This application note explains how to use a USB Flash device with the USB High Speed (HS) interfaces of the i.MX RT1170 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.
1. Hardware Platform
The hardware platform is the NXP i.MX RT1170 EVK board.
This demo assumes that a Micro-B to USB 2.0 A Female cable is plugged into the USB1 interface connector on the NXP i.MX RT1170 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.
2. 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 2023.04 (Sep 01 2023 - 17:34:47 +0000)
Trying to boot from MMC1
U-Boot 2023.04 (Sep 01 2023 - 17:34:47 +0000)
Model: NXP imxrt1170-evk board
DRAM: 960 KiB (effective 64.9 MiB)
Core: 72 devices, 15 uclasses, devicetree: separate
MMC: FSL_SDHC: 0
Loading Environment from MMC... OK
In: serial@4007c000
Out: serial@4007c000
Err: serial@4007c000
Net: eth0: ethernet@40424000
Hit any key to stop autoboot: 0
8489874 bytes read in 755 ms (10.7 MiB/s)
## Booting kernel from Legacy Image at 80007fc0 ...
Image Name: Linux-6.1.22
Image Type: ARM Linux Multi-File Image (uncompressed)
Data Size: 8489810 Bytes = 8.1 MiB
Load Address: 80008000
Entry Point: 80008001
Contents:
Image 0: 8480224 Bytes = 8.1 MiB
Image 1: 9574 Bytes = 9.3 KiB
Verifying Checksum ... OK
## Flattened Device Tree from multi component Image at 80007FC0
Booting using the fdt at 0x8081e5ec
Working FDT set to 8081e5ec
Loading Multi-File Image
Loading Device Tree to 2032a000, end 2032f565 ... OK
Working FDT set to 2032a000
Starting kernel ...
The kernel proceeds to boot-up, initializing the configured I/O interfaces and sub-systems:
Booting Linux on physical CPU 0x0 Linux version 6.1.22 (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 Fri Sep 1 14:20:49 UTC 2023 CPU: ARMv7-M [411fc272] revision 2 (ARMv7M), cr=00000000 CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache OF: fdt: Machine model: NXP IMXRT1170 EVK board Reserved memory: created DMA memory pool at 0x83f00000, size 1 MiB OF: reserved mem: initialized node dmapool@83f00000, compatible id shared-dma-pool Zone ranges: Normal [mem 0x0000000080000000-0x0000000083ffffff] Movable zone start for each node ... usbcore: registered new interface driver usbfs usbcore: registered new interface driver hub usbcore: registered new device driver usb pps_core: LinuxPPS API ver. 1 registered pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti < giometti@linux.it> PTP clock support registered Bluetooth: Core ver 2.22 NET: Registered PF_BLUETOOTH protocol family Bluetooth: HCI device and connection manager initialized Bluetooth: HCI socket layer initialized Bluetooth: L2CAP socket layer initialized Bluetooth: SCO socket layer initialized clocksource: Switched to clocksource mxc_timer1 NET: Registered PF_INET protocol family IP idents hash table entries: 2048 (order: 2, 16384 bytes, linear) tcp_listen_portaddr_hash hash table entries: 1024 (order: 0, 4096 bytes, linear) Table-perturb hash table entries: 65536 (order: 6, 262144 bytes, linear) TCP established hash table entries: 1024 (order: 0, 4096 bytes, linear) TCP bind hash table entries: 1024 (order: 1, 8192 bytes, linear) TCP: Hash tables configured (established 1024 bind 1024) UDP hash table entries: 256 (order: 0, 4096 bytes, linear) UDP-Lite hash table entries: 256 (order: 0, 4096 bytes, linear) NET: Registered PF_UNIX/PF_LOCAL protocol family RPC: Registered named UNIX socket transport module. RPC: Registered udp transport module. RPC: Registered tcp transport module. RPC: Registered tcp NFSv4.1 backchannel transport module. Bus freq driver module loaded Initialise