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-- Author: Raj Thilak Rajan : rajan at astron.nl: Nov 2009
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-- Copyright (C) 2009-2010
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-- ASTRON (Netherlands Institute for Radio Astronomy)
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-- P.O.Box 2, 7990 AA Dwingeloo, The Netherlands
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--
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-- This file is part of the UniBoard software suite.
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-- The file is free software: you can redistribute it and/or modify
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-- it under the terms of the GNU General Public License as published by
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-- the Free Software Foundation, either version 3 of the License, or
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-- (at your option) any later version.
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--
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-- This program is distributed in the hope that it will be useful,
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-- but WITHOUT ANY WARRANTY; without even the implied warranty of
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-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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-- GNU General Public License for more details.
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--
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-- You should have received a copy of the GNU General Public License
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-- along with this program. If not, see <http://www.gnu.org/licenses/>.
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--------------------------------------------------------------------------------
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-- Purpose: Shift register for control data bit
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-- Description:
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-- Delays input data by g_depth. The delay line shifts when in_val is
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-- indicates an active clock cycle.
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-- Remark:
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-- . This common_bit_delay can not use common_delay.vhd because it needs a reset.
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-- . Typically rst may be left not connected, because the internal power up
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-- state of the shift_reg is 0.
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-- . If dynamic restart control is needed then use in_clr for that. Otherwise
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-- leave in_clr also not connected.
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-- . For large g_depth Quartus infers a RAM block for this bitDelay even if
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-- the same signal is applied to both in_bit and in_val. It does not help
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-- to remove in_clr or to not use shift_reg(0) combinatorially.
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library IEEE;
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use IEEE.std_logic_1164.all;
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entity common_bit_delay is
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generic (
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g_depth : NATURAL := 16 --Quartus infers fifo for 4 to 4096 g_depth, 8 Bits.
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);
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port (
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clk : in std_logic;
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rst : in std_logic := '0'; -- asynchronous reset for initial start
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in_clr : in std_logic := '0'; -- synchronous reset for control of dynamic restart(s)
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in_bit : in std_logic;
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in_val : in std_logic := '1';
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out_bit : out std_logic
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);
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end entity common_bit_delay;
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architecture rtl of common_bit_delay is
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-- Use index (0) as combinatorial input and index(1:g_depth) for the shift
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-- delay, in this way the shift_reg type can support all g_depth >= 0
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signal shift_reg : std_logic_vector(0 to g_depth) := (others=>'0');
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begin
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shift_reg(0) <= in_bit;
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out_bit <= shift_reg(g_depth);
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gen_reg : if g_depth>0 generate
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p_clk : process(clk, rst)
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begin
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if rst='1' then
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shift_reg(1 to g_depth) <= (others=>'0');
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elsif rising_edge(clk) then
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if in_clr='1' then
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shift_reg(1 to g_depth) <= (others=>'0');
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elsif in_val='1' then
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shift_reg(1 to g_depth) <= shift_reg(0 to g_depth-1);
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end if;
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end if;
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end process;
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end generate;
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end rtl;
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