Reading data from SD card in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SD card read code?

Consider this Arduino code snippet that reads a line from a file on the SD card and prints it:

File dataFile = SD.open("test.txt");
if (dataFile) {
  while (dataFile.available()) {
    Serial.write(dataFile.read());
  }
  dataFile.close();
} else {
  Serial.println("Error opening file");
}

If the file test.txt contains the text Hello SD Card!, what will be printed on the Serial Monitor?

Arduino

File dataFile = SD.open("test.txt");
if (dataFile) {
  while (dataFile.available()) {
    Serial.write(dataFile.read());
  }
  dataFile.close();
} else {
  Serial.println("Error opening file");
}

A Hello SD Card!\n

B Error opening file

C Hello SD Card!

D HELLO SD CARD!

💡 Hint

Think about what Serial.write() does compared to Serial.println().

Serial.write() sends raw bytes without adding a newline. Since the file contains exactly "Hello SD Card!", that exact text is printed with no extra newline.

🧠 Conceptual

intermediate

Which Arduino library is required to read from an SD card?

When programming an Arduino to read data from an SD card, which library must you include to access SD card functions?

A #include <SD.h>

B #include <Wire.h>

C #include <SPI.h>

D #include <EEPROM.h>

💡 Hint

Think about the library that provides file system access on SD cards.

The SD.h library provides functions to open, read, write, and close files on an SD card. SPI.h is needed for communication but SD.h is the main library for SD card file operations.

🔧 Debug

advanced

Why does this SD card read code fail to open the file?

Look at this Arduino code snippet:

if (!SD.begin(4)) {
  Serial.println("Initialization failed!");
  return;
}
File myFile = SD.open("data.txt");
if (!myFile) {
  Serial.println("Error opening file");
}

The SD card is connected correctly, but the output is always "Error opening file". What is the most likely cause?

Arduino

if (!SD.begin(4)) {
  Serial.println("Initialization failed!");
  return;
}
File myFile = SD.open("data.txt");
if (!myFile) {
  Serial.println("Error opening file");
}

A The SD.open() function requires a second argument specifying read mode

B The file name "data.txt" is too long for SD cards

C The SD card is not formatted as FAT32

D The chip select pin number 4 is incorrect for the hardware setup

💡 Hint

Check the hardware connection and pin numbers.

If the chip select pin number passed to SD.begin() does not match the actual hardware wiring, the SD card will not initialize properly, causing file open to fail.

📝 Syntax

advanced

Which option correctly reads the first 10 bytes from an SD card file?

Choose the code snippet that correctly reads and prints the first 10 bytes from a file named "log.txt" on an SD card.

A

File file = SD.open("log.txt");
for (int i = 0; i < 10; i++) {
  if (file.available()) {
    Serial.print((char)file.read());
  }
}
file.close();

B

File file = SD.open("log.txt");
for (int i = 0; i <= 10; i++) {
  Serial.print(file.read());
}
file.close();

C

File file = SD.open("log.txt");
int count = 0;
while (count < 10) {
  Serial.print(file.read());
  count++;
}
file.close();

D

File file = SD.open("log.txt");
for (int i = 0; i < 10; i++) {
  Serial.print(file.read());
}
file.close();

💡 Hint

Consider checking if data is available before reading.

Option A checks file.available() before reading to avoid reading past the file end. It reads exactly 10 bytes or fewer if the file is shorter.

Options B, C, and D do not check availability and may read invalid data or cause errors.

🚀 Application

expert

How many bytes are read and printed by this code?

Analyze this Arduino code reading from an SD card file:

File file = SD.open("data.bin");
int bytesRead = 0;
while (file.available() && bytesRead < 15) {
  char c = file.read();
  if (c == '\n') {
    break;
  }
  Serial.print(c);
  bytesRead++;
}
file.close();
Serial.println(bytesRead);

If the file data.bin contains the text 12345\n67890abcdef, what will be the final number printed by Serial.println(bytesRead);?

Arduino

File file = SD.open("data.bin");
int bytesRead = 0;
while (file.available() && bytesRead < 15) {
  char c = file.read();
  if (c == '\n') {
    break;
  }
  Serial.print(c);
  bytesRead++;
}
file.close();
Serial.println(bytesRead);

A 10

B 5

C 15

D 0

💡 Hint

Count characters until newline or 15 bytes, whichever comes first.

The loop reads characters until it finds a newline character '\n' or reads 15 bytes. The file starts with "12345\n", so it reads 5 characters before breaking.