USB Mass Storage

Overview

This sample app demonstrates use of a USB Mass Storage driver by the Zephyr project. This very simple driver enumerates a board with either RAM or FLASH into an USB disk. This sample can be found under samples/subsys/usb/mass in the Zephyr project tree.

Requirements

This project requires a USB device driver, and either 96KiB of RAM or a FLASH device.

Building and Running

This sample can be built for multiple boards, customized through overlays found in samples/subsys/usb/mass/boards in the Zephyr project tree. The selection between a RAM-based or a FLASH-based disk and file system can be chosen passing Kconfig configuration via the -D command-line switch.

RAM-disk Example without any file system

The default configurations selects RAM-based disk without any file system. This example only needs additional 96KiB RAM for the RAM-disk and is intended for testing USB mass storage class implementation.

west build -b reel_board samples/subsys/usb/mass -- -DEXTRA_DTC_OVERLAY_FILE="ramdisk.overlay"

FAT FS Example

If more than 96KiB are available, FAT files system can be used with a RAM-disk. Alternatively it is possible with the FLASH-based disk. In this example we will build the sample with a RAM-based disk:

west build -b reel_board samples/subsys/usb/mass -- -DEXTRA_DTC_OVERLAY_FILE="ramdisk.overlay" -DCONFIG_APP_MSC_STORAGE_RAM=y

In this example we will build the sample with a FLASH-based disk and FAT file system for Adafruit Feather nRF52840 Express. This board configures to use the external 16 MiBi QSPI flash chip with a 2 MiBy FAT partition.

west build -b adafruit_feather/nrf52840 samples/subsys/usb/mass -- -DCONFIG_APP_MSC_STORAGE_FLASH_FATFS=y

After you have built and flashed the sample app image to your board, plug the board into a host device, for example, a PC running Linux. The board will be detected as shown by the Linux journalctl command:

$ journalctl -k -n 17
usb 2-2.4: new full-speed USB device number 29 using xhci_hcd
usb 2-2.4: New USB device found, idVendor=2fe3, idProduct=0008, bcdDevice= 2.03
usb 2-2.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 2-2.4: Product: Zephyr MSC sample
usb 2-2.4: Manufacturer: ZEPHYR
usb 2-2.4: SerialNumber: 86FE679A598AC47A
usb-storage 2-2.4:1.0: USB Mass Storage device detected
scsi host3: usb-storage 2-2.4:1.0
scsi 3:0:0:0: Direct-Access     ZEPHYR   ZEPHYR USB DISK  0.01 PQ: 0 ANSI: 0 CCS
sd 3:0:0:0: Attached scsi generic sg4 type 0
sd 3:0:0:0: [sdb] 256 512-byte logical blocks: (131 kB/128 KiB)
sd 3:0:0:0: [sdb] Write Protect is off
sd 3:0:0:0: [sdb] Mode Sense: 03 00 00 00
sd 3:0:0:0: [sdb] No Caching mode page found
sd 3:0:0:0: [sdb] Assuming drive cache: write through
 sdb:
sd 3:0:0:0: [sdb] Attached SCSI removable disk

The output to the console will look something like this (file system contents will be different):

*** Booting Zephyr OS build zephyr-v2.3.0-1991-g4c8d1496eafb  ***
Area 4 at 0x0 on GD25Q16 for 2097152 bytes
Mount /NAND:: 0
/NAND:: bsize = 512 ; frsize = 1024 ; blocks = 2028 ; bfree = 1901
/NAND: opendir: 0
  F 0 SAMPLE.TXT
End of files
[00:00:00.077,423] <inf> main: The device is put in USB mass storage mode.

On most operating systems the drive will be automatically mounted.

SD Card Example

This example requires SD card support, see Disk Access, and a SD card formatted with FAT filesystem.

If a board with SD card controller is available, the example can be built as follows:

west build -b mimxrt1050_evk samples/subsys/usb/mass -- -DCONFIG_APP_MSC_STORAGE_SDCARD=y

In case the board has no support for SD card controller, but the card can be connected to SPI using e.g. a shield, example can be built as follows:

west build -b nrf52840dk/nrf52840 samples/subsys/usb/mass -- -DSHIELD=waveshare_epaper_gdeh0154a07 -DCONFIG_APP_MSC_STORAGE_SDCARD=y

Depending on the size of the media it can take time until the file system has initialized the card and it is available via USB. It should also be noted that the transfer speed over SPI is very slow.

*** Booting Zephyr OS build v2.5.0-rc3-73-gd85067f0a759  ***
Mount /SD:: 0
[00:00:00.281,585] <inf> sdhc_spi: Found a ~3751 MiB SDHC card.
[00:00:00.282,867] <inf> sdhc_spi: Manufacturer ID=27 OEM='SM' Name='00000' Revision=0x10 Serial=0x16fdd47b
[00:00:00.308,654] <inf> sdhc_spi: Found a ~3751 MiB SDHC card.
[00:00:00.309,906] <inf> sdhc_spi: Manufacturer ID=27 OEM='SM' Name='00000' Revision=0x10 Serial=0x16fdd47b
/SD:: bsize = 512 ; frsize = 32768 ; blocks = 119776 ; bfree = 119773
/SD: opendir: 0
  D 0 42
  F 1111 TEST.TXT
End of files
[00:00:18.588,043] <inf> main: The device is put in USB mass storage mode.

