timing.cpp

/*
The millis() function known from Arduino
Calling millis() will return the milliseconds since the program started
Tested on atmega328p
Using content from http://www.adnbr.co.uk/articles/counting-milliseconds
Author: Monoclecat, https://github.com/monoclecat/avr-millis-function
REMEMBER: Add sei(); after init_millis() to enable global interrupts!
 */

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

#include "timing.h"

namespace timing
{
    namespace
    {
        volatile unsigned long timer1_millis;

        ISR(TIMER1_COMPA_vect)
        {
            timer1_millis++;
        }
    }

    void init(void)
    {
        unsigned long ctc_match_overflow;

        ctc_match_overflow = ((F_CPU / 1000) / 8); //when timer1 is this value, 1ms has passed

        // (Set timer to clear when matching ctc_match_overflow) | (Set clock divisor to 8)
        TCCR1B |= (1 << WGM12) | (1 << CS11);

        // high byte first, then low byte
        OCR1AH = (ctc_match_overflow >> 8);
        OCR1AL = ctc_match_overflow;

        // Enable the compare match interrupt
        TIMSK1 |= (1 << OCIE1A);
    }

    unsigned long millis (void)
    {
        unsigned long millis_return;

        // Ensure this cannot be disrupted
        ATOMIC_BLOCK(ATOMIC_FORCEON)
        {
            millis_return = timer1_millis;
        }
        return millis_return;
    }

    void delay_us(uint16_t us)
    {
        for (uint16_t t = 0; t < us; t++)
        {
            #if F_CPU <= 20000000 && F_CPU > 10000000
            __asm__ __volatile__ ("nop");
            __asm__ __volatile__ ("nop");
            #else
            __asm__ __volatile__ ("nop");
            #endif
        }
    }
}