#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

volatile uint8_t line_counter ;

#define WRITE_DISPLAY(c) while(!(UCSR0A & _BV(UDRE0)));\
                         UDR0 = c;

int main(void)
{
	// configure UART for SPI. 
	UCSR0C = _BV(UMSEL01) | _BV(UMSEL00);
	// set baudrate to Fosc/4
	UBRR0H = 0;
	UBRR0L = 1;
	// only enable tx, no rx.
	UCSR0B = _BV(TXEN0);
	// Use timer for selecting lines
	TCCR0A = _BV(WGM01);
	// run timer on Fosc/256 => 62.5us/count
	TCCR0B = _BV(CS02);
	// about 4ms/line, 60ms for entire display
	OCR0A = 60;		
	TIMSK0 = _BV(OCIE0A);	

	uint8_t i,j;
	uint8_t line_counter_prev;

	for(i=16; i<34; i++) {
		WRITE_DISPLAY(0x02);
		WRITE_DISPLAY(i);
		WRITE_DISPLAY(0x00);
		for (j=0; j<6; j++){	
			// 7 time 0xff is on, 6 times 0xef is on?
			// 6 time 0xff is off, 7 or 6 teime 0xfe is off
			WRITE_DISPLAY(0xef);
		}
	};
	sei();

	// this code unlocks the shift registers
	WRITE_DISPLAY(0x00);
	WRITE_DISPLAY(0x02);
	WRITE_DISPLAY(0x0f);
	WRITE_DISPLAY(0x00);
	
	sei();
	while (1) {
		if (line_counter != line_counter_prev) {
			cli();
			for (j=0; j < 8; j++) {
				line_counter_prev = line_counter;
				for (i=0; i<1; i++) {
					WRITE_DISPLAY(0);
				}
				WRITE_DISPLAY(0x00)
				for (i=0; i<(line_counter+1); i++) {
					WRITE_DISPLAY(0x03)
					WRITE_DISPLAY(0x00)
				}
				WRITE_DISPLAY(0x00)
				for (i=0; i<(23-line_counter); i++) {
					WRITE_DISPLAY(0x03)
					WRITE_DISPLAY(0x01)
				}
				WRITE_DISPLAY(0x00)
			}
			sei();

		}
		WRITE_DISPLAY(0x00)
	}
	return 666;
}

ISR(TIMER0_COMPA_vect) {
	// wait for UART free
	WRITE_DISPLAY(0x00);
	WRITE_DISPLAY(0x02);
	WRITE_DISPLAY(0xa0 | line_counter);
	if (line_counter == 0x0f) {
		line_counter = 0;
	} else {
		line_counter += 1;
	}
	WRITE_DISPLAY(0x80);
	return;
}