VHDL implementation of the 8255 PIO
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This code can be used to implement an 8255 PIO in a CPLD or a FPGA.
--
-- A simulation model of I82C55 PIA
-- Copyright (c) MikeJ - Feb 2007
--
-- All rights reserved
--
-- Redistribution and use in source and synthezised forms, with or without
-- modification, are permitted provided that the following conditions are met:
--
-- Redistributions of source code must retain the above copyright notice,
-- this list of conditions and the following disclaimer.
--
-- Redistributions in synthesized form must reproduce the above copyright
-- notice, this list of conditions and the following disclaimer in the
-- documentation and/or other materials provided with the distribution.
--
-- Neither the name of the author nor the names of other contributors may
-- be used to endorse or promote products derived from this software without
-- specific prior written permission.
--
-- THIS CODE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-- POSSIBILITY OF SUCH DAMAGE.
--
-- You are responsible for any legal issues arising from your use of this code.
--
-- The latest version of this file can be found at: www.fpgaarcade.com
--
-- Email support@fpgaarcade.com
--
-- Revision list
--
-- version 001 initial release
--
library ieee ;
use ieee.std_logic_1164.all ;
use ieee.std_logic_unsigned.all;
use ieee.numeric_std.all;
entity I82C55 is
port (
I_ADDR : in std_logic_vector(1 downto 0); -- A1-A0
I_DATA : in std_logic_vector(7 downto 0); -- D7-D0
O_DATA : out std_logic_vector(7 downto 0);
O_DATA_OE_L : out std_logic;
I_CS_L : in std_logic;
I_RD_L : in std_logic;
I_WR_L : in std_logic;
I_PA : in std_logic_vector(7 downto 0);
O_PA : out std_logic_vector(7 downto 0);
O_PA_OE_L : out std_logic_vector(7 downto 0);
I_PB : in std_logic_vector(7 downto 0);
O_PB : out std_logic_vector(7 downto 0);
O_PB_OE_L : out std_logic_vector(7 downto 0);
I_PC : in std_logic_vector(7 downto 0);
O_PC : out std_logic_vector(7 downto 0);
O_PC_OE_L : out std_logic_vector(7 downto 0);
RESET : in std_logic;
ENA : in std_logic; -- (CPU) clk enable
CLK : in std_logic
);
end;
architecture RTL of I82C55 is
-- registers
signal bit_mask : std_logic_vector(7 downto 0);
signal r_porta : std_logic_vector(7 downto 0);
signal r_portb : std_logic_vector(7 downto 0);
signal r_portc : std_logic_vector(7 downto 0);
signal r_control : std_logic_vector(7 downto 0);
--
signal porta_we : std_logic;
signal portb_we : std_logic;
signal porta_re : std_logic;
signal portb_re : std_logic;
--
signal porta_we_t1 : std_logic;
signal portb_we_t1 : std_logic;
signal porta_re_t1 : std_logic;
signal portb_re_t1 : std_logic;
--
signal porta_we_rising : boolean;
signal portb_we_rising : boolean;
signal porta_re_rising : boolean;
signal portb_re_rising : boolean;
--
signal groupa_mode : std_logic_vector(1 downto 0); -- port a/c upper
signal groupb_mode : std_logic; -- port b/c lower
--
signal porta_read : std_logic_vector(7 downto 0);
signal portb_read : std_logic_vector(7 downto 0);
signal portc_read : std_logic_vector(7 downto 0);
signal control_read : std_logic_vector(7 downto 0);
signal mode_clear : std_logic;
--
signal a_inte1 : std_logic;
signal a_inte2 : std_logic;
signal b_inte : std_logic;
--
signal a_intr : std_logic;
signal a_obf_l : std_logic;
signal a_ibf : std_logic;
signal a_ack_l : std_logic;
signal a_stb_l : std_logic;
signal a_ack_l_t1 : std_logic;
signal a_stb_l_t1 : std_logic;
--
signal b_intr : std_logic;
signal b_obf_l : std_logic;
signal b_ibf : std_logic;
signal b_ack_l : std_logic;
signal b_stb_l : std_logic;
signal b_ack_l_t1 : std_logic;
signal b_stb_l_t1 : std_logic;
--
signal a_ack_l_rising : boolean;
signal a_stb_l_rising : boolean;
signal b_ack_l_rising : boolean;
signal b_stb_l_rising : boolean;
--
signal porta_ipreg : std_logic_vector(7 downto 0);
signal portb_ipreg : std_logic_vector(7 downto 0);
begin
--
-- mode 0 - basic input/output
-- mode 1 - strobed input/output
-- mode 2/3 - bi-directional bus
--
-- control word (write)
--
-- D7 mode set flag 1 = active
-- D6..5 GROUPA mode selection (mode 0,1,2)
-- D4 GROUPA porta 1 = input, 0 = output
-- D3 GROUPA portc upper 1 = input, 0 = output
-- D2 GROUPB mode selection (mode 0 ,1)
-- D1 GROUPB portb 1 = input, 0 = output
-- D0 GROUPB portc lower 1 = input, 0 = output
--
-- D7 bit set/reset 0 = active
-- D6..4 x
-- D3..1 bit select
-- d0 1 = set, 0 - reset
--
-- all output registers including status are reset when mode is changed
--1. Port A:
--All Modes: Output data is cleared, input data is not cleared.
