/*********************************************
Laser Communication Text Data Echoer
Authors: Keith Carter, Michael Muccio
Cornell University, Electrical and Computer Engineering
ECE 476, Spring 2003

This program is used to program a Mega32 MCU to be the
receiver or transmitter for a serial text data laser communication
system.  It receives serial data and retransmits it.

Chip type           : ATmega32
Program type        : Application
Clock frequency     : 16.000000 MHz
Memory model        : Small
Internal SRAM size  : 2048
External SRAM size  : 0
Data Stack size     : 512
*********************************************/

#include <mega32.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <delay.h>  

//set baud to 38.4 kbps
#define baudnum 25
#define halfbaudnum 13

//global variables
char bitcnt;	// bit counter
char Txbyte;	// data to be transmitted
char Rxbyte;	// received data
char sb;		// number of stop bits (1, 2, ...)
char carry	// carry bit

//the data transmit and receive subroutines
void put(void);
void get(void); 

//Transmit Byte
void put(void)
{
	bitcnt=9+sb;      //initiate bit counter
	
	Txbyte=~Txbyte;   //invert byte for easier handling
	
	carry=1;          //set carry to 1 for start bit
	
	//while loop transmits the data bits using delay
	while(bitcnt>0)  
	{    
	     //transmit opposite of what is in carry
		if (carry)
		{
		  	PORTD.1=0;
		}
		else
		{
			PORTD.1=1;
			#asm("nop")
		}
		
		delay_us(baudnum);            //1 bit delay
		
		carry=(Txbyte&0b00000001);    //move LSB to carry1
		
		Txbyte=Txbyte>>1;             //shift transmit byte right
		
		bitcnt--;                     //decrement bitcnt, 1 bit sent
	}
	PORTD.1=1;
}

//Receive Byte
void get(void)
{
	Rxbyte = 0x00;        //clear receive byte
	
	bitcnt=9+sb;          //initialize bit counter 
	
	//while loop waits for start bit
	while(PIND.0 != 0)
	{
	// hang out	
	}
	
	//delay to middle of start bit            
	delay_us(halfbaudnum);
	
	//while loop samples at middle of bits
	//stops at bitcnt of 2 to avoid shifting 
	//in stop bit
	while(bitcnt>2)  
	{
		delay_us(baudnum);      //1 bit delay 
		
		Rxbyte=Rxbyte>>1;       //shift receive byte right 1
		
		//sample bit
		//move value to MSB of receive byte
		if (PIND.0)
		{
			Rxbyte=Rxbyte | 0b10000000;		
		}
		else
		{
			Rxbyte=Rxbyte & 0b01111111;
		}
		
		bitcnt--;            //decrement bitcnt, 1 bit received
	}
}


void main(void)
{    
	DDRD=0b00000010;        // direction for UART pins
	
	sb=1;                   // set to 1 stop bit
	
	Txbyte=12;              // clear the screen
	put();

	delay_ms(1500);

	Txbyte=12;              // clear the screen again
	put();
     
	//infinite loop receives data and echoes by immediately retransmitting
	while (1)
	{
        get();              // receive a byte
        Txbyte=Rxbyte;      // transfer data
        put();              // send a byte
        Rxbyte = 0x00;      // clear receive byte
	}
}