#include <avr/io.h>      // this contains all the IO port definitions
#include <avr/interrupt.h>
#include <avr/signal.h>

#define TIMER1_PRESCALE_1 1
#define TIMER1_PRESCALE_8 2
#define TIMER1_PRESCALE_64 3
#define TIMER1_PRESCALE_256 4
#define TIMER1_PRESCALE_1024 5

#define HEX__(n) 0x##n##UL

#define B8__(x) ((x&0x0000000FLU)?1:0)  \
               +((x&0x000000F0LU)?2:0)  \
               +((x&0x00000F00LU)?4:0)  \
               +((x&0x0000F000LU)?8:0)  \
               +((x&0x000F0000LU)?16:0) \
               +((x&0x00F00000LU)?32:0) \
               +((x&0x0F000000LU)?64:0) \
               +((x&0xF0000000LU)?128:0)

#define B8(d) ((unsigned char)B8__(HEX__(d)))

const static int image[] = {
  B8(00111100),
  B8(00000100),
  B8(00111000),
  B8(00000100),
  B8(00111000),
  B8(00000000),
  B8(00111101),
  B8(00000000),
  B8(00111100),
  B8(00000100),
  B8(00000100),
  B8(00111000),
  B8(00000000),
  B8(00111101),
  B8(00000000),
  B8(11111100),
  B8(00100100),
  B8(00100100),
  B8(00011000),
  B8(00000000),
  B8(00011000),
  B8(00100100),
  B8(00100100),
  B8(00011000),
  B8(00000000),
  B8(00000100),
  B8(00111000),
  B8(00010000),
  B8(00001100),
  B8(00000000),
  B8(00101111),
  B8(00000000),
  B8(00000000),
  B8(00000000),
  B8(00000000),
  B8(00000000),
  B8(00000000),
};

int imagesize = 35;


// This function basically wastes time, in case you want to
// do some other timing stuff
void delay_ms(long int ms) {
  unsigned long int timer;

  while (ms != 0) {
    // this number is dependant on the clock frequency
    for (timer=0; timer <= 4200; timer++);
    ms--;
  }
}

// this function is called when timer1 compare matches OCR1A

uint8_t j = 0;  // the current image pointer
uint8_t k = 0;  // the image subtype

// interrupt routine has been improved to blend adjacent pixel lines
// so as to improve the quality of the display; trebor -- 12/19/05

SIGNAL( SIG_TIMER1_COMPA ) {

  // wrap around one pixel line early, to prevent going off the edge
  // of the buffer

  if (j >= (imagesize - 1)) 
    j = 0;

  // alternate between current image and current+next.  step to
  // next image every other frame.
  
  if (k == 0) {
    PORTB = image[j];
    k = 1;
  } else {
    PORTB = image[j] | image [j+1];
	k = 0;
    j++;
  }
  
}

int main(void) {

  DDRB = 0xFF;       // set all 8 pins on port B to outputs

  /*
    the frequency of the interrupt overflow is determined by the 
    prescaler and overflow value.
    freq = clock_frequency / ( 2 * prescaler * overflow_val)
    where prescaler can be 1, 8, 64, 256, or 1024
    clock_freq is 8MHz
    and overflow_val is 16bit

    the overflow value is placed in OCR1A, the prescale is set in TCCR1B
    so for example:
    A good POV frequency is around 400Hz
    desired freq = 400Hz
    clock freq = 8MHz
    8MHz / (400Hz * 2) = 10000
    since 10000 is less than 655536 (largest 16 bit number)
    OCR1A = 10000 and the prescale is 1
  */

  TCCR1B = (1 << WGM12) | TIMER1_PRESCALE_1;
  OCR1A = (uint16_t)10000;

  TIMSK |= 1 << OCIE1A;   // Output Compare Interrupt Enable (timer 1, OCR1A) 

  sei();                 // Set Enable Interrupts

  while (1);
}