File Systems

The Arduino-Pico core supports using some of the onboard flash as a file system, useful for storing configuration data, output strings, logging, and more. It also supports using SD cards as another (FAT32) filesystem, with an API that’s compatible with the onboard flash file system.

Flash Layout

Even though file system is stored on the same flash chip as the program, programming new sketch will not modify file system contents (or EEPROM data).

The following diagram shows the flash layout used in Arduino-Pico:

|---------------------|-------------|----|
^                     ^             ^
Sketch                File system   EEPROM

The file system size is configurable via the IDE menus, rom 64k up to 15MB (assuming you have an RP2040 boad with that much flash)

Note: to use any of file system functions in the sketch, add the following include to the sketch:

#include "LittleFS.h" // LittleFS is declared
// #include <SDFS.h>
// #include <SD.h>

Compatible Filesystem APIs

LittleFS is an onboard filesystem that sets asidesome program flash for use as a filesystem without requiring any external hardware.

SDFS is a filesystem for SD cards, based on [SdFat 2.0](https://github.com/earlephilhower/ESP8266SdFat). It supports FAT16 and FAT32 formatted cards, and requires an external SD card reader.

SD is the Arduino supported, somewhat old and limited SD card filesystem. It is recommended to use SDFS for new applications instead of SD.

All three of these filesystems can open and manipulate File and Dir objects with the same code because the implement a common end-user filesystem API.

LittleFS File System Limitations

The LittleFS implementation for the RP2040 supports filenames of up to 31 characters + terminating zero (i.e. char filename[32]), and as many subdirectories as space permits.

Filenames are assumed to be in the root directory if no initial “/” is present.

Opening files in subdirectories requires specifying the complete path to the file (i.e. LittleFS.open("/sub/dir/file.txt", "r");). Subdirectories are automatically created when you attempt to create a file in a subdirectory, and when the last file in a subdirectory is removed the subdirectory itself is automatically deleted.

Uploading Files to the LittleFS File System

PicoLittleFS is a tool which integrates into the Arduino IDE. It adds a menu item to Tools menu for uploading the contents of sketch data directory into a new LittleFS flash file system.

  • Download the tool: https://github.com/earlephilhower/arduino-pico-littlefs-plugin/releases

  • In your Arduino sketchbook directory, create tools directory if it doesn’t exist yet.

  • Unpack the tool into tools directory (the path will look like <home_dir>/Arduino/tools/PicoLittleFS/tool/picolittlefs.jar) If upgrading, overwrite the existing JAR file with the newer version.

  • Restart Arduino IDE.

  • Open a sketch (or create a new one and save it).

  • Go to sketch directory (choose Sketch > Show Sketch Folder).

  • Create a directory named data and any files you want in the file system there.

  • Make sure you have selected a board, port, and closed Serial Monitor.

  • Double check theSerial Monitor is closed. Uploads will fail if the Serial Monitor has control of the serial port.

  • Select Tools > Pico LittleFS Data Upload. This should start uploading the files into the flash file system.

SD Library Information

The included SD library is the Arduino standard one. Please refer to the [Arduino SD reference](https://www.arduino.cc/en/reference/SD) for more information.

File system object (LittleFS/SD/SDFS)

setConfig

LittleFSConfig cfg;
cfg.setAutoFormat(false);
LittleFS.setConfig(cfg);

SDFSConfig c2;
c2.setCSPin(12);
SDFS.setConfig(c2);

This method allows you to configure the parameters of a filesystem before mounting. All filesystems have their own *Config (i.e. SDFSConfig or LittleFSConfig with their custom set of options. All filesystems allow explicitly enabling/disabling formatting when mounts fail. If you do not call this setConfig method before perforing begin(), you will get the filesystem’s default behavior and configuration. By default, SPIFFS will autoformat the filesystem if it cannot mount it, while SDFS will not.

begin

SDFS.begin()
or LittleFS.begin()

This method mounts file system. It must be called before any other FS APIs are used. Returns true if file system was mounted successfully, false otherwise. With no options it will format SPIFFS if it is unable to mount it on the first try.

