MicroSD Breakout With Level Shifter Hookup Guide

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Contributors: .Brent.
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Code Example

The example code for this product is a simple file logger that allows the user to write to a file on the µSD card using the Arduino IDE Serial Monitor (57600 baud). When the board boots you should see the following in the Serial Monitor:

Initializing SD card...Initialization done.
demoFile.txt doesn't exist. Creating.
Opening file: demoFile.txt
Enter text to be written to file. 'EOF' will terminate writing.

The last line of that bock of text is important to note. Your work is written to the µSD card every 20 characters, but to make sure everything is written append EOF to your writing. Doing so writes everything remaining in the buffer to the file and reads back the contents of the file.

EOF

demoFile.txt:
Test line of text.
Bacon ipsum dolor amet beef picanha drumstick alcatra brisket, short ribs sirloiBacon ipsum dolor amet beef picanha drumstick alcatra brisket, short ribs sirloin.

One thing to note is that the UART buffer on the Arduino might limit the number of characters entered on a single line. In one test I noticed that my Arduino only accepted 164 bytes before loosing data, but I've seen that vary a bit.

language:c
/*
 * A simple file logger that allows the user to write to a file on the µSD card
 * using the Arduino IDE Serial Monitor (57600 baud). Entered text is written
 * to the µSD card every 20 characters, but to make sure everything is written
 * append 'EOF' to your writing. Doing so writes everything remaining in the
 * buffer to the file and reads back the contents of the file.
 */

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

File fd;
const uint8_t BUFFER_SIZE = 20;
char fileName[] = "demoFile.txt"; // SD library only supports up to 8.3 names
char buff[BUFFER_SIZE+2] = "";  // Added two to allow a 2 char peek for EOF state
uint8_t index = 0;

const uint8_t chipSelect = 8;
const uint8_t cardDetect = 9;

enum states: uint8_t { NORMAL, E, EO };
uint8_t state = NORMAL;

bool alreadyBegan = false;  // SD.begin() misbehaves if not first call



////////////////////////////////////////////////////////////////////////////////
// Standard Arduino setup function
////////////////////////////////////////////////////////////////////////////////
void setup()
{
  Serial.begin(57600);
  while (!Serial);  // Wait for serial port to connect (ATmega32U4 type PCBAs)

  // Note: To satisfy the AVR SPI gods the SD library takes care of setting
  // SS_PIN as an output. We don't need to.
  pinMode(cardDetect, INPUT);

  initializeCard();
}



////////////////////////////////////////////////////////////////////////////////
// Arduino calls this function over and over again when running
////////////////////////////////////////////////////////////////////////////////
void loop()
{
  // Make sure the card is still present
  if (!digitalRead(cardDetect))
  {
    initializeCard();
  }

  if (Serial.available() > 0)
  {
    readByte();

    if (index == BUFFER_SIZE)
    {
      flushBuffer();  // Write full buffer to µSD card
    }
  }
}



////////////////////////////////////////////////////////////////////////////////
// Do everything from detecting card through opening the demo file
////////////////////////////////////////////////////////////////////////////////
void initializeCard(void)
{
  Serial.print(F("Initializing SD card..."));

  // Is there even a card?
  if (!digitalRead(cardDetect))
  {
    Serial.println(F("No card detected. Waiting for card."));
    while (!digitalRead(cardDetect));
    delay(250); // 'Debounce insertion'
  }

  // Card seems to exist.  begin() returns failure
  // even if it worked if it's not the first call.
  if (!SD.begin(chipSelect) && !alreadyBegan)  // begin uses half-speed...
  {
    Serial.println(F("Initialization failed!"));
    initializeCard(); // Possible infinite retry loop is as valid as anything
  }
  else
  {
    alreadyBegan = true;
  }
  Serial.println(F("Initialization done."));

  Serial.print(fileName);
  if (SD.exists(fileName))
  {
    Serial.println(F(" exists."));
  }
  else
  {
    Serial.println(F(" doesn't exist. Creating."));
  }

  Serial.print("Opening file: ");
  Serial.println(fileName);

  Serial.println(F("Enter text to be written to file. 'EOF' will terminate writing."));
}


////////////////////////////////////////////////////////////////////////////////
// This function is called after the EOF command is received. It writes the
// remaining unwritten data to the µSD card, and prints out the full contents
// of the log file.
////////////////////////////////////////////////////////////////////////////////
void eof(void)
{
  index -= 3; // Remove EOF from the end
  flushBuffer();

  // Re-open the file for reading:
  fd = SD.open(fileName);
  if (fd)
  {
    Serial.println("");
    Serial.print(fileName);
    Serial.println(":");

    while (fd.available())
    {
      Serial.write(fd.read());
    }
  }
  else
  {
    Serial.print("Error opening ");
    Serial.println(fileName);
  }
  fd.close();
}


////////////////////////////////////////////////////////////////////////////////
// Write the buffer to the log file. If we are possibly in the EOF state, verify
// that to make sure the command isn't written to the file.
////////////////////////////////////////////////////////////////////////////////
void flushBuffer(void)
{
  fd = SD.open(fileName, FILE_WRITE);
  if (fd) {
    switch (state)  // If a flush occurs in the 'E' or the 'EO' state, read more to detect EOF
    {
    case NORMAL:
      break;
    case E:
      readByte();
      readByte();
      break;
    case EO:
      readByte();
      break;
    }
    fd.write(buff, index);
    fd.flush();
    index = 0;
    fd.close();
  }
}



////////////////////////////////////////////////////////////////////////////////
// Reads a byte from the serial connection. This also maintains the state to
// capture the EOF command.
////////////////////////////////////////////////////////////////////////////////
void readByte(void)
{
  byte byteRead = Serial.read();
  Serial.write(byteRead); // Echo
  buff[index++] = byteRead;

  // Must be 'EOF' to not get confused with words such as 'takeoff' or 'writeoff'
  if (byteRead == 'E' && state == NORMAL)
  {
    state = E;
  }
  else if (byteRead == 'O' && state == E)
  {
    state = EO;
  }
  else if (byteRead == 'F' && state == EO)
  {
    eof();
    state = NORMAL;
  }
}