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Overview

The Free Fall Timer firmware measures the time interval between two PIR (Passive Infrared) sensors detecting an object in free fall. It uses hardware interrupts for precise timing and supports both Arduino (ATmega328) and ESP32 platforms.

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Pin Configuration

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Arduino Version

#define PIN_INICIO 2   // INT0 - Start sensor
#define PIN_FIN    3   // INT1 - End sensor

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ESP32 Version

#define PIN_INICIO 18  // Start sensor
#define PIN_FIN    26  // End sensor (safe pin)

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Hardware Timer Configuration

The Arduino version uses Timer1 configured for 1ms interrupts: 
// Timer1 Configuration (1ms interrupts)
noInterrupts();
TCCR1A = 0;
TCCR1B = 0;
TCNT1  = 0;

OCR1A = 249;                          // 250 ticks → 1 ms
TCCR1B |= (1 << WGM12);               // CTC mode
TCCR1B |= (1 << CS11) | (1 << CS10); // Prescaler 64
TIMSK1 |= (1 << OCIE1A);              // Enable interrupt

interrupts();

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Timer ISR

ISR(TIMER1_COMPA_vect) {
  contadorMs++;  // Increment every 1ms
}

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State Machine

The firmware uses a state-based approach with volatile flags: 
volatile bool esperandoInicio = true;  // Waiting for start
volatile bool esperandoFin = false;    // Waiting for end
volatile bool datoListo = false;       // Data ready flag
volatile unsigned long contadorMs = 0; // Millisecond counter

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Interrupt Service Routines

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Start Sensor ISR (Arduino)

void isrInicio() {
  if (esperandoInicio) {
    noInterrupts();
    contadorMs = 0;              // Reset counter
    esperandoInicio = false;
    esperandoFin = true;
    interrupts();
  }
}

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End Sensor ISR (Arduino)

void isrFin() {
  if (esperandoFin) {
    noInterrupts();
    esperandoFin = false;
    esperandoInicio = true;
    datoListo = true;            // Mark data ready
    interrupts();
  }
}

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ESP32 ISRs with FreeRTOS

The ESP32 version uses FreeRTOS critical sections: 
portMUX_TYPE mux = portMUX_INITIALIZER_UNLOCKED;

void IRAM_ATTR isrInicio() {
  portENTER_CRITICAL_ISR(&mux);
  
  if (esperandoInicio) {
    tInicio = millis();
    esperandoInicio = false;
    esperandoFin = true;
  }
  
  portEXIT_CRITICAL_ISR(&mux);
}

void IRAM_ATTR isrFin() {
  portENTER_CRITICAL_ISR(&mux);
  
  if (esperandoFin) {
    tFin = millis();
    delta = tFin - tInicio;
    esperandoFin = false;
    esperandoInicio = true;
    datoListo = true;
  }
  
  portEXIT_CRITICAL_ISR(&mux);
}

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Setup Function

void setup() {
  Serial.begin(115200);
  
  // Configure input pins with pull-up resistors
  pinMode(PIN_INICIO, INPUT_PULLUP);
  pinMode(PIN_FIN, INPUT_PULLUP);
  
  // Configure Timer1 (Arduino only)
  noInterrupts();
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1  = 0;
  OCR1A = 249;
  TCCR1B |= (1 << WGM12);
  TCCR1B |= (1 << CS11) | (1 << CS10);
  TIMSK1 |= (1 << OCIE1A);
  interrupts();
  
  // Attach interrupts on FALLING edge
  attachInterrupt(digitalPinToInterrupt(PIN_INICIO), isrInicio, FALLING);
  attachInterrupt(digitalPinToInterrupt(PIN_FIN), isrFin, FALLING);
  
  Serial.println("Sistema listo: Arduino + Timer 1 ms");
  Serial.println("Esperando INICIO...");
}

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Main Loop

void loop() {
  if (datoListo) {
    noInterrupts();
    unsigned long tiempoMs = contadorMs;
    datoListo = false;
    interrupts();
    
    Serial.print("TIEMPO = ");
    Serial.print(tiempoMs);
    Serial.println(" ms");
    Serial.println("------------------");
  }
}

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Serial Output Format

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Arduino Version

TIEMPO = 450 ms
------------------

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ESP32 Version

INICIO: 1234567 us
FIN: 1685567 us
DELTA = 451000 us
----------------------

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Three-Sensor Configuration

The firmware also supports a three-sensor configuration for measuring intermediate points: 
#define PIN_INICIO      2   // INT0
#define PIN_INTERMEDIO  3   // INT1  
#define PIN_FINAL       4   // PCINT20 (PORTD)

enum Estado {
  ESPERANDO_INICIO,
  ESPERANDO_INTERMEDIO,
  ESPERANDO_FINAL
};

volatile Estado estado = ESPERANDO_INICIO;
volatile unsigned long tInicio = 0;
volatile unsigned long tIntermedio = 0;
volatile unsigned long tFinal = 0;

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State Transitions

void isrInicio() {
  if (estado == ESPERANDO_INICIO) {
    noInterrupts();
    contadorMs = 0;
    tInicio = 0;
    estado = ESPERANDO_INTERMEDIO;
    interrupts();
  }
}

void isrIntermedio() {
  if (estado == ESPERANDO_INTERMEDIO) {
    noInterrupts();
    tIntermedio = contadorMs;
    estado = ESPERANDO_FINAL;
    interrupts();
  }
}

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Gravity Calculation

The three-sensor version calculates gravitational acceleration: 
float S1 = 0.68;  // Distance to intermediate sensor (meters)
float S2 = 1.75;  // Distance to final sensor (meters)

// Calculate gravity using: g = 2*distance / time²
double g1 = (2*S1)/(tm*tm*0.000001);
double g2 = (2*S2)/(tf*tf*0.000001);

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Key Features

  • Hardware interrupts for precise edge detection
  • Timer-based counting for accurate millisecond timing
  • State machine prevents spurious triggers
  • Critical sections ensure thread-safe access to shared variables
  • Support for multiple sensors for intermediate measurements
  • Gravity calculation in three-sensor configuration

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Platform Support

  • Arduino (ATmega328): Uses Timer1 with 1ms resolution
  • ESP32: Uses FreeRTOS primitives and millis() for timing

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Timing Resolution

  • Arduino: 1 millisecond (using Timer1)
  • ESP32: 1 millisecond (using millis())
  • Alternative ESP32: Microsecond resolution available with micros()

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