Normal Keypad Interface to PC Via PS2

#include <reg51.h> 

#define BYTE (7)
#define PARITY (1)
char D_Bus[BYTE+PARITY]={0};

sbit clockpin = P3^2; 
sbit datapin  = P3^7; 

void sendbit(char b){ 
char i; 
    datapin=b; 
     for (i=0; i<5; i++); // Clock Time for Hi 
    clockpin=0;      
     for (i=0; i<9; i++); // Clock Time for Lo 
    datapin=b; 
    clockpin=1;      
} 

void D_Buffer(void)
{
	char nbit=0, i=0;
	
	do
	{
		clockpin=1;
		for (i=0; i<5; i++) // Tck - Clock Time for Hi
		{
			D_Bus[nbit] = datapin;
		}
		clockpin=0;
		for (i=0; i<9; i++) // Tck - Clock Time for Lo
		{
			D_Bus[nbit] = datapin;
		}
	}while(nbit++ <= 8);

	sendbit(1); // HI
	sendbit(0); // ACK
}

void Host_To_Drive(void) interrupt 0
{
	char i=0;

	while(!INT0);
	datapin=0;

	clockpin=1;		     // Raising Edge
	for (i=0; i<5; i++); // Tck - Clock Time for Hi

	clockpin=0;			 // Falling Edge
	for (i=0; i<2; i++); // Tck - Clock Time for Lo

	D_Buffer();
}

void main (void) { 

	IT0 = 0;
	IE0 = 0;
	EX0 = 1;
	TF0 = 0;
	EA  = 1;

	while(1)
	{

	}

#include <reg51.h>

sbit clockpin = P3^2;
sbit datapin  = P3^7;
byte PCdata   = 0;

void sendbit(char b){
char i;
     datapin=b;
     for (i=0; i<5; i++); // Clock Time for Hi
     clockpin=0;     
     for (i=0; i<9; i++); // Clock Time for Lo
     datapin=b;
     clockpin=1;     
}

char recvbit(char b){
char i;
     for (i=0; i<5; i++); // Clock Time for Hi
     i = datapin;
     clockpin=0;     
     for (i=0; i<9; i++); // Clock Time for Lo
     clockpin=1;    
     return i<<7; 
}

void Host_To_Drive(void) interrupt 0
{
char i;
   EX0  = 0;     // disable interupt
   recvbit();    // start bit
   PCdata = 0;
   for (i=0; i<8; i++){
        PCdata += recvbit();
        PCdata >>= 1;
        }
   recvbit();    // parity bit
   sendbit(1);
   sendbit(0);
   sendbit(1);
}

void xmit(char a){
char i,mask;
char parity;
    parity = 0;
     sendbit(0);            // start bit
     mask = 1;
     for (i=0; i<=8; i++) { // shift right -> b0.. b7
         if (a & mask) {sendbit(1); parity++;}
         else sendbit(0);
         mask <<= 1;
     }
    parity &= 1;         // least significant bit only
    sendbit(!parity);    // convert to odd parity
    sendbit(1);          // stop bit
} 
void main (void) {

   IT0 = 1;
   EX0 = 1;
   EA  = 1;

   while(1)
   {
    if PCdata!=0 {xmit(0xFA); PCdata=0; EX0=1;}
   }
}

char recvbit(void){
char i,databit;
     // MCU generate clock, but PC transmit data bit
     for (i=0; i<5; i++);       // waiting for data stable
     databit = datapin;         // get PC data bit to i
     clockpin=0;                // negative going edge
     for (i=0; i<5; i++);       // PC will transmit new data bit
     clockpin=1;                // clockpin standy state
     return databit<<7;         // shift to bit 7 -> return value to caller
}

void PCsendCommand(void) interrupt 0
{
char i;
   EX0  = 0;                    // disable interupt
   recvbit();                   // start bit
   PCdata = 0;                  // if PCdata!=0 -> there is new byte from PC
   for (i=0; i<8; i++){
        PCdata >>= 1;           // shift right 1 position
        PCdata += recvbit();    // new data bit inserto into 7th position
        }
   recvbit();                   // parity bit, at this case I ignore it
   sendbit(1);                  // reply acknowledge bit
   sendbit(0);                  // reply acknowledge bit
   datapin = 1;                 // datapin standby state = 1
}

void initialize(void){
     LED = 0x00;
     xmit(0xFA); PCdata=0; EX0=1;
     LED = 0xFF;
}

if (PCdata) initialize();

clockpin = P3^2; // /INT0 (external interrup)