/*********************************************
Laser Communication System Transmitter File
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
transmitter for a serial voice data laser communication
system.  It utilizes a timer and ADC interrupt to ensure
3kHz voice sampling.

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 for 27.7 kbps
#define baudnum 36
#define halfbaudnum 18

//set ADC reference to AREF
#define ADC_VREF_TYPE 0x20

//timeout value for task1 voice sampler
//a value of 1 ensures the task executes at
//every overflow
#define t1 1

//global variables
char bitcnt;			// bit counter
char Txbyte;			// data to be transmitted
char sb;				// number of stop bits (1, 2, ...)
char carry;			// carry bit 
unsigned char reload;	// timer 0 reload
int time1;	          // timeout counter
unsigned char temp;      // temporary data holder

//the task subroutine
void task1(void);

//the data transmit subroutine
void put(void);

//**********************************************************
//timer 0 overflow ISR
interrupt [TIM0_OVF] void timer0_overflow(void)
{       
	//reload to force overflow period
	TCNT0=reload;
    
	//decrement the time if not zero
	if (time1>0) --time1;
}

// ADC interrupt service routine
interrupt [ADC_INT] void adc_isr(void)
{
	// Read the 8 most significant bits
	// of the AD conversion result and
	// transfer to the transmit byte
	Txbyte=ADCH;
	temp=Txbyte;
     put();
}

//**********************************************************          
//Task 1
void task1(void) 
{  
	time1=t1;    //reset the task timer
	ADCSR.6=1;   //initiate ADC conversion
	PORTB=temp;  //send result to PORTB for debugging
}  

//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
	}
}

void main(void)
{
	//set up timer 0
	reload=256-21; //value for about 1/3 Msec at 16 MHz 
	TCNT0=reload;  //initial reload
	TIMSK=1;	//turn on timer 0 overflow ISR
	TCCR0=4;	//prescalar to 256
    
	//init the task timers
	time1=t1;
	
	DDRB=0xff;            //direction for debugging port
	DDRD=0b00000010;      //direction for UART pins
	
	sb=1;  // set to 1 stop bit
      
	// ADC initialization
	// ADC Clock frequency: 125.000 kHz
	// ADC Voltage Reference: AREF pin
	// Only the 8 most significant bits of
	// the AD conversion result are used
	ADMUX=ADC_VREF_TYPE;
	ADCSR=0x8F;

	// Global enable interrupts
	#asm("sei")
	
	//infinite loop ensures task1 executes at evey overflow
	//ISRs do the rest of the work
	while (1)
	{
	 	//execute task1 when time1 is 0
		if (time1==0) task1();
	}
}