di2704.txt
;************************************************************************************************
; LISTING 1 - SOURCE CODE FOR A VIRTUAL-EEPROM BASED RANDOM-NUMBER GENERATOR
;
; "Rolling-code generator uses flash microprocessor," EDN, May 3, 2001, pg 94
;
; http://www.ednmag.com/ednmag/reg/2001/050301/designideas.htm#09di5
;************************************************************************************************


#include <8cbr.h>
#include <flash_op.h>


//***************************************************************
//
// Description: The following is a random number generator,
//              its implementation is the use of a 16 bit
//              LFSR, by XORing the correct bits we arrive
//              at a pseudo random number
//
// Implementation: The pseudo number is created by using the
//                 feedback equation (for a 16 bit word)
//                 bit16=bit5 XOR bit4 XOR bit3 XOR bit0
//
//***************************************************************

void random(){

        unsigned int seed_upper;  // The seed variables
        unsigned int seed_lower;

        // Seed the random number generator
        // Choose any random numbers for the MSB and
        // LSB of the initial random numbers

        readbf(0x1FFF);    // Do a read at the high location
        seed_upper=ISPRD;

        readbf(0x2000);    // Do a read at the low location
        seed_lower=ISPRD;

        if (!seed_upper && !seed_lower){      // If it is zero then continue

           }

        else {

          seed_upper=10;  // Otherwise give it a seed
          seed_lower=20;

          }

        bits temp1;		 // Some Xoring Bits
        bits temp2;


        char flag;
        bits carry;

        temp1=(bits)seed_upper;		 // Assign the upper & lower
        temp2=(bits)seed_lower;

        carry.0=temp2.5^temp2.4;	 // Now do the Xoring
        carry.0=carry.0^temp2.3;
        carry.0=carry.0^temp2.0;


        if (temp2.0)
            flag=1;

            else flag=0;


        temp1=temp1>>1;					 // Bit shift left
        temp2=temp2>>1;


        if (flag)
            temp2.7=1;


        temp1.7=carry.0;				 // Use the Carry Bit


        seed_upper=temp1;
        seed_lower=temp2;

        page_erase(0x1FFF);		// Erase the location
        cwritebf(0x1FFF,seed_upper);	// Save the upper byte
        cwritebf(0x2000,seed_lower);	// Save the lower byte


} // end of the random routine



void main(){

        unsigned int i;			// A counter variable
        unsigned int random_arr[2];	// A random number array

                 // Set the clock timer 
                 PGMTIM=0x7B;

   	
                                        // copy it back to the random numbers
                 readbf(0x1FFF);    // Read the result
                 random_arr[0]=ISPRD;	// From the ISPRD register

                 readbf(0x2000);
                 random_arr[1]=ISPRD;	// Both the Upper and Lower 
					// Seed
        while (1){
				 


		 page_erase(0x1FFF);		  // Page Erase

		 cwritebf(0x1FFF,random_arr[0]);  // Write the byte back
		 cwritebf(0x2000,random_arr[1]);  // Write the byte back


     		 // Get a number ramdomly generated through 100 iterations

        	 for (i=0;i<100;i++)		// Call the random number
            	{				// generator a hundred times
	            random();
        	}		 				 
		 
				 	// Application code goes over here

                }

  

} // end of main