Atom 8255 Input/Output
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This chip handles keyboard and tape I/O.

    
<->  PA3 1 U










      8255      		40 PA4 <->
<-> PA2 2 39 PA5 <->
<-> PA1 3 38 PA6 <->
<-> PA0 4 37 PA7 <->
--> !RD 5 36 VCC ---
--> !CS 6 35 RESET <--
--- GND 7 34 D0 <->
--> A1 8 33 D1 <->
--> A0 9 32 D2 <->
<-> PC7 10 31 D3 <->
<-> PC6 11 30 D4 <->
<-> PC5 12 29 D5 <->
<-> PC4 13 28 D6 <->
<-> PC3 14 27 D7 <->
<-> PC2 15 26 !WR <--
<-> PC1 16 25 PB7 <->
<-> PC0 17 24 PB6 <->
<-> PB0 18 23 PB5 <->
<-> PB1 19 22 PB4 <->
<-> PB2 20 21 PB3 <->
Software structure 
struct PPI	=	// a standard 8255 chip
{
	byte	PORTA;	// Port A
	byte	PORTB;	// Port B
	byte	PORTC;	// Port C
	byte	CTRL; 	//
};
Hardware structure

Approximate VHDL entity only!

entity PPI_8255 is	--
port(
	rst:	in	std_logic;	-- Reset, active high
	n_cs: 	in	std_logic;
	n_rd:	in	std_logic;	-- Not Read
	n_wr:	in	std_logic;	-- Not Write
	ph2:	in	std_logic;	-- CPU clock phase 2
	cs1:	in	std_logic;
	a: 	in	std_logic_vector(1 downto 0);

	d:	inout	std_logic_vector(7 downto 0);
	pa:	inout	std_logic_vector(7 downto 0);	-- Port A
	pb:	inout	std_logic_vector(7 downto 0);	-- Port B
	pc:	inout	std_logic_vector(7 downto 0);	-- Port C
	);
end entity PPI_8255;