LittleFS Example

This board configures to use the external 64 MiBi QSPI flash chip with a 128 KiBy littlefs partition compatible with the one produced by the LittleFS filesystem sample.

west build -b nrf52840dk/nrf52840 samples/subsys/usb/mass -- -DCONFIG_APP_MSC_STORAGE_FLASH_LITTLEFS=y

After you have built and flashed the sample app image to your board, connect the board’s two USB connectors (debug and nRF USB) to a host running a littlefs-FUSE-capable operating system. The output to the console will look something like this (file system contents will be different):

*** Booting Zephyr OS build zephyr-v2.2.0-1966-g7815942d5fc5  ***
Area 4 at 0x0 on MX25R64 for 65536 bytes
[00:00:00.005,310] <inf> main: The device is put in USB mass storage mode.

[00:00:00.009,002] <inf> littlefs: LittleFS version 2.2, disk version 2.0
[00:00:00.009,063] <inf> littlefs: FS at MX25R64:0x0 is 16 0x1000-byte blocks with 512 cye
[00:00:00.009,063] <inf> littlefs: sizes: rd 16 ; pr 16 ; ca 64 ; la 32
[00:00:00.011,718] <inf> littlefs: /lfs mounted
Mount /lfs: 0
/lfs: bsize = 16 ; frsize = 4096 ; blocks = 16 ; bfree = 13
/lfs opendir: 0
  F 8 hi
  F 128 linux
  F 5 newfile
End of files

For information on mounting littlefs file system on Linux or FreeBSD systems refer to the “littlefs Usage” section below.

littlefs Usage

While a FAT-based file system can be mounted by many systems automatically, mounting the littlefs file system on a Linux or FreeBSD system can be accomplished using the littlefs-FUSE utility.

First determine the local device name from the system log, e.g.:

Apr 25 08:10:25 tirzah kernel: [570310.921039] scsi 17:0:0:0: Direct-Access     ZEPHYR   ZEPHYR USB DISK  0.01 PQ: 0 ANSI: 0 CCS
Apr 25 08:10:25 tirzah kernel: [570310.921550] sd 17:0:0:0: Attached scsi generic sg4 type 0
Apr 25 08:10:25 tirzah kernel: [570310.922277] sd 17:0:0:0: [sdd] 256 512-byte logical blocks: (131 kB/128 KiB)
Apr 25 08:10:25 tirzah kernel: [570310.922696] sd 17:0:0:0: [sdd] Write Protect is off

This shows that the block device associated with the USB drive is /dev/sdd:

tirzah[447]$ ll /dev/sdd
brw-rw---- 1 root disk 8, 48 Apr 25 08:10 /dev/sdd

This can be mounted as a file system with the following commands:

sudo chmod a+rw /dev/sdd   # required to allow user access
mkdir /tmp/lfs
lfs \
       --read_size=16 \
       --prog_size=16 \
       --block_size=4096 \
       --block_count=32 \
       --cache_size=64 \
       --lookahead_size=32 \
       /dev/sdd /tmp/lfs

which produces output like this (disk contents will vary):

tirzah[467]$ ls -l /tmp/lfs
total 0
-rwxrwxrwx 0 root root   8 Dec 31  1969 hi
-rwxrwxrwx 0 root root 128 Dec 31  1969 linux
-rwxrwxrwx 0 root root   5 Dec 31  1969 newfile

lfs is a mount command and you should take care to unmount the device before removing the USB drive:

umount /tmp/lfs

littlefs parameter selection

Be aware that the parameters passed to lfs in the example above must exactly match the corresponding parameters used to initialize the file system. The required parameters can be observed from the Zephyr mount log messages:

[00:00:00.009,002] <inf> littlefs: LittleFS version 2.2, disk version 2.0
[00:00:00.009,063] <inf> littlefs: FS at MX25R64:0x0 is 16 0x1000-byte blocks with 512 cye
[00:00:00.009,063] <inf> littlefs: sizes: rd 16 ; pr 16 ; ca 64 ; la 32

  • --read_size corresponds to the rd size and is 16;

  • --prog_size corresponds to the pr size and is 16;

  • --block_size comes from 0x1000-byte blocks and is 4096 (0x1000);

  • --block_count comes from 16 0x1000-byte blocks and is 16;

  • --cache_size comes from the ca size and is 64;

  • --lookahead_size comes from the la size and is 32

If any of the parameters are inconsistent between the Zephyr and Linux specification the file system will not mount correctly.