high resolution frequency counter implementation

high resolution frequency counter implementation for atmega328p

part of my lcmeter project available here:

https://github.com/texane/lcmeter

/* high resolution frequency counter implementation
*/

/* timer2 interrupt handler. timer1 is an extended
32 bits register (16 bits hard + 16 softs)
incremented once per:
1 / (fcpu / prescal) <=> prescal / fcpu
thus, it will overflow at:
2^16 * prescal / fcpu
on tim2 overflow, the interrupt handler is called
and stores the tim1 current value in tim1_cur_counter.
thus, the tim1 value integrated over the whole
tim1 period is:
(tim1_ovf_counter * 2^16) + tim1_cur_counter.
tim2_is_ovf is set to notify the application.
*/

static volatile uint8_t tim2_ovf_counter;
static volatile uint8_t tim2_is_ovf;
static volatile uint16_t tim1_cur_counter;

ISR(TIMER2_OVF_vect)
{
  if ((tim2_ovf_counter--) == 0)
  {
    /* disable tim1 before reading */
    TCCR1B = 0;
    tim1_cur_counter = TCNT1;

    /* disable tim2 */
    TCCR2B = 0;

    tim2_is_ovf = 1;
  }
}

/* timer2 interrupt handler. timer2 is a 8 bits counter
incremented by the input signal rising edges. since
8 bits are not enough to integrate, an auxiliary
register (tim2_ovf_counter) is updated on overflow.
tim2_ovf_counter is an 8 bits register, and will
overflow without any notice past 0xff.
*/

static volatile uint8_t tim1_ovf_counter;

ISR(TIMER1_OVF_vect)
{
  ++tim1_ovf_counter;
}

static void hfc_start(void)
{
  /* resolution: 1.907349 hz per tick */
  /* fmax: 500 khz */
  /* acquisition time: 0.524288 seconds */

  /* disable interrupts */
  TIMSK1 = 0;
  TIMSK2 = 0;

  /* reset stuff */
  tim1_ovf_counter = 0;
  tim1_cur_counter = 0;
  tim2_is_ovf = 0;

  /* 0x100 overflows make 16 bits */
  tim2_ovf_counter = 0xff;

  /* configure tim2
normal operation
prescaler 128
enable interrupt on overflow
*/
  TCNT2 = 0;
  TIMSK2 = 1 << 0;
  TCCR2A = 0;
  TCCR2B = 0;

  /* configure tim1
t1 pin (pd5) rising edge as external clock
*/
  DDRD &= ~(1 << 5);
  TCNT1 = 0;
  TIMSK1 = 1 << 0;
  TCCR1A = 0;
  TCCR1B = 0;

  /* start tim1, tim2 */
  TCCR1B = 7 << 0;
  TCCR2B = 5 << 0;
}

static uint8_t hfc_poll(void)
{
  return tim2_is_ovf;
}

static uint32_t hfc_wait(void)
{
  /* busy wait for tim1 to overflow. returns the resulting
16 bits counter, to be multiplied by the frequency
resolution (refer to hfc_start) to get the actual
frequency.
*/

  /* force inline, do not use hfc_poll */
  while (tim2_is_ovf == 0) ;

  return ((uint32_t)tim1_ovf_counter << 16) | (uint32_t)tim1_cur_counter;
}

static inline uint32_t hfc_start_wait(void)
{
  hfc_start();
  return hfc_wait();
}

static inline double hfc_to_hz(uint32_t counter)
{
  return 1.907349 * (double)counter;
}