Reading data from SD card in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What library do you need to include to read data from an SD card in Arduino?

You need to include the SD library, which provides functions to access files on the SD card.

beginner

How do you initialize the SD card in your Arduino sketch?

Use SD.begin(chipSelectPin) where chipSelectPin is the pin connected to the SD card's chip select. It returns true if initialization succeeds.

beginner

How do you open a file for reading from the SD card?

Use File myFile = SD.open("filename.txt", FILE_READ). This opens the file named "filename.txt" for reading.

intermediate

How do you read data from an open file on the SD card?

You can use myFile.read() to read one byte at a time or myFile.readStringUntil('\n') to read a line until a newline character.

beginner

Why is it important to close a file after reading from the SD card?

Closing the file with myFile.close() frees up memory and ensures data integrity on the SD card.

Which Arduino library is used to read data from an SD card?

A SPI

B Wire

C SD

D EEPROM

The SD library is specifically designed for reading and writing files on SD cards.

What does SD.begin(chipSelectPin) do?

A Formats the SD card

B Reads data from the SD card

C Closes the SD card connection

D Initializes the SD card and SPI communication

It sets up the SD card and SPI bus so you can access files.

How do you open a file named "data.txt" for reading?

A File myFile = SD.open("data.txt", FILE_READ)

B File myFile = SD.read("data.txt")

C File myFile = SD.begin("data.txt")

D File myFile = SD.close("data.txt")

The open() function opens the file for reading or writing.

Which function reads one byte from an open file?

A myFile.open()

B myFile.read()

C myFile.close()

D myFile.write()

read() reads one byte from the file.

Why should you call myFile.close() after reading?

A To free memory and save changes

B To open the file again

C To delete the file

D To format the SD card

Closing the file frees resources and ensures data is saved properly.

Explain the steps to read a text file from an SD card using Arduino.

Think about setup, opening, reading, and closing.

Why is it important to check if SD.begin() returns true before reading files?

What happens if the SD card is missing or not working?

Practice

1. What is the correct function to open a file for reading from an SD card in Arduino?

easy

A. SD.open(filename, FILE_READ)

B. SD.read(filename)

C. SD.begin(filename)

D. SD.load(filename)

Solution

1. Step 1: Identify the function to open files on SD card

   The Arduino SD library uses SD.open() to open files.

2. Step 2: Specify the mode for reading

   To read a file, the mode FILE_READ must be passed as the second argument.

3. Final Answer:

   SD.open(filename, FILE_READ) -> Option A

4. Quick Check:

   Open file for reading = SD.open(filename, FILE_READ) [OK]

Hint: Use SD.open() with FILE_READ to open files for reading [OK]

Common Mistakes:

• Using SD.read() to open files
• Forgetting FILE_READ mode
• Using SD.begin() to open files
• Using SD.load() which doesn't exist

2. Which of the following is the correct syntax to check if a file opened successfully before reading?

easy

A. if (file != NULL) { /* read file */ }

B. if (file.isOpen()) { /* read file */ }

C. if (file) { /* read file */ }

D. if (file.exists()) { /* read file */ }

Solution

1. Step 1: Understand file object truthiness in Arduino

   In Arduino, File objects can be checked directly in if statements to see if they opened successfully.

2. Step 2: Recognize correct syntax

   Using if (file) is the standard way to check if the file opened.

3. Final Answer:

   if (file) { /* read file */ } -> Option C

4. Quick Check:

   Check file open with if (file) [OK]

Hint: Use if (file) to check if file opened successfully [OK]

Common Mistakes:

• Comparing file to NULL (not Arduino style)
• Using file.isOpen() which doesn't exist
• Using file.exists() which checks SD, not file open

3. What will be the output of this Arduino code snippet reading from an SD card file named "data.txt" containing the text "Hello"?

File file = SD.open("data.txt", FILE_READ);
if (file) {
  char c = file.read();
  Serial.println(c);
  file.close();
} else {
  Serial.println("Error opening file");
}

medium

A. H

B. Hello

C. Error opening file

D. 104

Solution

1. Step 1: Understand file.read() returns a byte

   The read() method returns the ASCII value of the first character, but when assigned to a char and printed with Serial.println(char), it prints the character itself.

2. Step 2: Check code carefully

   Since c is a char and Serial.println(c) prints the character, the output is 'H'.

3. Final Answer:

   H -> Option A

4. Quick Check:

   file.read() returns char 'H' printed as H [OK]

Hint: file.read() returns one char; Serial.println prints char [OK]

Common Mistakes:

• Thinking read() returns whole string
• Expecting ASCII number printed
• Confusing file.read() with readStringUntil()

4. Identify the error in this Arduino code snippet that reads a line from an SD card file:

File file = SD.open("log.txt", FILE_READ);
if (file) {
  String line = file.readStringUntil('\n');
  Serial.println(line);
}
file.close();

medium

A. readStringUntil() is not a valid File method

B. file.close() is called outside the if block

C. Missing SD.begin() before opening file

D. Using single quotes instead of double quotes in readStringUntil

Solution

1. Step 1: Check file.close() placement

   file.close() should be called only if file opened successfully, so it must be inside the if block.

2. Step 2: Understand consequences

   If file failed to open, calling file.close() causes error because file is invalid.

3. Final Answer:

   file.close() is called outside the if block -> Option B

4. Quick Check:

   Close file only if opened [OK]

Hint: Call file.close() only inside if (file) block [OK]

Common Mistakes:

• Calling file.close() unconditionally
• Assuming readStringUntil() is invalid
• Forgetting SD.begin() but not shown here
• Confusing single vs double quotes in char argument

5. You want to read all lines from a file "config.txt" on an SD card and print each line to Serial. Which code snippet correctly does this?

hard

A. File file = SD.open("config.txt", FILE_READ);
while (file) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

B. File file = SD.open("config.txt", FILE_READ);
while (!file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

C. File file = SD.open("config.txt", FILE_READ);
while (file.read()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

D. File file = SD.open("config.txt", FILE_READ);
while (file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

Solution

1. Step 1: Use file.available() to check for data

   file.available() returns true if there is data left to read, so loop while available() is true.

2. Step 2: Read each line with readStringUntil('\n')

   This method reads until newline, perfect for line-by-line reading.

3. Step 3: Close file after reading

   Call file.close() after finishing reading to free resources.

4. Final Answer:

   Use while (file.available()) with readStringUntil('\n') -> Option D

5. Quick Check:

   Loop with file.available() to read lines [OK]

Hint: Loop while file.available() to read all lines [OK]

Common Mistakes:

• Using while (!file.available()) which loops incorrectly
• Using while (file.read()) which reads bytes, not lines
• Using while (file) which doesn't check data availability