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Understanding why should avoid "String" and alternative solutions

Why "Strings" are bad for Arduino?
Which is the most efficient and fastest solution to read and store the data from Accelerometer and GPS?

Stings are evil for Arduino

An Uno or other ATmega328-based board only has 2048 bytes SRAM.

The SRAM is composed by three parts:

The Static
The Heap
The Stack

The Heap is where, randomly, a C++ string information is allocated.

If I add an information to my C++ String the system copies itself and deletes the old copy leaving a hole in the heap, that grows up with the cycles.

When I wrote my first sketch for reading and writing the information from an MPU6050 and stored on a SD (whithout connecting to a GPS) I noticed that the sampling time was not constant.

enter image description here

Thanks to the feedback obtained in this question on StackOverflow, I understood that the problem was my excessive use of strings, and now i understand why.

So I started to dig into this issue.

This was one of the first version of the code:

/******************************************************************************
 CSV_Logger_TinyGPSPlus.ino
 Log GPS data to a CSV file on a uSD card
 By Jim Lindblom @ SparkFun Electronics
 February 9, 2016
 https://github.com/sparkfun/GPS_Shield
 This example uses SoftwareSerial to communicate with the GPS module on
 pins 8 and 9, then communicates over SPI to log that data to a uSD card.
 It uses the TinyGPS++ library to parse the NMEA strings sent by the GPS module,
 and prints interesting GPS information - comma separated - to a newly created
 file on the SD card.
 Resources:
 TinyGPS++ Library - https://github.com/mikalhart/TinyGPSPlus/releases
 SD Library (Built-in)
 SoftwareSerial Library (Built-in)
 Development/hardware environment specifics:
 Arduino IDE 1.6.7
 GPS Logger Shield v2.0 - Make sure the UART switch is set to SW-UART
 Arduino Uno, RedBoard, Pro, Mega, etc.
******************************************************************************/
#include <SPI.h>
#include <SD.h>
#include <TinyGPS++.h>
#include<Wire.h>
#define ARDUINO_USD_CS 10 // uSD card CS pin (pin 10 on SparkFun GPS Logger Shield)
/////////////////////////
// Log File Defintions //
/////////////////////////
// Keep in mind, the SD library has max file name lengths of 8.3 - 8 char prefix,
// and a 3 char suffix.
// Our log files are called "gpslogXX.csv, so "gpslog99.csv" is our max file.
#define LOG_FILE_PREFIX "gpslog" // Name of the log file.
#define MAX_LOG_FILES 100 // Number of log files that can be made
#define LOG_FILE_SUFFIX "csv" // Suffix of the log file
char logFileName[13]; // Char string to store the log file name
// Data to be logged:
#define LOG_COLUMN_COUNT 15
char * log_col_names[LOG_COLUMN_COUNT] = {
 "longitude", "latitude", "altitude", "speed", "course", "date", "time", "satellites","Acc.X","Acc.Y","Acc.Z","Gy.X","Gy.Y","Gy.Z","Temp"
}; // log_col_names is printed at the top of the file.
//////////////////////
// Log Rate Control //
//////////////////////
#define LOG_RATE 1000 // Log every 1 seconds
unsigned long lastLog = 0; // Global var to keep of last time we logged
/////////////////////////
// TinyGPS Definitions //
/////////////////////////
TinyGPSPlus tinyGPS; // tinyGPSPlus object to be used throughout
#define GPS_BAUD 9600 // GPS module's default baud rate
/////////////////////////////////
// GPS Serial Port Definitions //
/////////////////////////////////
// If you're using an Arduino Uno, Mega, RedBoard, or any board that uses the
// 0/1 UART for programming/Serial monitor-ing, use SoftwareSerial:
#include <SoftwareSerial.h>
#define ARDUINO_GPS_RX 9 // GPS TX, Arduino RX pin
#define ARDUINO_GPS_TX 8 // GPS RX, Arduino TX pin
SoftwareSerial ssGPS(ARDUINO_GPS_TX, ARDUINO_GPS_RX); // Create a SoftwareSerial
// Set gpsPort to either ssGPS if using SoftwareSerial or Serial1 if using an
// Arduino with a dedicated hardware serial port
#define gpsPort ssGPS // Alternatively, use Serial1 on the Leonardo
// Define the serial monitor port. On the Uno, Mega, and Leonardo this is 'Serial'
// on other boards this may be 'SerialUSB'
#define SerialMonitor Serial
const int MPU=0x68; // I2C address of the MPU-6050
int16_t AcX,AcY,AcZ,Tmp,GyX,GyY,GyZ;
void setup()
{
 Wire.begin();
 Wire.beginTransmission(MPU);
 Wire.write(0x6B); // PWR_MGMT_1 register
 Wire.write(0); // set to zero (wakes up the MPU-6050)
