthezised forms, with or without
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-- modification, are permitted provided that the following conditions are met:
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--
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-- Redistributions of source code must retain the above copyright notice,
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-- this list of conditions and the following disclaimer.
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--
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-- Redistributions in synthesized form must reproduce the above copyright
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-- notice, this list of conditions and the following disclaimer in the
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-- documentation and/or other materials provided with the distribution.
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--
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-- Neither the name of the author nor the names of other contributors may
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-- be used to endorse or promote products derived from this software without
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-- specific prior written permission.
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--
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-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
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-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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-- POSSIBILITY OF SUCH DAMAGE.
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--
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-- Please report bugs to the author, but before you do so, please
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-- make sure that this is not a derivative work and that
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-- you have the latest version of this file.
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--
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-------------------------------------------------------------------------------
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library ieee;
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use IEEE.STD_LOGIC_1164.ALL;
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use IEEE.STD_LOGIC_ARITH.ALL;
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use IEEE.STD_LOGIC_UNSIGNED.ALL;
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entity sn76489_attenuator is
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port (
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attenuation_i : in std_logic_vector(0 to 3);
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factor_i : in std_logic;
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product_o : out std_logic_vector(0 to 7)
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);
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end sn76489_attenuator;
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architecture rtl of sn76489_attenuator is
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type volume_t is array (natural range 0 to 15) of natural;
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constant volume_c : volume_t := (31, 25, 20, 16, 12, 10, 8, 6, 5, 4, 3, 2, 2, 2, 1, 0);
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begin
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-----------------------------------------------------------------------------
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-- Process attenuate
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--
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-- Purpose:
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-- Determine the attenuation and generate the resulting product.
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--
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-- The maximum attenuation value is 31 which corresponds to volume off.
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-- As described in the data sheet, the maximum "playing" attenuation is
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-- 28 = 16 + 8 + 4
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--
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-- The table for the volume constants is derived from the following
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-- formula (each step is 2dB voltage):
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-- v(0) = 31
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-- v(n+1) = v(n) * 0.79432823
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--
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product_o <= conv_std_logic_vector(volume_c(conv_integer(attenuation_i)), product_o'length) when factor_i = '1'
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else (others => '0');
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end rtl;
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