Note that LittleFS will automatically format the filesystem if one is not detected. This is configurable via setConfig

end

SDFS.end()
or LittleFS.end()

This method unmounts the file system.

format

SDFS.format()
or LittleFS.format()

Formats the file system. May be called either before or after calling begin. Returns true if formatting was successful.

open

SDFS.open(path, mode)
or LittleFS.open(path, mode)

Opens a file. path should be an absolute path starting with a slash (e.g. /dir/filename.txt). mode is a string specifying access mode. It can be one of “r”, “w”, “a”, “r+”, “w+”, “a+”. Meaning of these modes is the same as for fopen C function.

r      Open text file for reading.  The stream is positioned at the
       beginning of the file.

r+     Open for reading and writing.  The stream is positioned at the
       beginning of the file.

w      Truncate file to zero length or create text file for writing.
       The stream is positioned at the beginning of the file.

w+     Open for reading and writing.  The file is created if it does
       not exist, otherwise it is truncated.  The stream is
       positioned at the beginning of the file.

a      Open for appending (writing at end of file).  The file is
       created if it does not exist.  The stream is positioned at the
       end of the file.

a+     Open for reading and appending (writing at end of file).  The
       file is created if it does not exist.  The initial file
       position for reading is at the beginning of the file, but
       output is always appended to the end of the file.

Returns File object. To check whether the file was opened successfully, use the boolean operator.

File f = LittleFS.open("/f.txt", "w");
if (!f) {
    Serial.println("file open failed");
}

exists

SDFS.exists(path)
or LittleFS.exists(path)

Returns true if a file with given path exists, false otherwise.

mkdir

SDFS.mkdir(path)
or LittleFS.mkdir(path)

Returns true if the directory creation succeeded, false otherwise.

rmdir

SDFS.rmdir(path)
or LittleFS.rmdir(path)

Returns true if the directory was successfully removed, false otherwise.

openDir

SDFS.openDir(path)
or LittleFS.openDir(path)

Opens a directory given its absolute path. Returns a Dir object. Please note the previous discussion on the difference in behavior between LittleFS and SPIFFS for this call.

remove

SDFS.remove(path)
or LittleFS.remove(path)

Deletes the file given its absolute path. Returns true if file was deleted successfully.

rename

SDFS.rename(pathFrom, pathTo)
or LittleFS.rename(pathFrom, pathTo)

Renames file from pathFrom to pathTo. Paths must be absolute. Returns true if file was renamed successfully.

info DEPRECATED

FSInfo fs_info;
or LittleFS.info(fs_info);

Fills FSInfo structure with information about the file system. Returns true if successful, false otherwise. Because this cannot report information about filesystemd greater than 4MB, don’t use it in new code. Use info64 instead which uses 64-bit fields for filesystem sizes.

Filesystem information structure

struct FSInfo {
    size_t totalBytes;
    size_t usedBytes;
    size_t blockSize;
    size_t pageSize;
    size_t maxOpenFiles;
    size_t maxPathLength;
};

This is the structure which may be filled using FS::info method. - totalBytes — total size of useful data on the file system - usedBytes — number of bytes used by files - blockSize — filesystem block size - pageSize — filesystem logical page size - maxOpenFiles — max number of files which may be open simultaneously - maxPathLength — max file name length (including one byte for zero termination)

info64

FSInfo64 fsinfo;
SDFS.info(fsinfo);
or LittleFS(fsinfo);

Performs the same operation as info but allows for reporting greater than 4GB for filesystem size/used/etc. Should be used with the SD and SDFS filesystems since most SD cards today are greater than 4GB in size.

