Practical case: SD Event Data Logger with Arduino UNO

/*
 * SD Event Data Logger
 * Target: Arduino UNO R3 (ATmega328P)
 * Objective: Log debounced button presses to a microSD card over SPI.
 */

#include <SPI.h>
#include <SD.h>

// Pin Definitions
const int chipSelect = 4;
const int buttonPin = 2;
const int ledPin = 8;

// State Variables
unsigned long eventCount = 0;
int buttonState;
int lastButtonState = HIGH; // HIGH because we use INPUT_PULLUP

// Timing Variables for Debounce
unsigned long lastDebounceTime = 0;
const unsigned long debounceDelay = 50; // 50 milliseconds

void setup() {
  // Initialize Serial for debugging
  Serial.begin(9600);
  while (!Serial) {
    ; // Wait for serial port to connect
  }

  // Configure Pins
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);

  Serial.println("Initializing SD card...");

  // Initialize SD Card
  if (!SD.begin(chipSelect)) {
    Serial.println("Critical Error: SD card initialization failed!");
    Serial.println("Check wiring, formatting (FAT16/32), and card insertion.");
    // Trap execution in an infinite loop and blink LED rapidly
    while (true) {
      digitalWrite(ledPin, HIGH);
      delay(100);
      digitalWrite(ledPin, LOW);
      delay(100);
    }
  }

  Serial.println("SD card initialized successfully.");

  // Optional: Write a CSV header if the file does not exist
  if (!SD.exists("events.csv")) {
    File dataFile = SD.open("events.csv", FILE_WRITE);
    if (dataFile) {
      dataFile.println("Event_ID,Uptime_ms");
      dataFile.close();
      Serial.println("Created new events.csv with headers.");
    } else {
      Serial.println("Error: Could not create events.csv");
    }
  }
}

void loop() {
  // Read the state of the switch into a local variable:
  int reading = digitalRead(buttonPin);

  // Check to see if you just pressed the button
  // (i.e. the input went from HIGH to LOW), and you've waited long enough
  // since the last press to ignore any noise:

  // If the switch changed, due to noise or pressing:
  if (reading != lastButtonState) {
    // reset the debouncing timer
    lastDebounceTime = millis();
  }

  if ((millis() - lastDebounceTime) > debounceDelay) {
    // whatever the reading is at, it's been there for longer than the debounce
    // delay, so take it as the actual current state:

    // if the button state has changed:
    if (reading != buttonState) {
      buttonState = reading;

      // only trigger an event if the new button state is LOW (pressed)
      if (buttonState == LOW) {
        logEventToSD();
      }
    }
  }

  // save the reading. Next time through the loop, it'll be the lastButtonState:
  lastButtonState = reading;
}

void logEventToSD() {
  eventCount++;
  unsigned long currentTimestamp = millis();

  // Open the file. Note that only one file can be open at a time.
  // The filename must follow the 8.3 format (max 8 chars name, 3 chars extension)
  File dataFile = SD.open("events.csv", FILE_WRITE);

  // If the file is available, write to it:
  if (dataFile) {
    dataFile.print(eventCount);
    dataFile.print(",");
    dataFile.println(currentTimestamp);
    dataFile.close();

    // Print to the serial port too:
    Serial.print("Logged -> Event: ");
    Serial.print(eventCount);
    Serial.print(" | Time: ");
    Serial.println(currentTimestamp);

    // Visual feedback: Quick pulse on the status LED
    digitalWrite(ledPin, HIGH);
    delay(150); // Short blocking delay is acceptable here to stretch the visual flash
    digitalWrite(ledPin, LOW);
  } 
  else {
    // If the file isn't open, pop up an error:
    Serial.println("Error: Failed to open events.csv for writing.");

    // Visual feedback: Three slow flashes indicating a write error
    for (int i = 0; i < 3; i++) {
      digitalWrite(ledPin, HIGH);
      delay(300);
      digitalWrite(ledPin, LOW);
      delay(300);
    }
  }
}