ardupilot/libraries/AP_PeriodicProcess/AP_TimerAperiodicProcess.cpp

#include "AP_TimerAperiodicProcess.h"

extern "C" {
#include <inttypes.h>
#include <stdint.h>
#include "WConstants.h"
}
// TCNT2 values for various interrupt rates,
// assuming 256 prescaler. Note that these values
// assume a zero-time ISR. The actual rate will be a
// bit lower than this
#define TCNT2_781_HZ   (256-80)
#define TCNT2_1008_HZ  (256-62)
#define TCNT2_1302_HZ  (256-48)

uint8_t AP_TimerAperiodicProcess::_timer_offset;

void AP_TimerAperiodicProcess::init( Arduino_Mega_ISR_Registry * isr_reg )
{
	// Enable Timer2 Overflow interrupt to trigger process.
	TIMSK2 = 0;                 // Disable interrupts
	TCCR2A = 0;                 // normal counting mode
	TCCR2B = _BV(CS21) | _BV(CS22);	// Set prescaler of clk/256
	TCNT2  = 0;                 // Set count to zero, so it goes off right away.
	TIFR2  = _BV(TOV2);	        // clear pending interrupts;
	TIMSK2 = _BV(TOIE2);        // enable the overflow interrupt

    isr_reg->register_signal(ISR_REGISTRY_TIMER2_OVF, AP_TimerAperiodicProcess::run);
}

void AP_TimerAperiodicProcess::run(void)
{
    _timer_offset = (_timer_offset + 49) % 32;
    _period = TCNT2_781_HZ + _timer_offset;
    TCNT2 = _period;
    for (int i = 0; i < _pidx; i++) {
        if (_proc[i] != NULL)
            _proc[i]();
    }
}