AppleIISd/VHDL/AddressDecoder.vhd

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-- Company: 
-- Engineer: 
-- 
-- Create Date:    22:03:22 10/10/2017 
-- Design Name: 
-- Module Name:    AddressDecoder - Behavioral 
-- Project Name: 
-- Target Devices: 
-- Tool versions: 
-- Description: 
--
-- Dependencies: 
--
-- Revision: 
-- Revision 0.01 - File Created
-- Additional Comments: 
--
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library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
--use IEEE.NUMERIC_STD.ALL;

-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;

entity AddressDecoder is
    Port ( A : in  std_logic_vector (10 downto 8);
           B : out  std_logic_vector (10 downto 8); -- to EPROM
           CLK : in std_logic;
           PHI0 : in std_logic;
           RNW : in  std_logic;
           NDEV_SEL : in  std_logic;    -- $C0n0 - $C0nF
           NIO_SEL : in  std_logic;     -- $Cs00 - $CsFF
           NIO_STB : in  std_logic;     -- $C800 - $CFFF
           NRESET : in std_logic;
           DATA_EN : out  std_logic;    -- to CPLD
           NG : out  std_logic;         -- to bus transceiver
           NOE : out  std_logic);       -- to EPROM
end AddressDecoder;

architecture Behavioral of AddressDecoder is

    signal cfxx : std_logic;            -- $C800 - $CFFF disable
    signal noe_int : std_logic;
    signal ndev_sel_int : std_logic;
    signal nio_sel_int : std_logic;
    signal nio_stb_int : std_logic;
    signal ncs : std_logic;             -- $C800 - $CFFF enabled

begin

    -- According to Apple IIgs Tech Note #68
    -- in order to prevent bus fights with video data,
    -- data from peripheral to CPU shall be valid on the bus
    -- only from the first rising edge of 7M when any select
    -- line is low (Phi0 high) to the falling edge of Phi0
    
    B <= A when (NIO_STB = '0') else (others => '0');
    DATA_EN <= RNW and not ndev_sel_int and PHI0;
    --NG <= (ndev_sel_int and noe_int) or not PHI0;
    --NOE <= noe_int or not PHI0;
    --noe_int <= not RNW or not ndev_sel_int
    --        or (nio_sel_int and nio_stb_int)
    --        or (nio_sel_int and ncs);
            
    NG <= (NDEV_SEL and NIO_SEL and NIO_STB)
        or (ncs and NDEV_SEL and NIO_SEL);
    NOE <= not RNW or not NDEV_SEL
        or (not NIO_STB and ncs);
    
    cfxx <= A(8) and A(9) and A(10) and not nio_stb_int;
    
    process(NRESET, nio_sel_int, cfxx)
    begin
        if (NRESET = '0' or cfxx = '1') then
            ncs <= '1';
        elsif falling_edge(nio_sel_int) then
            ncs <= '0';
        end if;
    end process;
    
    process(NRESET, CLK)
    begin
        if(NRESET = '0') then
            ndev_sel_int <= '1';
            nio_sel_int <= '1';
            nio_stb_int <= '1';
        elsif rising_edge(CLK) then
            ndev_sel_int <= NDEV_SEL;
            nio_sel_int <= NIO_SEL;
            nio_stb_int <= NIO_STB;
        end if;
    end process;
    
end Behavioral;