Getting started with the Mbed OS file system

This example demonstrates how to use the Mbed OS file system.

You can find more information about the Mbed OS file system and other related pieces of the Mbed OS storage stack in the storage overview.

Table of contents:

Hardware requirements
Usage
Changing the file system
Changing the block device
Tested configurations

Hardware requirements

This example uses a block device as storage. This can be either an external block device (one of SPI flash, DataFlash or an SD card) or simulated on a heap block device on boards with enough RAM.

This example uses an instance of the LittleFileSystem API (LittleFS) on external SPI flash. The changing the block device section describes how to change the file system or block device in the example.

Usage

Import the example

Make sure you have an Mbed development environment set up. Get started with Mbed OS to set everything up.

From the command-line, import the example:

mbed import mbed-os-example-filesystem
cd mbed-os-example-filesystem

Compile the example

Invoke mbed compile, and specify the name of your platform and your favorite toolchain (GCC_ARM, ARM, IAR). For example, for the ARM Compiler 5:

mbed compile -m K64F -t ARM

Your PC may take a few minutes to compile your code. At the end, you see the following result:

[snip]
+--------------------------+-------+-------+-------+
| Module                   | .text | .data |  .bss |
+--------------------------+-------+-------+-------+
| Fill                     |   164 |     0 |  2136 |
| Misc                     | 54505 |  2556 |   754 |
| drivers                  |   640 |     0 |    32 |
| features/filesystem      | 15793 |     0 |   550 |
| features/storage         |    42 |     0 |   184 |
| hal                      |   418 |     0 |     8 |
| platform                 |  2355 |    20 |   582 |
| rtos                     |   135 |     4 |     4 |
| rtos/rtx                 |  5861 |    20 |  6870 |
| targets/TARGET_Freescale |  8382 |    12 |   384 |
| Subtotals                | 88295 |  2612 | 11504 |
+--------------------------+-------+-------+-------+
Allocated Heap: 24576 bytes
Allocated Stack: unknown
Total Static RAM memory (data + bss): 14116 bytes
Total RAM memory (data + bss + heap + stack): 38692 bytes
Total Flash memory (text + data + misc): 91947 bytes

Image: ./BUILD/K64F/ARM/mbed-os-example-filesystem.bin

Run the example

Connect your Mbed Enabled device to the computer over USB.
Copy the binary file to the Mbed Enabled device.
Press the reset button to start the program.
Open the UART of the board in your favorite UART viewing program. For example, screen /dev/ttyACM0.

Note: The default serial port baud rate is 9600 bit/s.

Expected output:

--- Mbed OS filesystem example ---
Mounting the filesystem... Fail :(
No filesystem found, formatting... OK
Opening "/fs/numbers.txt"... Fail :(
No file found, creating a new file... OK
Writing numbers (10/10)... OK
Seeking file... OK
Incrementing numbers (10/10)... OK
Closing "/fs/numbers.txt"... OK
Opening the root directory... OK
root directory:
    .
    ..
    numbers.txt
Closing the root directory... OK
Opening "/fs/numbers.txt"...OK
numbers:
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
Closing "/fs/numbers.txt"... OK
Unmounting... OK
Mbed OS filesystem example done!

You can also reset the board to see the data persist across boots. Each boot increments the numbers stored on disk:

--- Mbed OS filesystem example ---
Mounting the filesystem... OK
Opening "/fs/numbers.txt"... OK
Incrementing numbers (10/10)... OK
Closing "/fs/numbers.txt"... OK
Opening the root directory... OK
root directory:
    .
    ..
    numbers.txt
Closing the root directory... OK
Opening "/fs/numbers.txt"...OK
numbers:
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
Closing "/fs/numbers.txt"... OK
Unmounting... OK
Mbed OS filesystem example done!

If you find yourself with a corrupted file system, you can reset the storage by pressing BUTTON1:

Initializing the block device... OK
Erasing the block device... OK
Deinitializing the block device... OK

Note that if you press the reset button at the wrong time, you may corrupt a file system that is not power resilient!

Troubleshooting

If you have problems, you can review the documentation for suggestions on what could be wrong and how to fix it.

Changing the file system

In Mbed OS, a C++ classes that inherits from the FileSystem interface represents each file system. You can change the file system in the example by changing the class declared in main.cpp.

