//https://forum.arduino.cc/t/gravando-dados-cartao-sd/1271808
#include <SD.h>
long count = 0;
const int chipSelect = 15;
File myFile;
//------------------------------------------------------------------
void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(115200);
  // wait for Serial Monitor to connect. Needed for native USB port boards only:
  while (!Serial);
  Serial.print("Initializing SD card...");
  if (!SD.begin(chipSelect)) {
    Serial.println("initialization failed");
    while (true);
  }
  Serial.println("initialization done.");
  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  if (!SD.exists("test.txt")) {
    myFile = SD.open("test.txt", FILE_WRITE); //If no exist, create file
    if (myFile) {           // if the file opened okay, write to it:
      myFile.close();
    } else {
      // if the file didn't open, print an error:
      Serial.println("error opening test.txt");
    }
  }
}
//------------------------------------------------------------------
void loop() {
  delay(2000);
  count += 25;
  readSD();
}
//------------------------------------------------------------------
void readSD() {
  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  myFile = SD.open("test.txt", O_RDWR); // Open no append
  if (myFile) {     // if the file opened okay, write to it:
    myFile.println(count);
    myFile.close();
  } else {
    Serial.println("error opening test.txt");
  }
  myFile = SD.open("test.txt");
  if (myFile) {
    while (myFile.available()) {
      Serial.write(myFile.read());
    }
    myFile.close();
  } else {
    Serial.println("error opening test.txt");
  }
}