#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/power.h>
#include <avr/sleep.h>

/* Pinout used:
 *         -----
 * !reset - MCU - Vcc
 * NA     - MCU - NA
 * NA     - MCU - NA
 * GND    - MCU - (PB0) powerbutton
 *         -----
 */

static const uint8_t SLEPT_TIMEOUT = 25;  /* ~2.5s */

static void init_timer()
{
    /* timer_resolution = 1 / (clock_speed / prescaler)
     * timer_resolution = 1 / (10**6 / 1024)
     *
     * in ctc mode, target counts:
     *
     * timer_counts = (target_time / timer_resolution) - 1
     * timer_counts = (0.1         / (1/(10**6/1024))) - 1
     * timer_counts = 96.65625000000001 =~ 97
     *
     * Why did we add the extra +1 to our number of timer counts? In CTC mode,
     * when the timer matches our desired count it will reset itself to zero.
     * This takes one clock cycle to perform, so we need to factor that into
     * our calculations.
     *
     * VG notes: more simply 0 to 97 = 98 values = 98 cycles.
     *
     * trigger_time = (97+1) * (1/(10**6 / 1024)) =~ 0.100352s
     *
     * WGM0[2:0] = 010 = CTC
     */
    TCCR0A =
            (1 << WGM01) |     // CTC
            (0 << WGM00);      // CTC
    TCCR0B =
            (0 << WGM02) |     // CTC
            (1 << CS02)  |     // CS0[2:0] = 101 => prescaler clk/1024
            (0 << CS01)  |     // CS0[2:0] = 101 => prescaler clk/1024
            (1 << CS00);       // CS0[2:0] = 101 => prescaler clk/1024
    OCR0A = 97;
    TIMSK =
            (1 << OCIE0A) |    // enable CTC interrupt for compare match 0A
            (0 << OCIE0B) |    // disable CTC interrupt for compare match 0B
            (0 << TOIE0);      // disable interrupt for timer0 overflow

}

static volatile uint8_t sleep_time;

ISR(TIMER0_COMPA_vect)
{
    ++sleep_time;
}

static uint8_t get_sleep_time(void)
{
    return sleep_time;
}

static void reset_sleep_time(void)
{
    sleep_time = 0;
}

int main()
{
    cli();
    PORTB = 0; // outputs set to low and input not set to internal pull-up
    init_timer();
    sei();

    // Normally a loop is not required but if somehow the mcu woke up from its
    // sleep state this ensure we can do the operation again.
    for (;;) {
        reset_sleep_time();

        // digital output, pull-down for a time then go to default PB0 = Hi-Z
        DDRB |= (1 << PB0);
        while (get_sleep_time() <= SLEPT_TIMEOUT);
        DDRB &= ~(1 << PB0);

        /* set low power mode */
        power_all_disable();    // disable all peripherals
        set_sleep_mode(SLEEP_MODE_PWR_DOWN); // sleep mode
        sleep_enable();         // set register to enable sleep
        sleep_bod_disable();    // disable bod to consume even less
        //sei();                // normally set sei to ensure wakeup but I do
                                // not want to wake up appart from a power
                                // reset
        sleep_cpu();            // put cpu to sleep = low power mode now
        sleep_disable();        // clear register (if somehow mcu woke up)
    }

    return 0;
}