/***********************************************************
	Chris Foster and Jeff Puchalski
	ECE 476 Spring 2003
	Project - Xmodem Protocol Test
***********************************************************/

#include <Mega32.h>
#include <delay.h>
#include <string.h>
       
/* States */
#define IDLE 1
#define RECV 2
#define DONE 3

/* Xxmodem codes */
#define SOH 0x01 // start of header
#define EOT 0x04 // end of transmission
#define ACK 0x06 // acknowledgement
#define NAK 0x15 // negative acknowledgement
#define CAN 0x18 // cancel
#define EOF 0x1a // end of file

#define PACKET_SIZE 128     

#define Rxmode 1
#define Ready 2 

#define Baud 103 // 9600 baud w/ 16 MHz crystal

/* Xxmodem packet format
		Byte	Description
		1		SOH (start of header)
		2		Packet number
		3		~(Packet number)
		4-131	Packet data (128 bytes)
		132		checksum
*/ 

// Global variables
unsigned char packet_num;
unsigned int byte_num;
unsigned char eot_num;
unsigned char xmode;
unsigned char buf[PACKET_SIZE];		// stores the current packet being recieved
unsigned char data[PACKET_SIZE];
unsigned char data_loaded;
unsigned char state;
unsigned char checksum;

void x_init(void) {
	unsigned char i;
           
	byte_num = 0; 
	packet_num = 1;  
	eot_num = 0;
	OCR1A = 313;
	TCNT1 = 0;
	xmode = Ready;
	
	for (i=0; i < PACKET_SIZE; i++)
		buf[i] = data[i] = 0;
}                                                 
   
void x_receive(void) {
	unsigned char c;
	
	while (xmode == Rxmode) {     
		if (byte_num == 0 && packet_num == 1) {
			delay_ms(20000);
			UDR = NAK;
			byte_num = 1; 
		}
		else if (UCSRA.7) {    
			c = UDR;	// current byte sent
			
			if (byte_num == 1) {	// SOH      
				switch (c) {
					case SOH : // have the start byte
						checksum = 0;
						byte_num++;
						break;
					case EOT :
						// send a NAK, and expect a EOT as the next byte
						// we do not increment byte_num because we want 
						// to check if the next byte is also a EOT
						if (eot_num == 0) { // 1st EOT byte
							eot_num++;
							while (!UCSRA.5); // wait for output buffer
							UDR = NAK;
						}
						else { // 2nd EOT byte
							// need to end transmission
							// turn off Xmodem transfer
							eot_num = 2;
							while (!UCSRA.5);
							UDR = ACK;
							xmode = Ready;
							return;
						}
						break;  
					default:
						byte_num = 1;
						break;    
				}
				continue;
			}                
			if (byte_num == 2) { // packet_num
				if (c == packet_num) byte_num++;
				else byte_num = 1;       
				continue;
			}
			if (byte_num == 3) { // ~(packet_num)
				if ((c + packet_num) == 255) byte_num++;                  
				else byte_num = 1;
				continue;
			}
			if ((byte_num >= 4) && (byte_num < (4 + PACKET_SIZE))) { // packet data
				buf[byte_num-4] = c;
                checksum += c; // add to the checksum calculation			
				byte_num++;
				continue;
			}
			if (byte_num == (4 + PACKET_SIZE)) { // checksum
  				byte_num = 1; // packet ended
  				
  				if (checksum == c) { // checksum correct, accept packet
  					if (packet_num == 1) strncpy(data,buf,PACKET_SIZE);
  				
					packet_num++;      
					while (!UCSRA.5);
					UDR = ACK;
				}
				else { // checksum is wrong, need to resend packet
					while (!UCSRA.5);     
					UDR = NAK;
				}
				
		  		data_loaded = 1;		  
				continue;
			}
		}
	}
}

void uart_echo(char *str) {
	char *temp;	
	temp = str;   
	                                                      
	// xmodem protocol uses ^Z for padding packets
	while (*temp != 0x1a) {	// 0x1a = SUB (substitute, ^Z)
		while (!UCSRA.5);
		UDR = *temp;
		temp++; 
	}
} 

void main(void) {
	unsigned char c;

	// set up the ports
	DDRD=0x00;		// PORT C is the UART serial port
	DDRB=0xff;		// PORT B is LEDs
	PORTB=0x00;
	
	// set up the UART
	UBRRH = Baud >> 8; 
	UBRRL = Baud;	
	UCSRB = 0x18;	// enable receiver and transmitter
	UCSRC = 0x86;	// async mode, 1 stop bit, 8-bit char size
                  
	state = IDLE;
 	data_loaded = 0;
 	x_init();

	while (1) {
		PORTB= ~state;	
	
		switch (state) {
			case IDLE :
				// poll RX done bit
	   			if (UCSRA.7) {
		 			c = UDR;
		 			UDR = c;
			
		 			if (c == 'l' && xmode == Ready) {
						state = RECV;
						xmode = Rxmode;
					 	x_receive();
					}
		 		}
		 		break;
		 	case RECV :
		 		while (!data_loaded);
		 		state = DONE;
		 		break;
		 	case DONE :       
		 		uart_echo(data);
		 		while (1);
		 		break;
		}				
   	}
}