 Wire.endTransmission(true);
 SerialMonitor.begin(9600);
 gpsPort.begin(GPS_BAUD);
 SerialMonitor.println("Setting up SD card.");
 // see if the card is present and can be initialized:
 if (!SD.begin(ARDUINO_USD_CS))
 {
 SerialMonitor.println("Error initializing SD card.");
 }
 updateFileName(); // Each time we start, create a new file, increment the number
 printHeader(); // Print a header at the top of the new file
}
void loop()
{
 Wire.beginTransmission(MPU);
 Wire.write(0x3B); // starting with register 0x3B (ACCEL_XOUT_H)
 Wire.endTransmission(false);
 Wire.requestFrom(MPU,14,true); // request a total of 14 registers
 AcX=Wire.read()<<8|Wire.read(); // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L) 
 AcY=Wire.read()<<8|Wire.read(); // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
 AcZ=Wire.read()<<8|Wire.read(); // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
 Tmp=Wire.read()<<8|Wire.read(); // 0x41 (TEMP_OUT_H) & 0x42 (TEMP_OUT_L)
 GyX=Wire.read()<<8|Wire.read(); // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
 GyY=Wire.read()<<8|Wire.read(); // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
 GyZ=Wire.read()<<8|Wire.read(); // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)
{
 if ((lastLog + LOG_RATE) <= millis())
 { // If it's been LOG_RATE milliseconds since the last log:
 if (tinyGPS.location.isUpdated()) // If the GPS data is vaild
 {
 if (logGPSData()) // Log the GPS data
 {
 SerialMonitor.println("GPS logged."); // Print a debug message
 lastLog = millis(); // Update the lastLog variable
 }
 else // If we failed to log GPS
 { // Print an error, don't update lastLog
 SerialMonitor.println("Failed to log new GPS data.");
 }
 }
 else // If GPS data isn't valid
 {
 // Print a debug message. Maybe we don't have enough satellites yet.
 SerialMonitor.print("No GPS data. Sats: ");
 SerialMonitor.println(tinyGPS.satellites.value());
 }
 }
 // If we're not logging, continue to "feed" the tinyGPS object:
 while (gpsPort.available())
 tinyGPS.encode(gpsPort.read());
}
}
byte logGPSData()
{
 File logFile = SD.open(logFileName, FILE_WRITE); // Open the log file
 if (logFile)
 { // Print longitude, latitude, altitude (in feet), speed (in mph), course
 // in (degrees), date, time, and number of satellites.
 logFile.print(tinyGPS.location.lng(), 6);
 logFile.print(',');
 logFile.print(tinyGPS.location.lat(), 6);
 logFile.print(',');
 logFile.print(tinyGPS.altitude.feet(), 1);
 logFile.print(',');
 logFile.print(tinyGPS.speed.mph(), 1);
 logFile.print(',');
 logFile.print(tinyGPS.course.deg(), 1);
 logFile.print(',');
 logFile.print(tinyGPS.date.value());
 logFile.print(',');
 logFile.print(tinyGPS.time.value());
 logFile.print(',');
 logFile.print(tinyGPS.satellites.value());
 logFile.print(AcX);
 logFile.print(',');
 logFile.print(AcY);
 logFile.print(',');
 logFile.print(AcZ);
 logFile.print(',');
 logFile.print(GyX);
 logFile.print(',');
 logFile.print(GyY);
 logFile.print(',');
 logFile.print(GyZ);
 logFile.print(',');
 logFile.print(Tmp);
 logFile.println();
 logFile.close();
 return 1; // Return success
 }
 return 0; // If we failed to open the file, return fail
}
// printHeader() - prints our eight column names to the top of our log file
void printHeader()
{
 File logFile = SD.open(logFileName, FILE_WRITE); // Open the log file
 if (logFile) // If the log file opened, print our column names to the file
 {
 int i = 0;
 for (; i < LOG_COLUMN_COUNT; i++)
 {
 logFile.print(log_col_names[i]);
 if (i < LOG_COLUMN_COUNT - 1) // If it's anything but the last column
 logFile.print(','); // print a comma
 else // If it's the last column
 logFile.println(); // print a new line
 }
 logFile.close(); // close the file
 }
}
// updateFileName() - Looks through the log files already present on a card,
// and creates a new file with an incremented file index.
void updateFileName()
{
 int i = 0;
 for (; i < MAX_LOG_FILES; i++)
 {
 memset(logFileName, 0, strlen(logFileName)); // Clear logFileName string
 // Set logFileName to "gpslogXX.csv":
 sprintf(logFileName, "%s%d.%s", LOG_FILE_PREFIX, i, LOG_FILE_SUFFIX);
 if (!SD.exists(logFileName)) // If a file doesn't exist
 {
 break; // Break out of this loop. We found our index
 }
 else // Otherwise:
 {
 SerialMonitor.print(logFileName);
 SerialMonitor.println(" exists"); // Print a debug statement
 }
 }
 SerialMonitor.print("File name: ");
 SerialMonitor.println(logFileName); // Debug print the file name
}

