// xmodem interface file

#include <Mega32.h>
#include <stdio.h>       
#include <delay.h>

#define CLK 16000000L
            
#define SOH 0x01
#define EOT 0x04
#define ACK 0x06
#define NAK 0x15
#define CAN 0x18
#define _C_	0x43

void initcomm(int baud) {
	unsigned short ubrr;
	ubrr=CLK/16/baud-1;	// initialize baud rate
	UBRRH=ubrr>>8;
	UBRRL=ubrr&0xff;
	UCSRB=0b00011000; // RX&TX enabled, no interrupts, 8 data bits
}      

#define byte unsigned char

typedef struct {
	byte start;
	byte blockno;
	byte _blockno;
	byte data[128];
	byte crc1; // hibyte
	byte crc2; // lobyte
} xmodem_pkt;

xmodem_pkt pkt;

#define SENDNAK putchar(NAK)
#define SENDACK putchar(ACK)
#define SENDC   putchar('C')

// this function calculates a crc for 128 byte packet
unsigned short int getcrc(byte *buffer) {          
	int i,j;
	unsigned short int crc=0;
	for(i=0;i<128;i++) {      
		crc=crc^(((unsigned short int)buffer[i])<<8);
		for(j=0;j<8;j++) {
			if(crc & 0x8000)
				crc=(crc<<1)^0x1021;
			else
				crc=crc<<1;
		}
	}                         
	return crc;
}
 
int xmodem_receive(int (*callback)(byte *)) { // receive an xmodem file
	int i,j;             
	int errcount=0;   
	byte curblock=1;
	byte check;          
	int eof=0; 
	unsigned short int crc,crcpkt;
 	SENDC;   
	while(!UCSRA.7) { 
		for(i=0;i<1000;i++) {
			delay_ms(1);
			if(UCSRA.7)
				break;
		}
		SENDC;            
	}
//	DDRB=0xff;     
//	PORTB=0xff;
	while(1) {
//   	PORTB=~1;  
		pkt.start=getchar();    
//		PORTB=~2;
		switch(pkt.start) {
			case SOH: // header          
				eof=0;
				pkt.blockno=getchar();
//				PORTB=~4;
				if(pkt.blockno!=curblock) {
					break;
				}        // make sure we're getting the current block
//				PORTB=~8;
				pkt._blockno=getchar();    
//				PORTB=~16;
				crc=0;                                               
				// read the packet and calculate CRC on the fly
				for(i=0;i<128;i++) {
					pkt.data[i]=getchar();
					crc=crc^(((unsigned short int)pkt.data[i])<<8);
                                                                               
					for(j=0;j<8;j++) {
						if(crc & 0x8000)
							crc=(crc<<1)^0x1021;
						else
							crc=crc<<1;
					}   
				}             
//				PORTB=~32;
				// grab the CRC and check
				pkt.crc1=getchar();
				pkt.crc2=getchar();
				crcpkt=(((unsigned short int)pkt.crc1)<<8)|pkt.crc2;
				// check for validity of block number
				if(pkt.blockno!=(~pkt._blockno)) {
					SENDNAK;
					break;
				}        
				// check for CRC
				if(crcpkt!=crc) {
					SENDNAK;
					break;
				}         
//				PORTB=~64;     
				// call our project
				if((*callback)(pkt.data)) {
					// bad packet
					SENDNAK;
				}     
				else         {     
					// good packet
					SENDACK;  
					curblock++;
				}         
//				PORTB=~0;
				break;
			case EOT: // end of transmission
//  				PORTB=~3;
				if(eof==0) {  
  					SENDNAK;
    					eof=1;
	 			} else {
	   				SENDACK;  	
					return 0;
		  		}
				break;
			default: // erroneous      
				eof=0;
//				PORTB=~7;
		}
	}
	return 0;
}    

int xmodem_send(int (*callback)(byte *)) {
	byte resp;
	byte pktno=1;
	byte i,j;   
	int keep=1;
	unsigned short int crc;
	while((resp=getchar())!='C') { 
		if(resp==CAN || resp==EOT)
			return -1;
	}                          
	// fill the 128 byte buffer for the first packet
	keep=(*callback)(pkt.data);
	while(keep) {
		//PORTB=~pktno;
		// generate the packet
		pkt.start=SOH;
		pkt.blockno=pktno;
		pkt._blockno=~pktno;
		pktno++;            
//		crc=getcrc(pkt.data);
//		pkt.checksum=0;
//		for(i=0;i<128;i++) {
//			pkt.checksum+=pkt.data[i];
//		}       
		do {                                       
			// write the packet
			putchar(pkt.start);
			putchar(pkt.blockno);
			putchar(pkt._blockno);
			
	  		crc=0;
			// write the actual data and calculate CRC on the fly
			for(i=0;i<128;i++) {
				crc=crc^(((unsigned short int)pkt.data[i])<<8);
				putchar(pkt.data[i]);
				for(j=0;j<8;j++) {
					if(crc & 0x8000)
						crc=(crc<<1)^0x1021;
					else
						crc=crc<<1;
				}
				
			}         
			pkt.crc1=crc>>8;
			pkt.crc2=crc&0xff;
			// write the CRC			
			       
//			putchar(pkt.checksum);
			putchar(pkt.crc1);
			putchar(pkt.crc2); 
			// grab the next packet
			keep=(*callback)(pkt.data);
			resp=getchar();
			if(resp!=NAK && resp!=ACK) {
				//PORTB=~0b11110000;
				return -1;            
			}
		} while(resp==NAK);
	}         
	putchar(EOT);
	return 0;
}
#ifdef XMODEMDEBUG // for XMODEM testing only
// callback function that is called with the 128 byte buffer
// callback returns nonzero if there's an error
int sample_callback(byte *buf) {
	return 0;
}      

byte state=0;
byte count=100;
                    
// callback for sender
// this function fills a 128 byte buffer
// return 0 when finished
int sample_callback2(byte *buf) {      
	int i;
	if(count==0) return 0;
	for(i=0;i<128;i++) {
		buf[i]=state;  
		state=state*0xa6;
		state=state+1;
	}	     
	count--;               
	return 1;
}    

void main(void) {
	int i;
	initcomm(19200);       
	DDRB=0b11111111;
	//PORTB=0;
	for(i=0;i<1000;i++);
	//PORTB=0b11111111;              
	putsf("XMODEM TEST\r\n");
	while(1) {            
		putsf("MENU:\r\n");
		putsf(" 1) Upload file\r\n");
		putsf(" 2) Download file\r\n");
		i=getchar();  
		switch(i) {
			case '1':  
				putsf("Start sending on your end with XModem CRC\r");
				xmodem_receive(sample_callback);
				putsf("TRANSFER DONE\r\n");    
				i=UDR;
				break;
			case '2':
				putsf("Start receiving on your end with XModem CRC\r");
				state=0;
				count=100;
				xmodem_send(sample_callback2);
				putsf("TRANSFER COMPLETE\r\n");
				i=UDR;
				break;
			default:
				putsf("Unknown command\r\n");
				break;
		}
	}
}
#endif