Slow USB MSC transfer speed on nRF5340 with SD card

[Shuhao Dong]

In our design, we wired the SD card in SPI mode on SPI bus 4 wich a maximum SPI frequency of 50 MHz or 25 MHz as these are supported by the SD card. We used FAT32 file system, but later enabled support for exFAT and formated the SD card with exFAT. For internal logging from our device to the SD card, there is no issue. However, when we try to offload the data from SD card to a host machine, we are having an extremely slow transfer speed: When offloading a 10.5 MB binary file, it takes around 420 seconds, which effectively is only 90 KB/s. We have tried both 25 and 50 MHz SPI frequency but the result is very similar.

We are using USB legacy stack with NCS 3.0.2. I attached here our Kconfigs related to USB.

# Configure USB device
CONFIG_USB_DEVICE_STACK=y
CONFIG_USB_DEVICE_PRODUCT="BORUS"
CONFIG_USB_DEVICE_PID=0x0001
CONFIG_USB_DEVICE_VID=0x0001
CONFIG_USB_DRIVER_LOG_LEVEL_ERR=y
CONFIG_USB_DEVICE_LOG_LEVEL_ERR=y
CONFIG_SERIAL=n
CONFIG_USB_CDC_ACM=n
CONFIG_UART_INTERRUPT_DRIVEN=n
CONFIG_UART_LINE_CTRL=n
CONFIG_BOARD_SERIAL_BACKEND_CDC_ACM=n
CONFIG_USB_DEVICE_INITIALIZE_AT_BOOT=n

# Configure USB device to appear as DFU class
# Configure DFU autoupdate and reset
CONFIG_USB_DFU_CLASS=y
CONFIG_USB_REQUEST_BUFFER_SIZE=128
CONFIG_USB_DFU_ENABLE_UPLOAD=y
CONFIG_USB_DFU_PERMANENT_DOWNLOAD=y
CONFIG_USB_DFU_REBOOT=y
CONFIG_IMG_MANAGER=y
CONFIG_STREAM_FLASH=y
CONFIG_BOOTLOADER_MCUBOOT=y

# Configure file system 
CONFIG_FLASH_MAP=y
CONFIG_FLASH=y
CONFIG_FILE_SYSTEM=y
CONFIG_FILE_SYSTEM_LITTLEFS=n
CONFIG_FAT_FILESYSTEM_ELM=y
CONFIG_FS_FATFS_LFN=y
CONFIG_FS_FATFS_EXFAT=y
CONFIG_FS_LOG_LEVEL_INF=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y
CONFIG_NORDIC_QSPI_NOR=y
CONFIG_NORDIC_QSPI_NOR_FLASH_LAYOUT_PAGE_SIZE=4096

# Configure mass storage
CONFIG_USB_MASS_STORAGE=y
CONFIG_DISK_ACCESS=y
CONFIG_USB_MASS_STORAGE_LOG_LEVEL_ERR=y
CONFIG_DISK_LOG_LEVEL_DBG=y
CONFIG_DISK_DRIVERS=y
CONFIG_DISK_DRIVER_SDMMC=y
CONFIG_DISK_DRIVER_FLASH=y
CONFIG_MASS_STORAGE_DISK_NAME="SD"

[dejans]

Hi,

Which SD card do you use?

Can you show your SPI related configuration?

You can try to increase the value of CONFIG_SPI_NRFX_RAM_BUFFER_SIZE. What is the result?

Best regards,
Dejan

[Shuhao Dong]

Hello,

I have tried to increase the buffer size to 128 or 1024. I can confirm this has limited help on increasing transfer speed. Specifically, when copy 10.5 MB file, the time is decreased from previous 131 seconds to now 125 seconds (buffer size = 1024). There is an increase here but not a game changer. Can you suggest other posibilities to debug this issue?

[dejans]

Hi,

Which operating system have you used in your testing?

How did you calculate your transfer speed to be around 90 KB/s?

We used FAT32 file system, but later enabled support for exFAT and formated the SD card with exFAT.

Were transfer speed results similar in both cases - FAT32 vs exFAT?

Have you tried to test when logging is disabled? What is the result?

Best regards,
Dejan

[Shuhao Dong]

I used MacOS as host machine. I have a single 10.5 MB binary file in the SD card, I copy the file and start counting time then a simple divide. Transfer speed is very similar with FAT32 and exFAT. The only difference I can observe from exFAT is increased stability. When we use FAT32 at the beginning, copying large files like 10 MB will sometime trigger an automatic ejection of SD card from host machine. While exFAT won't cause this issue. 

[dejans]

Hi,

Have you observed speed difference when transferring small vs large files?

dejans said:

Have you tried to test when logging is disabled? What is the result?

Have you tried to disable logging?

Best regards,
Dejan

[Shuhao Dong]

Hi, I have tried to disable logging, and the speed is doubled: a 10 MB file takes 54 seconds to finish copy, that is effectively 192 kB/s. Not as fast as the previous post, but I think this is due to the QSPI flash in the post vs SPI flash in my design. Please suggest if SD card in SPI mode with this speed is the expected behaviour. Many thanks. 

[dejans]

Hi,

Shuhao Dong said:

Please suggest if SD card in SPI mode with this speed is the expected behaviour.

Obtained values for transfer speed seem to be quite right in terms of what should be achievable.

Best regards,
Dejan