--2. Port B:
--Mode 0: Output data is cleared, input data is not cleared.
--Mode 1 and 2: Both output and input data are cleared.
--3. Port C:
--Mode 0:Output data is cleared, input data is not cleared.
--Mode 1 and 2: IBF and INTR are cleared and OBF# is set.
--Outputs in Port C which are not used for handshaking or interrupt signals are cleared.
--Inputs such as STB#, ACK#, or "spare" inputs are not affected. The interrupts for Ports A and B are disabled.
p_bit_mask : process(I_DATA)
begin
bit_mask <= x"01";
case I_DATA(3 downto 1) is
when "000" => bit_mask <= x"01";
when "001" => bit_mask <= x"02";
when "010" => bit_mask <= x"04";
when "011" => bit_mask <= x"08";
when "100" => bit_mask <= x"10";
when "101" => bit_mask <= x"20";
when "110" => bit_mask <= x"40";
when "111" => bit_mask <= x"80";
when others => null;
end case;
end process;
p_write_reg_reset : process(RESET, CLK)
variable r_portc_masked : std_logic_vector(7 downto 0);
variable r_portc_setclr : std_logic_vector(7 downto 0);
begin
if (RESET = '1') then
r_porta <= x"00";
r_portb <= x"00";
r_portc <= x"00";
r_control <= x"9B"; -- 10011011
mode_clear <= '1';
elsif rising_edge(CLK) then
r_portc_masked := (not bit_mask) and r_portc;
for i in 0 to 7 loop
r_portc_setclr(i) := bit_mask(i) and I_DATA(0);
end loop;
if (ENA = '1') then
mode_clear <= '0';
if (I_CS_L = '0') and (I_WR_L = '0') then
case I_ADDR is
when "00" => r_porta <= I_DATA;
when "01" => r_portb <= I_DATA;
when "10" => r_portc <= I_DATA;
when "11" => if (I_DATA(7) = '0') then -- set/clr
r_portc <= r_portc_masked or r_portc_setclr;
else
--mode_clear <= '1';
--r_porta <= x"00";
--r_portb <= x"00"; -- clear port b input reg
--r_portc <= x"00"; -- clear control sigs
r_control <= I_DATA; -- load new mode
end if;
when others => null;
end case;
end if;
end if;
end if;
end process;
p_decode_control : process(r_control)
begin
groupa_mode <= r_control(6 downto 5);
groupb_mode <= r_control(2);
end process;
p_oe : process(I_CS_L, I_RD_L)
begin
O_DATA_OE_L <= '1';
if (I_CS_L = '0') and (I_RD_L = '0') then
O_DATA_OE_L <= '0';
end if;
end process;
p_read : process(I_ADDR, porta_read, portb_read, portc_read, control_read)
begin
O_DATA <= x"00"; -- default
--if (I_CS_L = '0') and (I_RD_L = '0') then -- not required
case I_ADDR is
when "00" => O_DATA <= porta_read;
when "01" => O_DATA <= portb_read;
when "10" => O_DATA <= portc_read;
when "11" => O_DATA <= control_read;
when others => null;
end case;
--end if;
end process;
control_read(7) <= '1'; -- always 1
control_read(6 downto 0) <= r_control(6 downto 0);
p_rw_control : process(I_CS_L, I_RD_L, I_WR_L, I_ADDR)
begin
porta_we <= '0';
portb_we <= '0';
porta_re <= '0';
portb_re <= '0';
if (I_CS_L = '0') and (I_ADDR = "00") then
porta_we <= not I_WR_L;
porta_re <= not I_RD_L;
end if;
if (I_CS_L = '0') and (I_ADDR = "01") then
portb_we <= not I_WR_L;
portb_re <= not I_RD_L;
end if;
end process;
p_rw_control_reg : process
begin
wait until rising_edge(CLK);