system trusted keyrings workingset: timestamp_bits=30 max_order=14 bucket_order=0 fuse: init (API version 7.37) Key type asymmetric registered Asymmetric key parser 'x509' registered Block layer SCSI generic (bsg) driver version 0.4 loaded (major 251) io scheduler mq-deadline registered io scheduler kyber registered 4007c000.serial: ttyLP0 at MMIO 0x4007c010 (irq = 447, base_baud = 1500000) is a FSL_LPUART fsl-lpuart 4007c000.serial: Serial: Console lpuart rounded baud ratefrom 187500 to 115200 printk: console [ttyLP0] enabled PPP generic driver version 2.4.2 PPP BSD Compression module registered PPP Deflate Compression module registered usbcore: registered new interface driver rt2800usb usbcore: registered new interface driver cdc_acm cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters usbcore: registered new interface driver uas usbcore: registered new interface driver usb-storage i2c_dev: i2c /dev entries driver usbcore: registered new interface driver btusb sdhci: Secure Digital Host Controller Interface driver sdhci: Copyright(c) Pierre Ossman sdhci-pltfm: SDHCI platform and OF driver helper usbcore: registered new interface driver usbhid usbhid: USB HID core driver NET: Registered PF_PACKET protocol family mmc0 bounce up to 128 segments into one, max segment size 65536 bytes Bluetooth: RFCOMM TTY layer initialized Bluetooth: RFCOMM socket layer initialized Bluetooth: RFCOMM ver 1.11 ARMv7-M VFP coprocessor found VFP: Double precision floating points are supported Loading compiled-in X.509 certificates mmc0: SDHCI controller on 40418000.usdhc [40418000.usdhc] using DMA input: gpio-keys as /devices/platform/gpio-keys/input/input0 cfg80211: Loading compiled-in X.509 certificates for regulatory database cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7' platform regulatory.0: Direct firmware load for regulatory.db failed with error -2 cfg80211: failed to load regulatory.db Freeing unused kernel image (initmem) memory: 3556K This architecture does not have kernel memory protection. Run /init as init process [72] Jan 01 00:00:01 Running in background / # mmc0: host does not support reading read-only switch, assuming write-enable mmc0: new high speed SDHC card at address 59b4 mmcblk0: mmc0:59b4 USD 7.39 GiB mmcblk0: p1 Micrel KSZ8081 or KSZ8091 40424000.ethernet-1:02: attached PHY driver (mii_bus:phy_addr=40424000.ethernet-1:02, irq=POLL) fec 40424000.ethernet eth0: Link is Up - 100Mbps/Full - flow control off
2.1. USB OTG 1 Controller (USB OTG)
Connect just a Micro-B to USB 2.0 A Female cable to the USB1 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 1 ci_hdrc ci_hdrc.0: USB 2.0 started, EHCI 1.00 hub 1-0:1.0: USB hub found hub 1-0:1.0: 1 port detected usb 1-1: new high-speed USB device number 2 using ci_hdrc usb-storage 1-1:1.0: USB Mass Storage device detected scsi host0: usb-storage 1-1:1.0 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 1-1: USB disconnect, device number 2 ci_hdrc ci_hdrc.0: remove, state 1 usb usb1: USB disconnect, device number 1 ci_hdrc ci_hdrc.0: USB bus 1 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:
ci_hdrc ci_hdrc.1: EHCI Host Controller ci_hdrc ci_hdrc.1: new USB bus registered, assigned bus number 1 ci_hdrc ci_hdrc.1: USB 2.0 started, EHCI 1.00 hub 1-0:1.0: USB hub found hub 1-0:1.0: 1 port detected usb 1-1: new high-speed USB device number 2 using ci_hdrc usb-storage 1-1:1.0: USB Mass Storage device detected scsi host0: usb-storage 1-1:1.0 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
3. 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.