Error writing to file with SD card shield on Mega2560

Hi everyone,

I am using Arduino 1.8.5 software and Arduino MEGA2560 with the SD card shield. For my project, I have to read the voltage and temperature from the pins connected to the board, which later should be saved as a .csv file.

Here is my code:

//SWITCH part of the code
int inPin = 2; // the number of the input pin
int outPin = 13; // the number of the output pin

int state = HIGH; // the current state of the output pin
int reading; // the current reading from the input pin
int previous = LOW; // the previous reading from the input pin
int goButton; // press inPin = 2 to go

// the follow variables are long's because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long time = 0; // the last time the output pin was toggled
long debounce = 200; // the debounce time, increase if the output flickers

#include "SD.h" //includes SD library
#include"SPI.h" //includes SPI library

const int CSpin = 53;
String dataString = ""; // holds the data to be written to the SD card

const float referenceVolts = 5.0; // the default reference on a 5-volt board
const int batteryPin = 0; // battery is connected to analog pin 0
const int temperaturePin = 5; // temperature probe is connected to analogue pin 5 (amp needed)
const int fluxMeterPin = 6; // Fluxmeter (a TEM) is connected to analogue pin 6 (?)
const int pwmPin = 7; // Output voltage to TEM is connected to analogue pin 7 (amp needed)
const int pwmSwitchPin = 4; // Digital to TEM - switches between heating and cooling.

File sensorData;
File myFile;
int myTime = 0;
void setup()

pinMode(inPin, INPUT);
pinMode(outPin, OUTPUT);
Serial.print("Initializing SD card...");
pinMode(CSpin, OUTPUT);

Serial.println ("Start of program ");
// Check make sure that go switch is off
// The "go button" gives LOW when depressed and HIGH when not depressed.
Serial.println ("Depress and then release the go button to start");
goButton = HIGH;
while (goButton==HIGH) {
goButton = digitalRead(inPin);
Serial.print ("Start button value is ");
Serial.println (goButton);
// Serial.println("Switch the start button off");

// Now wait until it is on.
Serial.println ("Awaiting release of go button");
// Serial.println (goButton);
while (goButton == LOW){
goButton = digitalRead(inPin);
// Serial.print ("Start button value is ");
// Serial.println (goButton);
// Serial.println ("Awaiting go button.");

Serial.println ("Operation started");

// see if the card is present and can be initialized:

if (!SD.begin(CSpin)) {
Serial.println("Card failed, or not present");
// don't do anything more:
Serial.println("card initialized.");
sensorData ="myData2.csv", FILE_WRITE);
if (sensorData) {
Serial.print("Writing file header...");
sensorData.println("TEM TEST DATA");
Serial.println("TEM TEST DATA");
// close the file:
} else {
// if the file didn't open, print an error:
Serial.println("error 10 opening data file");


void loop()

reading = digitalRead(inPin);

// if the input just went from LOW and HIGH and we've waited long enough
// to ignore any noise on the circuit, toggle the output pin and remember
// the time
if (reading == HIGH && previous == LOW && millis() - time > debounce) {
if (state == HIGH)
state = LOW;
state = HIGH;

time = millis();

digitalWrite(outPin, state);

previous = reading;

int val = analogRead(batteryPin); // read the value from the sensor
float volts = (val / 1023.0) * referenceVolts; // calculate the ratio
// *******************************************************************
// Start logging
float mjtTime = millis(); // Time in milli-seconds
val = analogRead(temperaturePin); // read the value from the temperature probe.
float temperVolts = (val / 1023.0) * 80; // Wild guess that f.s.d. = 80 deg C. Check this!
val = analogRead(fluxMeterPin); // read the value from the sensor
float fmVolts = (val / 1023.0) * referenceVolts; // Wild guess that f.s.d. = 80 deg C. Check this!

// Implement simple proportional control with an arbitrary coefficient.
float spError = setPointTemp(millis())- temperVolts; // set point error.
float TEMvolts= spError * 0.1;

if (TEMvolts > 0) {
digitalWrite (pwmSwitchPin, 1); // Set hardware to heating mode.
else {
digitalWrite (pwmSwitchPin, 0); // Set hardware to cooling mode.
TEMvolts = abs(TEMvolts); // Lose any negative sign
TEMvolts = max (2, TEMvolts); // Restrict to two volts max
digitalWrite (pwmPin, TEMvolts); // Write voltage to TEM.

// Determine output string

dataString = String(mjtTime)+","+String(temperVolts)+","+String(fmVolts) ;

while (millis() < mjtTime + 1000){
// delay loop
Serial.println ("End of delay loop");

myTime = myTime + 1;
// build the data string
//dataString = String(myTime) + "," + String(volts); // convert to CSV
if (myTime < 30) {
else {
Serial.println ("You can remove the SD card now.");

// delay(5000); // delay before next write to SD Card, adjust as required
void saveData() {
// if (SD.exists("myData2.csv")) { // check the card is still there
// now append new data file
sensorData ="myData2.csv", FILE_WRITE);
if (sensorData) {
sensorData.close(); // close the file
// }
else {
Serial.println("Error 20 writing to file !");


float setPointTemp(float myTime){
// This returns the set point temp as a function of time
float amplitude = 0;
float mean = 25;
float timePeriod = 2400;
// At the moment, the set point is a constant 25 deg C. Eventually we shall make this a sinusoidal function.
// (But lets test it thoroughly first!
float s = mean + sin (6.28amplitudemyTime/timePeriod);
return s;

The error is:

Operation started
Card failed, or not present
End of delay loop
Error 20 writing to file !

I will be very grateful for your help.

MJTplayJunklast.ino (5.97 KB)

Card failed, or not present

Bad wiring, bad card, bad reader or interference with other attached hardware..

I very much dislike the output of wrong/misleading messages,
locking loops and delays longer than a couple of ms.

Did you write that code, or is it based on something you found somewhere?

I have just started coding on Arduino and I started the code and then it was modified by my supervisor. It used to work with a PushButton, however, once I replaced it with a toggle switch, it started to show this error. At the moment, the person who helped me with the code is away and I don't know which bits of code to change in order to get rid of the error. Help will be very much appreciated.


I have a similar problem and suspect that the SD card shield itself is the problem

your code seems OK to me- but I agree that long delays are not very elegant in such an application
I read somewhere that supply voltage is an issue if the card reader runs on 3.3 V and you are using a 5V microcontroller (uno for instance)

anyway I hope you get your system to work and I would be very interested to hear the outcome

best regards