This application note explains how to use a USB Flash device with the USB High Speed (HS) interfaces of the i.MX RT1050 microcontroller running uCLinux. The chip has Universal Serial Bus 2.0 Integrated PHY which contains two integrated USB 2.0 PHY macrocells capable of connecting to USB host/device systems at the USB low-, full-, and high-speed rates.
Hardware Platform
The hardware platform is the NXP i.MX RT1050 EVK board.
This demo assumes that a Micro-B to USB 2.0 A Female cable is plugged into the USB1 J9 interface connector on the NXP i.MX RT1050 EVK board and that a pre-formatted USB Flash disk with an FAT32 partition is plugged into the USB 2.0 A Female connector of the above USB cable.
Logging Data onto USB Flash
On power-up or reset, U-Boot loads the Linux and Device Tree images from the SD Card to the SDRAM and passes control to the kernel entry point:
U-Boot SPL 2022.04 (Sep 25 2023 - 08:16:21 +0000)
Trying to boot from MMC1
U-Boot 2022.04 (Sep 25 2023 - 08:16:21 +0000)
DRAM: 32 MiB
Core: 71 devices, 13 uclasses, devicetree: separate
MMC: FSL_SDHC: 0
Loading Environment from MMC... OK
In: serial@40184000
Out: serial@40184000
Err: serial@40184000
Net: eth0: ethernet@402D8000
Hit any key to stop autoboot: 0
8277462 bytes read in 1466 ms (5.4 MiB/s)
## Booting kernel from Legacy Image at 80007fc0 ...
Image Name: Linux-5.15.71
Image Type: ARM Linux Multi-File Image (uncompressed)
Data Size: 8277398 Bytes = 7.9 MiB
Load Address: 80008000
Entry Point: 80008001
Contents:
Image 0: 8269856 Bytes = 7.9 MiB
Image 1: 7530 Bytes = 7.4 KiB
Verifying Checksum ... OK
## Flattened Device Tree from multi component Image at 80007FC0
Booting using the fdt at 0x807eb02c
Loading Multi-File Image
Loading Device Tree to 81e81000, end 81e85d69 ... OK
Starting kernel ...
Booting Linux on physical CPU 0x0
Linux version 5.15.71 (sasha@workbench.emcraft.com) (arm-none-eabi-gcc
(GNU Arm Embedded Toolchain 10.3-2021.10) 10.3.1 20210824 (release),
GNU ld (GNU Arm Embedded Toolchain 10.3-2021.10) 2.36.1.20210621) #2
Mon Sep 25 08:03:11 UTC 2023
CPU: ARMv7-M [411fc271] revision 1 (ARMv7M), cr=00000000
CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
OF: fdt: Machine model: NXP IMXRT1050-evk board
Reserved memory: created DMA memory pool at 0x81f00000, size 1 MiB
OF: reserved mem: initialized node dmapool@81f00000, compatible id
shared-dma-pool
Zone ranges:
Normal [mem 0x0000000080000000-0x0000000081ffffff]
Movable zone start for each node
Early memory node ranges
node 0: [mem 0x0000000080000000-0x0000000081efffff]
node 0: [mem 0x0000000081f00000-0x0000000081ffffff]
Initmem setup node 0 [mem 0x0000000080000000-0x0000000081ffffff]
Built 1 zonelists, mobility grouping off. Total pages: 8128
Kernel command line: ip=192.168.1.86:192.168.1.96:192.168.1.254:255.255.
0.0::eth0:off
...
Freeing unused kernel image (initmem) memory: 3544K
This architecture does not have kernel memory protection.
Run /init as init process
[71] Jan 01 00:00:01 Running in background
/ # mmc0: new high speed SDHC card at address 0007
mmcblk0: mmc0:0007 SD8GB 7.42 GiB
mmcblk0: p1
Micrel KSZ8081 or KSZ8091 402d8000.ethernet-1:02: attached PHY driver
(mii_bus:phy_addr=402d8000.ethernet-1:02, irq=POLL)
fec 402d8000.ethernet eth0: Link is Up - 100Mbps/Full - flow control off
USB OTG 1 Controller (USB OTG)
Connect just a Micro-B to USB 2.0 A Female cable to the USB1 J9 connector.
