Getting started with Fat filesystem on mbed OS

This guide reviews the steps to get the FAT filesystem working on an mbed OS platform.

Please install mbed CLI.

Hardware requirements

This exmaple uses a RAM backed FAT filesystem. The FAT filesystem requires at least 128 512-byte blocks for a total of 64KB of space. This space is backed by the HeapBlockDevice, which uses a target's heap for storage. Therefore, to support this example, a target must have at least 64KB of space usable by as heap. As there is more than just the heap in a device's ram this translates to the requirement that a target's RAM must be at least 96KB large.

Import the example application

From the command-line, import the example:

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

Now compile

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/gcc_arm/mbed-os-example-fat-filesystem.bin

Program your board

Connect your mbed device to the computer over USB.
Copy the binary file to the mbed 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.

You see the following output:

Welcome to the filesystem example.
Formatting a FAT, RAM-backed filesystem. done.
Mounting the filesystem on "/fs". done.
Opening a new file, numbers.txt. done.
Writing decimal numbers to a file (20/20) done.
Closing file. done.
Re-opening file read-only. done.
Dumping file to screen.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
EOF.
Closing file. done.
Opening root directory. done.
Printing all filenames:
  numbers.txt
Closeing root directory. done.
Filesystem Demo complete.

Switch from RAM backed block device to an SD card

From the command-line, run the following command:

mbed add sd-driver

Then change the code on line 3 of main.cpp to import the SD card header:

#include "SDBlockDevice.h"

Change the block device declaration on line 7 of main.cpp to use the SD card by replacing the PinNames with the pins connected to the SD card:

SDBlockDevice bd(PinName mosi, PinName miso, PinName sclk, PinName cs);

Troubleshooting

Make sure mbed-cli is working correctly and its version is newer than 1.0.0.

mbed --version

If not, update it:

pip install mbed-cli --upgrade