if (ENA = '1') then
porta_we_t1 <= porta_we;
portb_we_t1 <= portb_we;
porta_re_t1 <= porta_re;
portb_re_t1 <= portb_re;
a_stb_l_t1 <= a_stb_l;
a_ack_l_t1 <= a_ack_l;
b_stb_l_t1 <= b_stb_l;
b_ack_l_t1 <= b_ack_l;
end if;
end process;
porta_we_rising <= (porta_we = '0') and (porta_we_t1 = '1'); -- falling as inverted
portb_we_rising <= (portb_we = '0') and (portb_we_t1 = '1'); -- "
porta_re_rising <= (porta_re = '0') and (porta_re_t1 = '1'); -- falling as inverted
portb_re_rising <= (portb_re = '0') and (portb_re_t1 = '1'); -- "
--
a_stb_l_rising <= (a_stb_l = '1') and (a_stb_l_t1 = '0');
a_ack_l_rising <= (a_ack_l = '1') and (a_ack_l_t1 = '0');
b_stb_l_rising <= (b_stb_l = '1') and (b_stb_l_t1 = '0');
b_ack_l_rising <= (b_ack_l = '1') and (b_ack_l_t1 = '0');
--
-- GROUP A
-- in mode 1
--
-- d4=1 (porta = input)
-- pc7,6 io (d3=1 input, d3=0 output)
-- pc5 output a_ibf
-- pc4 input a_stb_l
-- pc3 output a_intr
--
-- d4=0 (porta = output)
-- pc7 output a_obf_l
-- pc6 input a_ack_l
-- pc5,4 io (d3=1 input, d3=0 output)
-- pc3 output a_intr
--
-- GROUP B
-- in mode 1
-- d1=1 (portb = input)
-- pc2 input b_stb_l
-- pc1 output b_ibf
-- pc0 output b_intr
--
-- d1=0 (portb = output)
-- pc2 input b_ack_l
-- pc1 output b_obf_l
-- pc0 output b_intr
-- WHEN AN INPUT
--
-- stb_l a low on this input latches input data
-- ibf a high on this output indicates data latched. set by stb_l and reset by rising edge of RD_L
-- intr a high on this output indicates interrupt. set by stb_l high, ibf high and inte high. reset by falling edge of RD_L
-- inte A controlled by bit/set PC4
-- inte B controlled by bit/set PC2
-- WHEN AN OUTPUT
--
-- obf_l output will go low when cpu has written data
-- ack_l input - a low on this clears obf_l
-- intr output set when ack_l is high, obf_l is high and inte is one. reset by falling edge of WR_L
-- inte A controlled by bit/set PC6
-- inte B controlled by bit/set PC2
-- GROUP A
-- in mode 2
--
-- porta = IO
--
-- control bits 2..0 still control groupb/c lower 2..0
--
--
-- PC7 output a_obf
-- PC6 input a_ack_l
-- PC5 output a_ibf
-- PC4 input a_stb_l
-- PC3 is still interrupt out
p_control_flags : process(RESET, CLK)
variable we : boolean;
variable set1 : boolean;
variable set2 : boolean;
begin
if (RESET = '1') then
a_obf_l <= '1';
a_inte1 <= '0';
a_ibf <= '0';
a_inte2 <= '0';
a_intr <= '0';
--
b_inte <= '0';
b_obf_l <= '1';
b_ibf <= '0';
b_intr <= '0';
elsif rising_edge(CLK) then
we := (I_CS_L = '0') and (I_WR_L = '0') and (I_ADDR = "11") and (I_DATA(7) = '0');
if (ENA = '1') then
if (mode_clear = '1') then
a_obf_l <= '1';
a_inte1 <= '0';
a_ibf <= '0';
a_inte2 <= '0';
a_intr <= '0';
--
b_inte <= '0';
b_obf_l <= '1';
b_ibf <= '0';
b_intr <= '0';
else
if (bit_mask(7) = '1') and we then
a_obf_l <= I_DATA(0);
else
if porta_we_rising then
a_obf_l <= '0';
elsif (a_ack_l = '0') then
a_obf_l <= '1';
end if;
end if;
--
if (bit_mask(6) = '1') and we then a_inte1 <= I_DATA(0); end if; -- bus set when mode1 & input?
