#include<reg52.h>

sbit PIN_RXD = P3^0;
sbit PIN_TXD = P3^1;

bit RxdEnd = 0;
bit RxdOrTxd = 0;
bit TxdEnd = 0;
unsigned char RxdBuf = 0;
unsigned char TxdBuf = 0;

void ConfigUART(unsigned int baud);
void StartRXD();
void StartTXD(unsigned char dat);

void main()
{
	EA = 1;
	ConfigUART(9600);
	
	while(1)
	{
		while(PIN_RXD);
		StartRXD();
		while(!RxdEnd);
		StartTXD(RxdBuf+1);
		while(!TxdEnd);
	}
	
}

void ConfigUART(unsigned int baud)
{
	TMOD &= 0xF0;
	TMOD |= 0x02;
	TH0 = 256 - (11059200/12)/baud;
}

void StartRXD()
{
	TL0 = 256 - ((256 - TH0)>>1);
	ET0 = 1;
	TR0 = 1;
	RxdEnd = 0;
	RxdOrTxd = 0;
}

void  StartTXD(unsigned char dat)
{
	TxdBuf = dat;
	TL0 = TH0;
	ET0 = 1;
	TR0 = 1;
	PIN_TXD = 0;
	TxdEnd = 0;
	RxdOrTxd = 1;
}

void InterruptTimer0() interrupt 1
{
	static unsigned char cnt = 0;

	if(RxdOrTxd)
	{
		cnt++;
		if(cnt <= 8)
		{
			PIN_TXD = TxdBuf & 0x01;
			TxdBuf >>= 1;
		}
		else if(cnt == 9)
		{
		 	PIN_TXD = 1;
		}
		else
		{
			cnt = 0;
			TR0 = 0;
			TxdEnd = 1;
		}
	}
	else
	{
		if(cnt == 0)
		{
			if(!PIN_RXD)
			{
				RxdBuf = 0;
				cnt++;
			}
			else
			{
				TR0 = 0;
			}
		}
		else if(cnt <= 8)
		{
			RxdBuf >>= 1;	     
			if(PIN_RXD)
			{
				RxdBuf |= 0x80;
			}
			cnt++;
		}
		else
		{
			cnt = 0;
			TR0 = 0;
			if(PIN_RXD)
			{
				RxdEnd = 1;
			}
		}
	}
}