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--
-- Copyright 2021 Ettus Research, a National Instruments Brand
--
-- SPDX-License-Identifier: LGPL-3.0-or-later
--
-- Module: duc_saturate
--
-- Description:
--
-- Saturation logic for reducing 2x24 bit words to 2x16 bit words. See
-- comments below for full description.
--
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity duc_saturate is
port(
Clk : in std_logic;
cDataIn : in std_logic_vector(47 downto 0);
cDataValidIn : in std_logic;
cReadyForInput : out std_logic;
cDataOut : out std_logic_vector(31 downto 0);
cDataValidOut : out std_logic := '0'
);
end duc_saturate;
architecture RTL of duc_saturate is
signal cDataOutI : std_logic_vector(15 downto 0) := (others => '0');
signal cDataOutQ : std_logic_vector(15 downto 0) := (others => '0');
begin
-----------------------------------------------------------------------------
-- Saturation
--
-- The output of the Xilinx FIR Compiler has already been rounded on the LSB
-- side, but hasn't been saturated on the MSB side.
-- Coefficients = 18 bit, 1 integer bit (1.17)
-- Data In = 16 bits, 1 integer bit (1.15)
-- Xilinx FIR core rounds to output to 3.31. The filter coefficients has a
-- gain of 3 to compensate for the amplitude loss in interpolation, the
-- Xilinx FIR core rounds the output to 3.15.
-- Data Out = 18 bits, 3 integer bits (3.15), with 16 LSBs already rounded
-- off inside the FIR core.
-- We need to manually saturate the 3.15 number back to a 1.15 number
--
-- If 3 MSBs = 000, output <= input without MSB, e.g. positive number < 1
-- If 3 MSBs = 0x1/01x, output <= 0.111111111111111, e.g. positive number >= 1
-- If 3 MSBs = 1x0/10x, output <= 1.000000000000000, e.g. negative number < -1
-- If 3 MSBs = 111, output <= input without MSB, e.g. negative number >= -1
-----------------------------------------------------------------------------
Saturation:
process(Clk)
begin
if rising_edge(Clk) then
-- Pipeline data valid to match the data
cDataValidOut <= cDataValidIn;
-- I, from cDataIn(17 downto 0)
if cDataIn(17) = '0' and cDataIn(16 downto 15) /= "00" then
cDataOutI <= "0111111111111111";
elsif cDataIn(17) = '1' and cDataIn(16) /= cDataIn(15) then
cDataOutI <= "1000000000000000";
else
cDataOutI <= cDataIn(15 downto 0);
end if;
-- Q, from cDataIn(41 downto 24)
if cDataIn(41) = '0' and cDataIn(40 downto 39) /= "00" then
cDataOutQ <= "0111111111111111";
elsif cDataIn(41) = '1' and
(not (cDataIn(40 downto 39) = "11")) then
cDataOutQ <= "1000000000000000";
else
cDataOutQ <= cDataIn(39 downto 24);
end if;
end if;
end process Saturation;
cDataOut <= cDataOutQ & cDataOutI;
cReadyForInput <= '1';
end RTL;
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