- LittleFileSystem fs("fs");
+ FATFileSystem fs("fs");

Note: Different file systems require different minimum numbers of storage blocks to function. For the FATFileSystem, this example requires a minimum of 256 blocks, and for the LittleFileSystem, this example requires a minimum of 6 blocks. You can find the number of blocks on a block device by dividing the block device's size by its erase size.

Mbed OS has two options for the file system:

LittleFileSystem - The little file system is a fail-safe file system we designed for embedded systems, specifically for microcontrollers that use flash storage.
```
LittleFileSystem fs("fs");
```
- Bounded RAM/ROM - This file system works with a limited amount of memory. It avoids recursion and limits dynamic memory to configurable buffers.
- Power-loss resilient - We designed this for operating systems that may have random power failures. It has strong copy-on-write guarantees and keeps storage on disk in a valid state.
- Wear leveling - Because the most common form of embedded storage is erodible flash memories, this file system provides a form of dynamic wear leveling for systems that cannot fit a full flash translation layer.
FATFileSystem - The FAT file system is a well-known file system that you can find on almost every system, including PCs. The Mbed OS implementation of the FAT file system is based on ChanFS and is optimized for small embedded systems.
```
FATFileSystem fs("fs");
```
- Portable - Almost every operating system supports the FAT file system, which is the most common file system found on portable storage, such as SD cards and flash drives. The FAT file system is the easiest way to support access from a PC.

Changing the block device

In Mbed OS, a C++ classes that inherits from the BlockDevice interface represents each block device. You can change the filesystem in the example by changing the class declared in main.cpp.

-SPIFBlockDevice bd(
-        MBED_CONF_SPIF_DRIVER_SPI_MOSI,
-        MBED_CONF_SPIF_DRIVER_SPI_MISO,
-        MBED_CONF_SPIF_DRIVER_SPI_CLK,
-        MBED_CONF_SPIF_DRIVER_SPI_CS);
+SDBlockDevice bd(
+        MBED_CONF_SD_SPI_MOSI,
+        MBED_CONF_SD_SPI_MISO,
+        MBED_CONF_SD_SPI_CLK,
+        MBED_CONF_SD_SPI_CS);

Note: Most block devices require pin assignments. Double check that the pins in <driver>/mbed_lib.json are correct. For example, to change the pins for the SD driver, open sd-driver/config/mbed_lib.json, and change your target platform to the correct pin-out in the target_overrides configuration:

   "target_overrides": {
         ...
         "NUCLEO_F429ZI": {
             "SPI_MOSI": "PC_12",
             "SPI_MISO": "PC_11",
             "SPI_CLK":  "PC_10",
             "SPI_CS":   "PA_15"
         },
         ...
     }

Mbed OS has several options for the block device:

SPIFBlockDevice - Block device driver for NOR-based SPI flash devices that support SFDP. NOR-based SPI flash supports byte-sized read and writes, with an erase size of about 4kbytes. An erase sets a block to all 1s, with successive writes clearing set bits.
```
SPIFBlockDevice bd(
        MBED_CONF_SPIF_DRIVER_SPI_MOSI,
        MBED_CONF_SPIF_DRIVER_SPI_MISO,
        MBED_CONF_SPIF_DRIVER_SPI_CLK,
        MBED_CONF_SPIF_DRIVER_SPI_CS);
```
DataFlashBlockDevice - Block device driver for NOR-based SPI flash devices that support the DataFlash protocol, such as the Adesto AT45DB series of devices. DataFlash is a memory protocol that combines flash with SRAM buffers for a programming interface. DataFlash supports byte-sized read and writes, with an erase size of about 528 bytes or sometimes 1056 bytes. DataFlash provides erase sizes with an extra 16 bytes for error correction codes (ECC), so a flash translation layer (FTL) may still present 512 byte erase sizes.
```
DataFlashBlockDevice bd(
        MBED_CONF_DATAFLASH_SPI_MOSI,
        MBED_CONF_DATAFLASH_SPI_MISO,
        MBED_CONF_DATAFLASH_SPI_CLK,
        MBED_CONF_DATAFLASH_SPI_CS);
```
SDBlockDevice - Block device driver for SD cards and eMMC memory chips. SD cards or eMMC chips offer a full FTL layer on top of NAND flash. This makes the storage well-suited for systems that require a about 1GB of memory. Additionally, SD cards are a popular form of portable storage. They are useful if you want to store data that you can access from a PC.
```
SDBlockDevice bd(
        MBED_CONF_SD_SPI_MOSI,
        MBED_CONF_SD_SPI_MISO,
        MBED_CONF_SD_SPI_CLK,
        MBED_CONF_SD_SPI_CS);
```
HeapBlockDevice - Block device that simulates storage in RAM using the heap. Do not use the heap block device for storing data persistently because a power loss causes complete loss of data. Instead, use it fortesting applications when a storage device is not available.
```
HeapBlockDevice bd(1024*512, 512);
```