--
if (bit_mask(5) = '1') and we then
a_ibf <= I_DATA(0);
else
if porta_re_rising then
a_ibf <= '0';
elsif (a_stb_l = '0') then
a_ibf <= '1';
end if;
end if;
--
if (bit_mask(4) = '1') and we then a_inte2 <= I_DATA(0); end if; -- bus set when mode1 & output?
--
set1 := a_ack_l_rising and (a_obf_l = '1') and (a_inte1 = '1');
set2 := a_stb_l_rising and (a_ibf = '1') and (a_inte2 = '1');
--
if (bit_mask(3) = '1') and we then
a_intr <= I_DATA(0);
else
if (groupa_mode(1) = '1') then
if (porta_we = '1') or (porta_re = '1') then
a_intr <= '0';
elsif set1 or set2 then
a_intr <= '1';
end if;
else
if (r_control(4) = '0') then -- output
if (porta_we = '1') then -- falling ?
a_intr <= '0';
elsif set1 then
a_intr <= '1';
end if;
elsif (r_control(4) = '1') then -- input
if (porta_re = '1') then -- falling ?
a_intr <= '0';
elsif set2 then
a_intr <= '1';
end if;
end if;
end if;
end if;
--
if (bit_mask(2) = '1') and we then b_inte <= I_DATA(0); end if; -- bus set?
if (bit_mask(1) = '1') and we then
b_obf_l <= I_DATA(0);
else
if (r_control(1) = '0') then -- output
if portb_we_rising then
b_obf_l <= '0';
elsif (b_ack_l = '0') then
b_obf_l <= '1';
end if;
else
if portb_re_rising then
b_ibf <= '0';
elsif (b_stb_l = '0') then
b_ibf <= '1';
end if;
end if;
end if;
if (bit_mask(0) = '1') and we then
b_intr <= I_DATA(0);
else
if (r_control(1) = '0') then -- output
if (portb_we = '1') then -- falling ?
b_intr <= '0';
elsif b_ack_l_rising and (b_obf_l = '1') and (b_inte = '1') then
b_intr <= '1';
end if;
else
if (portb_re = '1') then -- falling ?
b_intr <= '0';
elsif b_stb_l_rising and (b_ibf = '1') and (b_inte = '1') then
b_intr <= '1';
end if;
end if;
end if;
end if;
end if;
end if;
end process;
p_porta : process(r_porta, r_control, groupa_mode, r_porta, I_PA, porta_ipreg, a_ack_l)
begin
-- D4 GROUPA porta 1 = input, 0 = output
O_PA <= x"FF"; -- if not driven, float high
O_PA_OE_L <= x"FF";
porta_read <= x"00";
if (groupa_mode = "00") then -- simple io
if (r_control(4) = '0') then -- output
O_PA <= r_porta;
O_PA_OE_L <= x"00";
end if;
porta_read <= I_PA;
elsif (groupa_mode = "01") then -- strobed
if (r_control(4) = '0') then -- output
O_PA <= r_porta;
O_PA_OE_L <= x"00";
end if;
porta_read <= porta_ipreg;
else -- if (groupa_mode(1) = '1') then -- bi dir
if (a_ack_l = '0') then -- output enable
O_PA <= r_porta;
O_PA_OE_L <= x"00";
end if;
porta_read <= porta_ipreg; -- latched data
end if;
end process;
p_portb : process(r_portb, r_control, groupb_mode, r_portb, I_PB, portb_ipreg)
begin
O_PB <= x"FF"; -- if not driven, float high
O_PB_OE_L <= x"FF";
portb_read <= x"00";
if (groupb_mode = '0') then -- simple io
if (r_control(1) = '0') then -- output
O_PB <= r_portb;
O_PB_OE_L <= x"00";
end if;
portb_read <= I_PB;
else -- strobed mode
if (r_control(1) = '0') then -- output
O_PB <= r_portb;
O_PB_OE_L <= x"00";
end if;
portb_read <= portb_ipreg;
end if;
end process;
p_portc_out : process(r_portc, r_control, groupa_mode, groupb_mode,
a_obf_l, a_ibf, a_intr,b_obf_l, b_ibf, b_intr)
begin
O_PC <= x"FF"; -- if not driven, float high
O_PC_OE_L <= x"FF";
-- bits 7..4
if (groupa_mode = "00") then -- simple io
if (r_control(3) = '0') then -- output
