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Diffstat (limited to 'sn76489-1.0/sn76489_attenuator.vhd')
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1 files changed, 114 insertions, 0 deletions
diff --git a/sn76489-1.0/sn76489_attenuator.vhd b/sn76489-1.0/sn76489_attenuator.vhd new file mode 100644 index 0000000..444064e --- /dev/null +++ b/sn76489-1.0/sn76489_attenuator.vhd @@ -0,0 +1,114 @@ +------------------------------------------------------------------------------- +-- +-- Synthesizable model of TI's SN76489AN. +-- +-- $Id: sn76489_attenuator.vhd,v 1.7 2006/02/27 20:30:10 arnim Exp $ +-- +-- Attenuator Module +-- +------------------------------------------------------------------------------- +-- +-- 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_attenuator is + + port ( + attenuation_i : in std_logic_vector(0 to 3); + factor_i : in signed(0 to 1); + product_o : out signed(0 to 7) + ); + +end sn76489_attenuator; + + +architecture rtl of sn76489_attenuator is + +begin + + ----------------------------------------------------------------------------- + -- Process attenuate + -- + -- Purpose: + -- Determine the attenuation and generate the resulting product. + -- + -- The maximum attenuation value is 31 which corresponds to volume off. + -- As described in the data sheet, the maximum "playing" attenuation is + -- 28 = 16 + 8 + 4 + -- + -- The table for the volume constants is derived from the following + -- formula (each step is 2dB voltage): + -- v(0) = 31 + -- v(n+1) = v(n) * 0.79432823 + -- + attenuate: process (attenuation_i, + factor_i) + + type volume_t is array (natural range 0 to 15) of natural; + constant volume_c : volume_t := + (31, 25, 20, 16, 12, 10, 8, 6, 5, 4, 3, 2, 2, 2, 1, 0); + + variable attenuation_v : unsigned(attenuation_i'range); + variable volume_v : signed(product_o'range); + + begin + + attenuation_v := unsigned(attenuation_i); + + -- volume look-up table + volume_v := to_signed(volume_c(to_integer(attenuation_v)), + product_o'length); + + -- this replaces a multiplier and consumes a bit fewer + -- resources + case to_integer(factor_i) is + when +1 => + product_o <= volume_v; + when -1 => + product_o <= -volume_v; + when others => + product_o <= (others => '0'); + end case; + + end process attenuate; + -- + ----------------------------------------------------------------------------- + +end rtl; |