MicroSD Card Module Pin	ESP32 Pin
3.3V
GND
GPIO 5
GPIO 23
GPIO 18
GPIO 19

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

#define SIGNAL_PIN 4 // GPIO pin for digital signal
#define CS_PIN 5     // Chip Select pin for SD card module
#define BUFFER_SIZE 1024 // Buffer size for data storage

File dataFile;
volatile unsigned long timestamps[BUFFER_SIZE];
volatile int index = 0;

void IRAM_ATTR handleSignalChange() {
  if (index < BUFFER_SIZE) {
    timestamps[index++] = micros(); // Record timestamp in microseconds
  }
}

void setup() {
  Serial.begin(115200);

  // Configure signal pin
  pinMode(SIGNAL_PIN, INPUT);
  
  // Attach interrupt to capture changes in signal
  attachInterrupt(digitalPinToInterrupt(SIGNAL_PIN), handleSignalChange, CHANGE);

  // Initialize SD card
  if (!SD.begin(CS_PIN)) {
    Serial.println("Failed to initialize SD card!");
    while (1);
  }
  
  Serial.println("SD card initialized.");
  
  // Open file for writing
  dataFile = SD.open("/signal_data.txt", FILE_WRITE);
  if (!dataFile) {
    Serial.println("Failed to open file for writing!");
    while (1);
  }
  
  Serial.println("Ready to capture data...");
}

void loop() {
  if (index >= BUFFER_SIZE) {
    detachInterrupt(digitalPinToInterrupt(SIGNAL_PIN)); // Stop capturing when buffer is full
    
    Serial.println("Writing data to SD card...");
    for (int i = 0; i < BUFFER_SIZE; i++) {
      dataFile.println(timestamps[i]); // Write each timestamp to the file
    }
    
    dataFile.flush(); // Ensure all data is written to the SD card
    Serial.println("Data written successfully!");
    
    while (1); // Halt execution
  }
}




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

#define SIGNAL_PIN 4 // GPIO pin for digital signal output
#define CS_PIN 5     // Chip Select pin for SD card

File dataFile;
unsigned long previousTime = 0;

void setup() {
  Serial.begin(115200);

  // Configure the signal pin as output
  pinMode(SIGNAL_PIN, OUTPUT);
  
  // Initialize SD card
  if (!SD.begin(CS_PIN)) {
    Serial.println("SD card initialization failed!");
    while (1);
  }
  
  // Open the file for reading
  dataFile = SD.open("/signal_data.txt", FILE_READ);
  if (!dataFile) {
    Serial.println("Failed to open file for reading!");
    while (1);
  }
  
  Serial.println("Ready to replay recorded signal...");
}

void loop() {
  static unsigned long currentTime;
  
  if (dataFile.available()) {
    // Read the next timestamp from the file
    unsigned long timestamp = dataFile.parseInt();
    
    // Wait until the correct time to toggle the signal
    while (micros() < timestamp);

    // Toggle the signal pin
    digitalWrite(SIGNAL_PIN, !digitalRead(SIGNAL_PIN));
    
    previousTime = timestamp;
  } else {
    dataFile.close();
    Serial.println("Playback complete!");
    while (1); // Halt execution after playback is done
  }
}