Then I started:

Studying how Arduino Memory works
Breaking the problem, focusing only to learn how to read and write on a SD the accelerometer information (the device is not connected to the PC)

The following code is my latest version:

 void Read_Write()
 // function that reads the MPU and writes the data to the SD.
 {
 // Local variables:
int16_t AcX,AcY,AcZ,Tmp,GyX,GyY,GyZ; // Variables read from MPU6050
// Read data:
Wire.beginTransmission(MPU);
Wire.write(0x3B); // starting with register 0x3B (ACCEL_XOUT_H)
Wire.endTransmission(false);
Wire.requestFrom(MPU,14,true); // request a total of 14 registers
AcX = Wire.read()<<8|Wire.read(); // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L) 
AcY = Wire.read()<<8|Wire.read(); // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
AcZ = Wire.read()<<8|Wire.read(); // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
GyX = Wire.read()<<8|Wire.read(); // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
GyY = Wire.read()<<8|Wire.read(); // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
GyZ = Wire.read()<<8|Wire.read(); // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)
// Data preparation for file saving:
String dataString = ""; // string for assembling the data to log:
// Add time tag:
dataString += String(Time0); dataString += ",";
// Append the MPU6050 data to the string:
dataString += String(AcX); dataString += ",";
dataString += String(AcY); dataString += ",";
dataString += String(AcZ); dataString += ",";
dataString += String(GyX); dataString += ",";
dataString += String(GyY); dataString += ",";
dataString += String(GyZ);
// Open the file in append mode:
File dataFile = SD.open("datalog.txt", FILE_WRITE);
// If the file is available, write to it:
if (dataFile) 
{
dataFile.println(dataString);
dataFile.close();
if (Serial_plus_SD)
 Serial.println(dataString);
}
// if the file does not open, pop up an error:
else 
errorFW();
return;
}

I recived other suggestions such as "create a Buffer", but without a technical description and motivation I am having difficulties understand how to do it.

(I don’t want to simply copy&paste the code as i have done intitially with the first version)

I have to use C Strings instead of C++ Strings?

I have a civil engineer background, so is my first time with coding.

Thank you for your patience.

