uint32_t count = 0;  // sample counter
uint32_t output = 0; // output reading. The period is stored here

int64_t integral = 0; // integral of the signal starting from 0-crossing
int32_t dx = 0;       // derivative at zero crossing
int64_t ramp = 0;     //"tolerance" ramp

bool sign_old = 0; // sign of previous sample
bool sign = 0;     // sign of current sample

int32_t in_old; // old input

outlet_smp = output; // output the reading
outlet_trig = 0;

count++;                     // increase the counter by 1
sign = inlet_in > 0 ? 1 : 0; // calculate the sign of curent input
integral += inlet_in;        // integrate the input
ramp += ___SMMUL(
    dx, param_tol
            << 2); // integrate the ramp, using the derivative at zero crossing

// too early
bool trig = (count > attr_min) && ((ramp > 0) ? (integral < ramp ? 1 : 0)
                                              : (integral > ramp ? 0 : 1));
// this condition serves to filter out meaningless zero crossings:
// if count<attr_min it means it's too early to provide a reading (maybe there's
// some noise in the signal) the second condition serves to test if the mean
// value of the signal is sufficiently close to 0 let's be honest: we'll never be
// so lucky to get it exactly to 0, so we'll settle at a sufficiently small value
// how small? it depends on how steep was the signal at zero crossing and on how
// happy we are with approximate readings

if (sign != sign_old) // a zero crossing has occurred
{
  if (trig) // if the "condition zero" i've explained before is met, we have a
            // readout!
  {
    output = count;         // readout
    count = 0;              // resetting the counter
    integral = 0;           // resetting the integrator
    ramp = 0;               // resetting the ramp integrator
    dx = inlet_in - in_old; // calculating a new derivative at 0x
    outlet_trig = 1 << 27;
  }
  if (count > attr_max) // the reading is taking too long. I'll start counting
                        // from here without passing the readout.
  {
    count = 0;              // resetting the counter
    integral = 0;           // resetting the integrator
    ramp = 0;               // resetting the ramp integrator
    dx = inlet_in - in_old; // calculating a new derivative at 0x
  }
}

sign_old = sign;
in_old = inlet_in;