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

const int chipSelect = 4;
int interval = 5;  //Log to SD Card every 5 seconds

long ttimer;
long timer;
String mvalue;
String dataString;
String runTime;

void setup() {
  Serial.begin(9600);
  delay(3000);
  Serial.println("Initializing SD card...");
  if (!SD.begin(chipSelect)) {
    Serial.println("SD Card error");
    return;
  }
  Serial.println("card initialized");
}

void loop() {
  runTime = millis() / 1000;
  ttimer = millis();
  if ((timer + interval * 1000) < ttimer) {
    timer = millis();
    youraction();
    write_data();
  }
}

void youraction() {
  mvalue = "test"; //mvalue is your log parameter eg. Temperature


}


void write_data() {
  dataString = mvalue + "," + runTime;
  File dataFile = SD.open("datalog.txt", FILE_WRITE);
  if (dataFile) {
    dataFile.println(dataString);
    dataFile.close();
    Serial.println(dataString);
  }
  else {
    Serial.println("error opening datalog.txt");
  }
}