Answer*

First, let me point out a problem in your code, completely unrelated to
your question:

 AcX = Wire.read()<<8|Wire.read();

The C++ standard does not specify in which order the two reads will be
performed. This may work well with the particular version of the
particular compiler you are using, but it can break the day you (or an
update to the Arduino IDE) changes the compiler version or a compiler
flag. You should instead do a single read per statement, e.g.:

 AcX = Wire.read() << 8;
 AcX |= Wire.read();

Next, I would like to say that the simplest Arduino way of doing what
you are trying to do is to use neither a `String` nor a character array.
Instead, open the file first, then format and send the data on the fly
using `print()` or `println()`:

 dataFile.print(Time0); dataFile.print(",");
 dataFile.print(AcX); dataFile.print(",");
 [...]

This way you never need to store the whole string in memory. It is then
a very memory-friendly solution.

Now, I realize this seems inconvenient if you want to print the data to
both the SD file and the serial port. In such a case you could put the
printing code inside a function that can print to any `Print` object
(for instance the data file and `Serial`), and you can then call the
function twice:

 static void printImuData(Print &printer, int16_t Time0,
 int16_t AcX, int16_t AcY, int16_t AcZ,
 int16_t GyX, int16_t GyY, int16_t GyZ)
 {
 printer.print(Time0); printer.print(",");
 printer.print(AcX); printer.print(",");
 [...]
 }

 [...]

 printImuData(dataFile, Time0, AcX, AcY, AcZ, GyX, GyY, GyZ);
 if (Serial_plus_SD)
 printImuData(Serial, Time0, AcX, AcY, AcZ, GyX, GyY, GyZ);

Alternatively, you could use a kind of [tee][] to duplicate the data to
both streams. See for example [this implementation][].

[tee]: https://en.wikipedia.org/wiki/Tee_(command)
[this implementation]: https://arduino.stackexchange.com/a/26453

**Edit**: A more elegant way to achieve the same would be to wrap all
the IMU data into a `struct` (or a `class`), and then make the whole
thing [printable][]:

 struct ImuData : public Printable
 {
 uint16_t Time0, AcX, AcY, AcZ, GyX, GyY, GyZ;
 size_t printTo(Print& p) const;
 };

 size_t ImuData::printTo(Print& p) const
 {
 size_t bytes = 0;
 bytes += p.print(Time0); bytes += p.write(',');
 [...]
 bytes += p.print(GyZ);
 return bytes;
 }

The you can print the `struct` like you would print the string:

 ImuData imu;
 imu.Time0 = ...;
 imu.AcX = Wire.read() << 8;
 imu.AcX |= Wire.read();
 [...]

 dataFile.println(imu);
 if (Serial_plus_SD)
 Serial.println(imu);

[printable]: https://arduino.stackexchange.com/a/30552

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This is actually a good answer.

Patrick Trentin
– Patrick Trentin

2017年03月20日 13:44:20 +00:00
Commented Mar 20, 2017 at 13:44
@Edgar why you use this Print &printer, int16_t Time0, [...] (or where i can find more (specific) information about the theory behind this) ?

Andrea Ciufo
– Andrea Ciufo

2017年03月23日 17:07:17 +00:00
Commented Mar 23, 2017 at 17:07
Please be more specific, and maybe ask a new question. As I understand it, you are asking for a tutorial on the "theory" behind the C++ language, which is a question far too broad for this site.

Edgar Bonet
– Edgar Bonet

2017年03月23日 18:29:12 +00:00
Commented Mar 23, 2017 at 18:29
@EdgarBonet yes, it is a noob question, i was trying to understand and studying the pointer, in this specific situation how &printer works :)

Andrea Ciufo
– Andrea Ciufo

2017年03月24日 17:08:58 +00:00
Commented Mar 24, 2017 at 17:08
@uomodellamansarda: It's a reference, similar to a pointer but safer.

Edgar Bonet
– Edgar Bonet

2017年03月24日 17:59:44 +00:00
Commented Mar 24, 2017 at 17:59

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