void write_to_flash(unsigned char flash_data)
{
    static unsigned int buffer_counter;
    static unsigned int page_counter;
    unsigned char temp = 0x80;
    
    if((ACSR & 0x02))                       // if flag is set that new data has to be written
    {
        buffer_counter = 0;
        page_counter = 0;                   // reset the counter if new data has to be written
        ACSR &= 0xFD;                       // clear the signal flag
    }

    while(!(PINB & 0x02));                  // check if flash is busy

    PORTB &= ~DF_CHIP_SELECT;               // enable DataFlash
    
    SPDR = BUFFER_1_WRITE;
    while (!(SPSR & temp));                 // wait for data transfer to be completed
    SPDR = 0x00;                            // don't cares
    while (!(SPSR & temp));                 // wait for data transfer to be completed
    SPDR = (char)(buffer_counter>>8);       // don't cares plus first two bits of buffer address
    while (!(SPSR & temp));                 // wait for data transfer to be completed        
    SPDR = (char)buffer_counter;            // buffer address (max. 2^8 = 256 pages)
    while (!(SPSR & temp));                 // wait for data transfer to be completed    
    SPDR = flash_data;                      // write data into SPI Data Register
    while (!(SPSR & temp));                 // wait for data transfer to be completed
    
    PORTB |= DF_CHIP_SELECT;                // disable DataFlash
    
    buffer_counter++;
        
    if (buffer_counter > 528)               // if buffer full write buffer into memory page
    {
        buffer_counter = 0;
        if (page_counter < 4096)            // if memory is not full  
        {
            PORTB &= ~DF_CHIP_SELECT;       // enable DataFlash
        
            SPDR = B1_TO_MM_PAGE_PROG_WITHOUT_ERASE; // write data from buffer1 to page
            while (!(SPSR & temp));         // wait for data transfer to be completed
            SPDR = (char)(page_counter>>6);
            while (!(SPSR & temp));         // wait for data transfer to be completed
            SPDR = (char)(page_counter<<2);
            while (!(SPSR & temp));         // wait for data transfer to be completed
            SPDR = 0x00;                    // don't cares
            while (!(SPSR & temp));         // wait for data transfer to be completed                      
        
            PORTB |= DF_CHIP_SELECT;        // disable DataFlash
        
            page_counter++;
        }
        else
        {
            PORTB |= 0x08;                  // turn LED off
            while (!(PIND & 2));            // wait until button for recording (button 1) is released
        }
    }
}