Additionally, Mbed OS contains several utility block devices to give you better control over the allocation of storage.

SlicingBlockDevice - With the slicing block device, you can partition storage into smaller block devices that you can use independently.
ChainingBlockDevice - With the chaining block device, you can chain multiple block devices together and extend the usable amount of storage.
MBRBlockDevice - Mbed OS comes with support for storing partitions on disk with a Master Boot Record (MBR). The MBRBlockDevice provides this functionality and supports creating partitions at runtime or using preformatted partitions configured separately from outside the application.
ReadOnlyBlockDevice - With the read-only block device, you can wrap a block device in a read-only layer, ensuring that user of the block device does not modify the storage.
ProfilingBlockDevice - With the profiling block device, you can profile the quantity of erase, program and read operations that are incurred on a block device.
ObservingBlockDevice - The observing block device grants the user the ability to register a callback on block device operations. You can use this to inspect the state of the block device, log different metrics or perform some other operation.
ExhaustibleBlockDevice - Useful for evaluating how file systems respond to wear, the exhaustible block device simulates wear on another form of storage. You can configure it to expire blocks as necessary.

Tested configurations

K64F + Heap + LittleFS
K64F + Heap + FATFS
K64F + SD + LittleFS
K64F + SD + FATFS
K64F + SPIF (requires shield) + LittleFS
K64F + SPIF (requires shield) + FATFS
K64F + DataFlash (requires shield) + LittleFS
K64F + DataFlash (requires shield) + FATFS
UBLOX_EVK_ODIN_W2 [1] + Heap + LittleFS
UBLOX_EVK_ODIN_W2 [1] + Heap + FATFS
UBLOX_EVK_ODIN_W2 [1] + SD + LittleFS
UBLOX_EVK_ODIN_W2 [1] + SD + FATFS
UBLOX_EVK_ODIN_W2 [1] + SPIF (requires shield) + LittleFS
UBLOX_EVK_ODIN_W2 [1] + SPIF (requires shield) + FATFS
UBLOX_EVK_ODIN_W2 [1] + DataFlash (requires shield) + LittleFS
UBLOX_EVK_ODIN_W2 [1] + DataFlash (requires shield) + FATFS
NUCLEO_F429ZI + Heap + LittleFS
NUCLEO_F429ZI + Heap + FATFS
NUCLEO_F429ZI + SD (requires shield) + LittleFS
NUCLEO_F429ZI + SD (requires shield) + FATFS
NUCLEO_F429ZI + SPIF (requires shield) + LittleFS
NUCLEO_F429ZI + SPIF (requires shield) + FATFS
NUCLEO_F429ZI + DataFlash (requires shield) + LittleFS
NUCLEO_F429ZI + DataFlash (requires shield) + FATFS

[1]: Note: The UBLOX_EVK_ODIN_W2 SPI pins conflict with the default serial pins. A different set of serial pins must be selected to use SPI flash with serial output.

// Connect Tx, Rx, and ground pins to a separte board running the passthrough example:
// https://os.mbed.com/users/sarahmarshy/code/SerialPassthrough/file/2a3a62ee17fa/main.cpp/
Serial pc(TX, RX);   

pc.printf("...");    // Replace printf with pc.printf in the example

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README.md

README.md

Getting started with the Mbed OS file system

Hardware requirements

Usage

Import the example

Compile the example

Run the example

Troubleshooting

Changing the file system

Changing the block device

Tested configurations

Files

README.md

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README.md

File metadata and controls

Getting started with the Mbed OS file system

Hardware requirements

Usage

Import the example

Compile the example

Run the example

Troubleshooting

Changing the file system

Changing the block device

Tested configurations