Connect USB Flash device to the cable. Observe it is detected and configured:
ci_hdrc ci_hdrc.0: EHCI Host Controller
ci_hdrc ci_hdrc.0: new USB bus registered, assigned bus number 2
ci_hdrc ci_hdrc.0: USB 2.0 started, EHCI 1.00
hub 2-0:1.0: USB hub found
hub 2-0:1.0: 1 port detected
usb 2-1: new high-speed USB device number 2 using ci_hdrc
usb-storage 2-1:1.0: USB Mass Storage device detected
scsi host0: usb-storage 2-1:1.0
scsi 0:0:0:0: Direct-Access Generic Flash Disk 8.07 PQ: 0 ANSI: 2
sd 0:0:0:0: [sda] 8228864 512-byte logical blocks: (4.21 GB/3.92 GiB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] No Caching mode page found
sd 0:0:0:0: [sda] Assuming drive cache: write through
sda: sda1
sd 0:0:0:0: [sda] Attached SCSI removable disk
At this point, the USB Flash is accessible as a disk. The following command is used to examine the disk, which is detected as a 4GBytes disk partitioned to have a single empty FAT32 partition:
/ # fdisk -l /dev/sda
Disk /dev/sda: 4018 MB, 4213178368 bytes, 8228864 sectors
512 cylinders, 255 heads, 63 sectors/track
Units: sectors of 1 * 512 = 512 bytes
Device Boot StartCHS EndCHS StartLBA EndLBA Sectors Size Id
Type
/dev/sda1 * 0,32,33 511,254,63 2048 8228863 8226816 4017M c
Win95 FAT32 (LBA)
Partition 1 has different physical/logical end:
phys=(511,254,63) logical=(512,56,56)
Let's mount the FAT32 file system. As expected, it is empty at this point:
/ # mount /dev/sda1 /mnt/usbflash
/ # ls -l /mnt/usbflash/
drwxr-xr-x 2 root root 4096 May 25 2023 System Volume Information
/ #
Let's "harvest" some data and store what is collected into a file on the USB Flash disk. In this demo, we emulate a data stream by taking a snapshot of the system time each second:
~ # while true; do date >> /mnt/usbflash/data.log; sleep 1; done
Having let the "data harvesting" run for a few seconds, let's interrupt it (by pressing ^-C) and take a look at what data we have collected:
/ # cat /mnt/usbflash/data.log
Thu Jan 1 00:11:27 UTC 1970
Thu Jan 1 00:11:28 UTC 1970
Thu Jan 1 00:11:29 UTC 1970
Thu Jan 1 00:11:30 UTC 1970
Thu Jan 1 00:11:31 UTC 1970
Thu Jan 1 00:11:32 UTC 1970
Thu Jan 1 00:11:33 UTC 1970
Thu Jan 1 00:11:34 UTC 1970
Thu Jan 1 00:11:35 UTC 1970
Thu Jan 1 00:11:36 UTC 1970
/ #
Now, let's unmount the USB Flash and unplug the device from the USB cable:
/ # umount /mnt/usbflash
At this point, the USB Flash device can be taken to a PC for further data processing. Just plug in the USB Flash into a USB port on your PC and the PC software will be able to mount the device as a FAT32 file system.
usb 2-1: USB disconnect, device number 2
usb 2-1: new high-speed USB device number 3 using ci_hdrc
ci_hdrc ci_hdrc.0: remove, state 1
usb usb2: USB disconnect, device number 1
usb usb2-port1: attempt power cycle
ci_hdrc ci_hdrc.0: USB bus 2 deregistered
Note that the format of Windows and Unix text files differs slightly. In Windows, lines end with both the line feed and carriage return ASCII characters, but Unix uses only a line feed. As a consequence, some Windows applications will not show the line breaks in Unix-format files. Assuming that data is stored in a text file (vs a binary file) and Windows is a data processing host, Linux data harvesting applications should take care of the difference by adding a carriage return character to data logs.
Note further that you can hot plug your USB Flash device on the running system at any time:
usb 2-1: new high-speed USB device number 3 using ci_hdrc
ci_hdrc ci_hdrc.0: remove, state 1
usb usb2: USB disconnect, device number 1
usb usb2-port1: attempt power cycle
ci_hdrc ci_hdrc.0: USB bus 2 deregistered
ci_hdrc ci_hdrc.0: EHCI Host Controller
ci_hdrc ci_hdrc.0: new USB bus registered, assigned bus number 2
ci_hdrc ci_hdrc.0: USB 2.0 started, EHCI 1.00
hub 2-0:1.0: USB hub found
hub 2-0:1.0: 1 port detected
usb 2-1: new high-speed USB device number 2 using ci_hdrc
usb-storage 2-1:1.0: USB Mass Storage device detected
scsi host0: usb-storage 2-1:1.0
scsi 0:0:0:0: Direct-Access Generic Flash Disk 8.07 PQ: 0 ANSI: 2
sd 0:0:0:0: [sda] 8228864 512-byte logical blocks: (4.21 GB/3.92 GiB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] No Caching mode page found
sd 0:0:0:0: [sda] Assuming drive cache: write through
sda: sda1
sd 0:0:0:0: [sda] Attached SCSI removable disk
/ # mount /dev/sda1 /mnt/usbflash
/ # ls -l /mnt/usbflash
drwxr-xr-x 2 root root 4096 May 25 2023 System Volume Information
-rwxr-xr-x 1 root root 232 Jan 1 1980 data.log
/ # umount /mnt/usbflash
Data Synchronization Considerations
It is important to understand that VFAT supports write-back in Linux, which means that file changes do not go to the physical media straight away and instead are cached in memory and go to the Flash at a later time. This helps to reduce amount to I/O to the physical Flash, resulting in a better performance overall.
The write-back creates a certain issue for embedded devices however. If the power to the device is shut down unexpectedly, or the USB Flash is unplugged without a proper unmount or sync, some of latest file changes may be lost.
As it is typical with Linux, the issue can be handled in many ways. Data synchronization can be ensured on a per-file, per-subtree, per-filesystem or system-wide basis. Synchronization can be transparent for the user or may require issuing an explicit API call or a shell command.
The most obvious solution is to mount the file system in synchronous mode (note the -o sync parameter in the call below):
/ # mount -o sync /dev/sda1 /mnt/usbflash
/ # mount
rootfs on / type rootfs (rw)
proc on /proc type proc (rw,relatime)
sysfs on /sys type sysfs (rw,relatime)
devtmpfs on /dev type devtmpfs (rw,nosuid,relatime)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620,ptmxmode=000)
/dev/sda1 on /mnt/usbflash type vfat (rw,sync,relatime,fmask=0022,dmask=0022,
codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
When the file system is mounted for synchronous operation, Linux guarantees that data is written to the physical media before any write()returns to a calling application. The tradeoff is that written data is no longer cached in memory, which reduces the write performance substantially.
