   // This volatile variable sits in RAM and is modified by our fast interrupt
volatile uint32_t pulse_count = 0;
 
// ==========================================
// THE INTERRUPT SERVICE ROUTINE (ISR)
// IRAM_ATTR forces this into fast RAM to handle high speeds
// ==========================================
void IRAM_ATTR countPulse() 
{
  pulse_count++;
}
 
long readSensor() 
{
  const int NUM_SAMPLES = 5;
  uint32_t total_frequency = 0;
 
  Serial.println("--- Waking up Sensor for Batch Read ---");
 
  // 1. Parasitic Power: Power up the sensor ONCE for the whole batch
  digitalWrite(SENSOR_POWER_PIN, HIGH);
  delay(10); // Saturate traces
 
  // 2. The Throwaway Read (Clear startup noise)
  pulse_count = 0;
  attachInterrupt(digitalPinToInterrupt(SENSOR_SIGNAL_PIN), countPulse, RISING);
  delayMicroseconds(20000); 
  detachInterrupt(digitalPinToInterrupt(SENSOR_SIGNAL_PIN));
 
  // 3. The Batch Read
  for (int i = 0; i < NUM_SAMPLES; i++) {
    pulse_count = 0; 
    attachInterrupt(digitalPinToInterrupt(SENSOR_SIGNAL_PIN), countPulse, RISING);
    delayMicroseconds(20000); // Exactly 20ms
    detachInterrupt(digitalPinToInterrupt(SENSOR_SIGNAL_PIN));
 
    long current_freq = pulse_count <span className="italic"> 50; // Convert 20ms window to Hz (1 sec)
    total_frequency += current_freq;
  }
 
  // 4. Kill the power immediately to save battery
  digitalWrite(SENSOR_POWER_PIN, LOW); 
 
  long average_freq = total_frequency / NUM_SAMPLES;
 
  // 5. Sanity Check / Hardware Validation
  if (average_freq == 0 || average_freq > 800000) {
    Serial.println("ERROR: Frequency out of bounds!");
    return 0; 
  }
 
  return average_freq;
}
From Breadboard to Product: Designing a Low-Power IoT Capacitive Soil Sensor (Part 1)
