SD card writing speed issue


I am new to nrf9160 and zephyr. I was using the SD card and a breakout board on SPI bus to write the raw data coming from the PDM interface on nrf9160 Feather. Before that I want to do a SD write test to see how the speed is, so I wrote a simple test as attached.
The problem I am facing is:

  • I am able to mount, write and read the file on the SD card, but only on a limit speed of ~ 29KBytes/s, which according to this thread , is using a SPI speed of 4MHz. On my overlay file, I set spi-max-frequency = <8000000> . So I expect it should be at least faster than 29KBytes/s on SD write but not. Then I try changing the spi-max-frequency as different value, like 24MHz, 4MHz, even 800KHz. None of them change the speed and still keep the SD write speed as 29KBytes/s. I wonder if I have done in the wrong way of setting the SPI frequency?
  • Second thing I’ve found from the ‘disk_access_spi_sdhc.c’ under ‘zephyr/subsys/disk/’ path that it #define SDHC_SPI_SPEED 4000000, it kind of make sense that why it is running on 4MHz SPI speed, but again, it should be able to change the SPI frequency in some way, which back to my first question if I have done in the wrong way of setting the SPI frequency?
  • Third thing I want to ask, as I checked the data sheet of nrf9160, it doesn’t said anything on SPI frequency but from the register, it only list from 125kbps to 8Mbps. I wonder if it is correct to say SPI only run at a maximum speed of 8MHz on nrf9160?

Can anyone help with the issue please, thanks in advanced!!


my .overlay:

&spi2 {   
	compatible = "nordic,nrf-spim";
	status = "okay";
        sck-pin = <1>;
        mosi-pin = <30>;
        miso-pin = <0>;
	cs-gpios = <&gpio0 29 GPIO_ACTIVE_LOW>;
	sdhc0: sdhc@0 {
                compatible = "zephyr,mmc-spi-slot";
                reg = <0>;
                status = "okay";
                label = "SDHC0";
                spi-max-frequency = <8000000>;

my prj.conf:


#Disk access

#File system

# Stacks and heaps


My code:

#include <zephyr.h>
#include <device.h>
#include <disk/disk_access.h>
#include <logging/log.h>
#include <fs/fs.h>
#include <ff.h>

#include <nrf.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h> 

#define BUF_SIZE 16128
int16_t writebuff[BUF_SIZE];


char testbuff[] = "this is a test buffer";

struct fs_file_t filep;

static FATFS fat_fs;
/* mounting info */
static struct fs_mount_t mp = {
	.type = FS_FATFS,
	.fs_data = &fat_fs,

static const char *disk_mount_pt = "/SD:"; 

static int lsdir(const char *path)
	int res;
	struct fs_dir_t dirp;
	static struct fs_dirent entry;                                

	/* Verify fs_opendir() */
	res = fs_opendir(&dirp, path);
	if (res) {
		printk("Error opening dir %s [%d]\n", path, res);
		return res;

	printk("\nListing dir %s ...\n", path);
	for (;;) {
		/* Verify fs_readdir() */
		res = fs_readdir(&dirp, &entry);

		/*[0] == 0 means end-of-dir */
		if (res ||[0] == 0) {

		if (entry.type == FS_DIR_ENTRY_DIR) {
			printk("[DIR ] %s\n",;
		} else {
			printk("[FILE] %s (size = %zu)\n",, entry.size);

	/* Verify fs_closedir() */

	return res;

int FSInit(void)
    int res;
	static const char *disk_pdrv = "SD";
	uint64_t memory_size_mb;
	uint32_t block_count;
	uint32_t block_size;

	printk("Filesystem init...\n");
	if (disk_access_init(disk_pdrv) != 0) {
		LOG_ERR("Storage init ERROR!");
		return 0;

	if (disk_access_ioctl(disk_pdrv,
			DISK_IOCTL_GET_SECTOR_COUNT, &block_count)) {
		LOG_ERR("Unable to get sector count");
		return 0;
	LOG_INF("Block count %u", block_count);

	if (disk_access_ioctl(disk_pdrv,
			DISK_IOCTL_GET_SECTOR_SIZE, &block_size)) {
		LOG_ERR("Unable to get sector size");
		return 0;
	printk("Sector size %u\n", block_size);

	memory_size_mb = (uint64_t)block_count * block_size;
	printk("Memory Size(MB) %u\n", (uint32_t)(memory_size_mb >> 20));
	mp.mnt_point = disk_mount_pt;
	res = fs_mount(&mp);
	if (res == FR_OK) {
		printk("Disk mounted.\n");
	} else {
		printk("Error mounting disk.\n");
		return 0;
    return 1;

static int SDWriteSpeedTest(void)
	int res;
	for (int32_t i = 0; i < BUF_SIZE; i++)
		//Gernate random number into buffer
		writebuff[i] = rand() % 32768;
	LOG_INF("Opening file path");
	res = fs_open(&filep, "/SD:/test.bin", FS_O_CREATE | 
	if (res) {
		LOG_ERR("Error opening file [%d]", res);
		return 0;
	LOG_INF("Done opening file path");
	if (fs_write(&filep, writebuff, BUF_SIZE*sizeof(int16_t)) 
					!= BUF_SIZE*sizeof(int16_t))
		LOG_ERR("failed to wirte buff");
		return 0;
	LOG_INF("Done writing");
	return 1;

void main(void)
	int res;
	/* raw disk i/o */
	res = FSInit();
	if (!res)
		LOG_ERR("failed to init filesystem");

	res = SDWriteSpeedTest();
	if (!res)
		LOG_ERR("failed to write");

Hers is the result from the terminal:

[00:00:09.860,168] <inf> sdhc_spi: Found a 29664 MiB SDHC card.
[00:00:09.861,450] <inf> sdhc_spi: Manufacturer ID=39 OEM=‘PH’ Name=‘SD32G’ Revision=0×60 Serial=0xda44af6e
[00:00:09.861,480] <inf> main: Block count 60751872
[00:00:09.891,082] <inf> sdhc_spi: Found a 29664 MiB SDHC card.
[00:00:09.892,364] <inf> sdhc_spi: Manufacturer ID=39 OEM=‘PH’ Name=‘SD32G’ Revision=0×60 Serial=0xda44af6e
[00:00:10.551,940] <inf> main: Opening file path
[00:00:10.573,150] <inf> main: Done opening file path
[00:00:11.652,130] <inf> main: Done writing

In this way, speed is 32256 Bytes / (11.652130 - 10.573150) sec ~= 29.19KB/s