O_PC (7 downto 4) <= r_portc(7 downto 4);
O_PC_OE_L(7 downto 4) <= x"0";
end if;
elsif (groupa_mode = "01") then -- mode1
if (r_control(4) = '0') then -- port a output
O_PC (7) <= a_obf_l;
O_PC_OE_L(7) <= '0';
-- 6 is ack_l input
if (r_control(3) = '0') then -- port c output
O_PC (5 downto 4) <= r_portc(5 downto 4);
O_PC_OE_L(5 downto 4) <= "00";
end if;
else -- port a input
if (r_control(3) = '0') then -- port c output
O_PC (7 downto 6) <= r_portc(7 downto 6);
O_PC_OE_L(7 downto 6) <= "00";
end if;
O_PC (5) <= a_ibf;
O_PC_OE_L(5) <= '0';
-- 4 is stb_l input
end if;
else -- if (groupa_mode(1) = '1') then -- mode2
O_PC (7) <= a_obf_l;
O_PC_OE_L(7) <= '0';
-- 6 is ack_l input
O_PC (5) <= a_ibf;
O_PC_OE_L(5) <= '0';
-- 4 is stb_l input
end if;
-- bit 3 (controlled by group a)
if (groupa_mode = "00") then -- group a steals this bit
--if (groupb_mode = '0') then -- we will let bit 3 be driven, data sheet is a bit confused about this
if (r_control(0) = '0') then -- ouput (note, groupb control bit)
O_PC (3) <= r_portc(3);
O_PC_OE_L(3) <= '0';
end if;
--
else -- stolen
O_PC (3) <= a_intr;
O_PC_OE_L(3) <= '0';
end if;
-- bits 2..0
if (groupb_mode = '0') then -- simple io
if (r_control(0) = '0') then -- output
O_PC (2 downto 0) <= r_portc(2 downto 0);
O_PC_OE_L(2 downto 0) <= "000";
end if;
else
-- mode 1
-- 2 is input
if (r_control(1) = '0') then -- output
O_PC (1) <= b_obf_l;
O_PC_OE_L(1) <= '0';
else -- input
O_PC (1) <= b_ibf;
O_PC_OE_L(1) <= '0';
end if;
O_PC (0) <= b_intr;
O_PC_OE_L(0) <= '0';
end if;
end process;
p_portc_in : process(r_portc, I_PC, r_control, groupa_mode, groupb_mode, a_ibf, b_obf_l,
a_obf_l, a_inte1, a_inte2, a_intr, b_inte, b_ibf, b_intr)
begin
portc_read <= x"00";
a_stb_l <= '1';
a_ack_l <= '1';
b_stb_l <= '1';
b_ack_l <= '1';
if (groupa_mode = "01") then -- mode1 or 2
if (r_control(4) = '0') then -- port a output
a_ack_l <= I_PC(6);
else -- port a input
a_stb_l <= I_PC(4);
end if;
elsif (groupa_mode(1) = '1') then -- mode 2
a_ack_l <= I_PC(6);
a_stb_l <= I_PC(4);
end if;
if (groupb_mode = '1') then
if (r_control(1) = '0') then -- output
b_ack_l <= I_PC(2);
else -- input
b_stb_l <= I_PC(2);
end if;
end if;
if (groupa_mode = "00") then -- simple io
portc_read(7 downto 3) <= I_PC(7 downto 3);
elsif (groupa_mode = "01") then
if (r_control(4) = '0') then -- port a output
portc_read(7 downto 3) <= a_obf_l & a_inte1 & I_PC(5 downto 4) & a_intr;
else -- input
portc_read(7 downto 3) <= I_PC(7 downto 6) & a_ibf & a_inte2 & a_intr;
end if;
else -- mode 2
portc_read(7 downto 3) <= a_obf_l & a_inte1 & a_ibf & a_inte2 & a_intr;
end if;
if (groupb_mode = '0') then -- simple io
portc_read(2 downto 0) <= I_PC(2 downto 0);
else
if (r_control(1) = '0') then -- output
portc_read(2 downto 0) <= b_inte & b_obf_l & b_intr;
else -- input
portc_read(2 downto 0) <= b_inte & b_ibf & b_intr;
end if;
end if;
end process;
p_ipreg : process
begin
wait until rising_edge(CLK);
-- pc4 input a_stb_l
-- pc2 input b_stb_l
if (ENA = '1') then
if (a_stb_l = '0') then
porta_ipreg <= I_PA;
end if;
if (mode_clear = '1') then
portb_ipreg <= (others => '0');
elsif (b_stb_l = '0') then
portb_ipreg <= I_PB;
end if;
end if;
end process;
end architecture RTL;
