SD card writing speed issue

RE: SD card writing speed issue

liteyear — Thu, 29 Apr 2021 03:04:36 GMT

I hit this obstacle recently too. SD Card reads capped at about 27kB/s. My my, what a mission to get to the bottom of it. After a long battle, the issue is now resolved for me. I ended up using a few fixes that have been applied to the Zephyr upstream project, and back-porting them to the now very out of date nRF Connect SDK downstream project. I've summarised the changes in these three diffs:

devzone.nordicsemi.com/.../sdhc_2D00_speed_2D00_fix.patch

devzone.nordicsemi.com/.../spi_2D00_devicetree_2D00_fix_2D00_part1.patch

devzone.nordicsemi.com/.../spi_2D00_devicetree_2D00_fix_2D00_part2.patch

My next issue is that SD Card reading disrupts I2S comms... and so the saga continues.

RE: SD card writing speed issue

Z.Hong — Mon, 04 Jan 2021 17:21:02 GMT

Thank you so much for the help!

RE: SD card writing speed issue

Håkon Alseth — Mon, 04 Jan 2021 11:44:02 GMT

Hi,

[quote user="Z.Hong"]From my observation, the only way that the speed changed is changing the SDHC_SPI_INITIAL_SPEED to a different value, which is a hard coding define value. Additionally, this is the initial value, the speed should change to another value (SDHC_SPI_SPEED) after initialization here, but it doesn't. [/quote][quote user="Z.Hong"]- What is the correct way to change/define the SPI frequency in my case?[/quote][quote user="Z.Hong"]- Why the hard coding value SDHC_SPI_SPEED doesn't affect the actual read/write speed?[/quote]

As the frequency in the SDHC library is hard-coded, a change to the zephyr project is needed to allow other frequencies at initial speed and "normal speed".

By looking at the driver port, spi_nrfx_spim.c, it looks like a weakness there (or a intentional implementation), where you can only configure the device once:

https://github.com/nrfconnect/sdk-zephyr/blob/master/drivers/spi/spi_nrfx_spim.c#L112-L114

I would recommend that you either remove this check, or set the SDHC_SPI_INITIAL_SPEED equal to SDHC_SPI_SPEED, ie. up to 8 MHz.

Kind regards,

Håkon

RE: SD card writing speed issue

Z.Hong — Mon, 21 Dec 2020 17:50:47 GMT

Hi,

I understand what you mentioned in the answer. However, I just wonder why changing the SPI speed doesn't work, at least different frequency(or speed) will give you a different frequency in the scope, not matter what the value is. Or maybe I defined it in the wrong way.

From my observation, the only way that the speed changed is changing the SDHC_SPI_INITIAL_SPEED to a different value, which is a hard coding define value. Additionally, this is the initial value, the speed should change to another value (SDHC_SPI_SPEED) after initialization here, but it doesn't.

So my question is:

- What is the correct way to change/define the SPI frequency in my case?

- Why the hard coding value SDHC_SPI_SPEED doesn't affect the actual read/write speed?

My apology if my expression makes you confused.

Thanks,

Zirun

RE: SD card writing speed issue

Håkon Alseth — Mon, 21 Dec 2020 14:07:08 GMT

Hi,

The speeds you're getting (just below 30kB/s) seems to correspond with the numbers provided in this zephyr PR:

https://github.com/zephyrproject-rtos/zephyr/pull/26319

It looks like you are writing at 4MHz SPI speed, given the speed results you're seeing. As you can see from the comparison with 250k/4M, its not a linear speed difference, which indicates that it takes quite some processing to conform to the specific filesystem.

Kind regards,

Håkon

RE: SD card writing speed issue

Z.Hong — Thu, 17 Dec 2020 17:29:07 GMT

Hi,

I do probe the CLK line when transmitting the file, and it doesn't change to 4MHz. It remains at 250KHz for some reason.

So, I manually change the SDHC_SPI_INITIAL_SPEED to 4MHZ, not sure if it is correct to do so, but it initially at 4MHZ and remains at 4MHz when doing file writing. I guess it doesn't change the speed here when it finish initialization. Also I notice the speed is not changed/updated to new value in disk_access_spi_sdhc.c, maybe it is defined somewhere?

Zirun

RE: SD card writing speed issue

Håkon Alseth — Thu, 17 Dec 2020 09:14:20 GMT

Hi,

My apologies, the spi_nrfx_spim port reconfigures the SPI frequency after the application calls the first spi_transceive function, as explained in the comment here:

https://github.com/nrfconnect/sdk-zephyr/blob/v2.4.0-ncs2/drivers/spi/spi_nrfx_spim.c#L309-L311

[quote user="Z.Hong"] Also I probe the CLK line and find that it is at 250KHz.[/quote]

It looks like the initial communication is at 250 kHz, and it switches to 4M after successful init:

https://github.com/nrfconnect/sdk-zephyr/blob/master/subsys/disk/disk_access_spi_sdhc.c#L18-L21

Did you scope the communication when transferring a file?

Kind regards,

Håkon

RE: SD card writing speed issue

Z.Hong — Wed, 16 Dec 2020 21:34:51 GMT

Hi,

Thanks for your reply.

The driver sets the frequency configuration to 4MHz:

- So can I set it a different value by setting spi-max-frequency in the overlay file?

Or maybe, in another approach, setting

spi_nrfx_config.frequency = NRF_SPIM_FREQ_8M

You can try to change this to a higher value, lets say 64, and see if this affects the transfer speed & Could you also try to store a larger file to see if this speed is the same then as well?

- I try in both approach, the result is still the same, even worse when I store a larger file with a speed ~ 24KB/sec.

Also I probe the CLK line and find that it is at 250KHz.

Zirun

RE: SD card writing speed issue

Håkon Alseth — Wed, 16 Dec 2020 11:26:51 GMT

Hi,

The driver sets the frequency configuration to 4MHz:

https://github.com/nrfconnect/sdk-zephyr/blob/v2.4.0-ncs2/drivers/spi/spi_nrfx_spim.c#L431

The DMA length is 12 bit, meaning that you have 2047 bytes max per transaction: https://infocenter.nordicsemi.com/topic/ps_nrf9160/spim.html?cp=2_0_0_5_12_5_27#register.RXD.MAXCNT

The configuration "CONFIG_SPI_NRFX_RAM_BUFFER_SIZE=8" gives a chunk size of 8 bytes, as the DMA can only transfer from a RAM buffer. You can try to change this to a higher value, lets say 64, and see if this affects the transfer speed. That being said; you're using a filesystem on your SDHC card, which requires handling as well. Changing the SPI speed can reduce the physical delay between the nRF and SD-card, but you still have to conform to the fatfs filesystem.

Could you also try to store a larger file to see if this speed is the same then as well?

Kind regards,

Håkon