/*
See: https://www.gadgetronicx.com/attiny85-timer-tutorial-generating-time-delay-interrupts/$0
There are two timers on ATtiny85 Timer0 and Timer1. 
Timer0 is a 8 bit timer, it can count from 0 to 255.
Configuration of Timer0 register can be done using two registers TCCR0A and TCCR0B

** Steps to configure Timer and use 'Interrupt' to generate delay:
1. Select the normal mode of operation for timer0
2. Prescale the internal clock by a factor of 1024
3. Enable Global interrupt enable I bit in SREG register
4. Enable TOIE0 bit in TIMSK register to enable Timer0 interrupt.
5. Write the Interrupt service routine to repeat the counting operation 63 times to obtain a delay of 1 second.
6. Toggle the LED connected to PB1 pin.

Sketch assumes ATTiny85 at 8.0 MHz
Use calculator at https://eleccelerator.com/avr-timer-calculator/
*/

#include<avr/io.h>
#include <avr/interrupt.h>
#include <TinyDebug.h>

#define F_CPU 8000000UL
#define LED1 PB1 // output

volatile uint8_t intr_count = 0;
volatile uint8_t ISR_done = 0;

ISR (TIMER0_OVF_vect) { // Interrupt vector for Timer0
  intr_count++;
  if (intr_count == 30) { // 30 flags = 1 sec, value per https://eleccelerator.com/avr-timer-calculator/:
    //PORTB ^= (1 << PB1); //toggle on 1 sec, toggle off 1 sec
    ISR_done = 1;
    intr_count = 0; // set count back to zero
  }
}

void timer_setup() {
  DDRB |= (1 << LED1); // Set Port B, pin PB1 as output

  // TCCR0A – Timer/Counter Control Register A
  // We want 'Mode of Operation' set to 'Normal' so all bits are set to 0
  TCCR0A = 0x00; // Set bits WGM00 and WGM01 to 0 for 'Normal' mode

  // TCCR0B – Timer/Counter Control Register B
  // This line of code isn't really needed, or could use -
  // TCCR0B &= ~(1 << WGM02);
  TCCR0B = 0x00; // Set bit WGM02 to 0 for 'Normal' mode

  // CS02  CS01  CS00
  //  1     0     1
  TCCR0B |= (1 << CS00) | (1 << CS02); // prescaling with 1024

  /* 
  SREG – AVR Status Register
  Bit 7 – I: Global Interrupt Enable
  This bit must be enabled in order to use any interrupt on ATtiny85
  SREG |= (1 << 7);
  or
  SREG = SREG | 0b10000000;
  or
  */
  sei(); // enabling global interrupt

  // TCNT0 – Timer/Counter Register - 8 bit
  /*
  This is a 8 bit data register at which the timer value gets stored.
  Being a 8 bit register this register can hold values from 0 to 255.
  Upon completion of this count in TCNT0 register timer sets the Overflow flag
  */
  TCNT0 = 0;

  /*
  TIMSK – Timer/Counter Interrupt Mask Register
  Responsible for activating and deactivating interrupts associated with Timer0 and Timer1 of ATtiny85.
  The bit TOIE0 enables / disables Timer0 interrupts. Writing one to this bit enables the interrupt.
  */
  TIMSK |= (1 << TOIE0); // enabling timer0 interrupt

  PORTB |= (1 << PB1);
}

int main () {
  Debug.begin();
  timer_setup();
  while(1) {
    if (ISR_done) {
      Debug.print("if start: "); Debug.println(ISR_done);
      PORTB ^= (1 << PB1);
      ISR_done = 0;
      Debug.print("if end: "); Debug.println(ISR_done);
    }
  }
}