setTimeCallback(time_t (*cb)(void))

time_t myTimeCallback() {
    return 1455451200; // UNIX timestamp
}
void setup () {
    LittleFS.setTimeCallback(myTimeCallback);
    ...
    // Any files will now be made with Pris' incept date
}

The SD, SDFS, and LittleFS filesystems support a file timestamp, updated when the file is opened for writing. By default, the Pico will use the internal time returned from time(NULL) (i.e. local time, not UTC, to conform to the existing FAT filesystem), but this can be overridden to GMT or any other standard you’d like by using setTimeCallback(). If your app sets the system time using NTP before file operations, then you should not need to use this function. However, if you need to set a specific time for a file, or the system clock isn’t correct and you need to read the time from an external RTC or use a fixed time, this call allows you do to so.

In general use, with a functioning time() call, user applications should not need to use this function.

Directory object (Dir)

The purpose of Dir object is to iterate over files inside a directory. It provides multiple access methods.

The following example shows how it should be used:

Dir dir = LittleFS.openDir("/data");
// or Dir dir = LittleFS.openDir("/data");
while (dir.next()) {
    Serial.print(dir.fileName());
    if(dir.fileSize()) {
        File f = dir.openFile("r");
        Serial.println(f.size());
    }
}

next

Returns true while there are files in the directory to iterate over. It must be called before calling fileName(), fileSize(), and openFile() functions.

fileName

Returns the name of the current file pointed to by the internal iterator.

fileSize

Returns the size of the current file pointed to by the internal iterator.

fileTime

Returns the time_t write time of the current file pointed to by the internal iterator.

fileCreationTime

Returns the time_t creation time of the current file pointed to by the internal iterator.

isFile

Returns true if the current file pointed to by the internal iterator is a File.

isDirectory

Returns true if the current file pointed to by the internal iterator is a Directory.

openFile

This method takes mode argument which has the same meaning as for SDFS/LittleFS.open() function.

rewind

Resets the internal pointer to the start of the directory.

setTimeCallback(time_t (*cb)(void))

Sets the time callback for any files accessed from this Dir object via openNextFile. Note that the SD and SDFS filesystems only support a filesystem-wide callback and calls to Dir::setTimeCallback may produce unexpected behavior.

File object

SDFS/LittleFS.open() and dir.openFile() functions return a File object. This object supports all the functions of Stream, so you can use readBytes, findUntil, parseInt, println, and all other Stream methods.

There are also some functions which are specific to File object.

seek

file.seek(offset, mode)

This function behaves like fseek C function. Depending on the value of mode, it moves current position in a file as follows:

  • if mode is SeekSet, position is set to offset bytes from the beginning.

  • if mode is SeekCur, current position is moved by offset bytes.

  • if mode is SeekEnd, position is set to offset bytes from the end of the file.

Returns true if position was set successfully.

position

file.position()

Returns the current position inside the file, in bytes.

size

file.size()

Returns file size, in bytes.

name

String name = file.name();

Returns short (no-path) file name, as const char*. Convert it to String for storage.

fullName

// Filesystem:
//   testdir/
//           file1
Dir d = LittleFS.openDir("testdir/");
File f = d.openFile("r");
// f.name() == "file1", f.fullName() == "testdir/file1"

Returns the full path file name as a const char*.

getLastWrite

Returns the file last write time, and only valid for files opened in read-only mode. If a file is opened for writing, the returned time may be indeterminate.

getCreationTime

Returns the file creation time, if available.

isFile

bool amIAFile = file.isFile();

Returns true if this File points to a real file.

isDirectory

bool amIADir = file.isDir();

Returns true if this File points to a directory (used for emulation of the SD.* interfaces with the openNextFile method).

close

file.close()

Close the file. No other operations should be performed on File object after close function was called.

setTimeCallback(time_t (*cb)(void))

Sets the time callback for this specific file. Note that the SD and SDFS filesystems only support a filesystem-wide callback and calls to Dir::setTimeCallback may produce unexpected behavior.