Subversion Repositories common_components

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--

-- Copyright 2020

-- ASTRON (Netherlands Institute for Radio Astronomy) <http://www.astron.nl/>

-- P.O.Box 2, 7990 AA Dwingeloo, The Netherlands

-- 

-- Licensed under the Apache License, Version 2.0 (the "License");

-- you may not use this file except in compliance with the License.

-- You may obtain a copy of the License at

-- 

--     http://www.apache.org/licenses/LICENSE-2.0

-- 

-- Unless required by applicable law or agreed to in writing, software

-- distributed under the License is distributed on an "AS IS" BASIS,

-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

-- See the License for the specific language governing permissions and

-- limitations under the License.

--

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--   Purpose: Shift register for control data bit

--   Description:

--     Delays input data by g_depth. The delay line shifts when in_val is

--     indicates an active clock cycle.

--   Remark:

--   . This common_bit_delay can not use common_delay.vhd because it needs a reset.

--   . Typically rst may be left not connected, because the internal power up

--     state of the shift_reg is 0.

--   . If dynamic restart control is needed then use in_clr for that. Otherwise

--     leave in_clr also not connected.

--   . For large g_depth Quartus infers a RAM block for this bitDelay even if

--     the same signal is applied to both in_bit and in_val. It does not help

--     to remove in_clr or to not use shift_reg(0) combinatorially.

library IEEE;

use IEEE.std_logic_1164.all;

entity common_bit_delay is

  generic (

    g_depth : NATURAL := 16 --Quartus infers fifo for 4 to 4096 g_depth, 8 Bits.

  );

  port (

    clk     : in  std_logic;

    rst     : in  std_logic := '0';   -- asynchronous reset for initial start

    in_clr  : in  std_logic := '0';   -- synchronous reset for control of dynamic restart(s)

    in_bit  : in  std_logic;

    in_val  : in  std_logic := '1';

    out_bit : out std_logic

  );

end entity common_bit_delay;

architecture rtl of common_bit_delay is

  -- Use index (0) as combinatorial input and index(1:g_depth) for the shift

  -- delay, in this way the shift_reg type can support all g_depth >= 0

  signal shift_reg : std_logic_vector(0 to g_depth) := (others=>'0');

begin

  shift_reg(0) <= in_bit;

  out_bit <= shift_reg(g_depth);

  gen_reg : if g_depth>0 generate

    p_clk : process(clk, rst)

    begin

      if rst='1' then

        shift_reg(1 to g_depth) <= (others=>'0');

      elsif rising_edge(clk) then

        if in_clr='1' then

          shift_reg(1 to g_depth) <= (others=>'0');

        elsif in_val='1' then

          shift_reg(1 to g_depth) <= shift_reg(0 to g_depth-1);

        end if;

      end if;

    end process;

  end generate;

end rtl;