AppleIISd/VHDL/AddressDecoder.vhd
2017-10-16 22:53:41 +02:00

100 lines
3.0 KiB
VHDL

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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 (11 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(8) <= A(8) or not A(11);
B(9) <= A(9) or not A(11);
B(10) <= A(10) or not A(11);
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);
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;