Storage & SD Card

The ESP32-P4-WIFI6-DEV-KIT provides two storage options beyond the main firmware flash: a high-speed microSD card slot and 16 MB of onboard NOR flash. This guide covers setting up both for file I/O in your applications.

Storage overview

Storage	Interface	Capacity	Speed	Use case
microSD card	SDIO 3.0 (4-bit)	Up to 128 GB+	Up to ~50 MB/s	Large files, logs, media, OTA images
Onboard NOR flash	QSPI	16 MB	~40 MB/s read	Configuration, small assets, SPIFFS/LittleFS

microSD card

Supported cards

The SDIO 3.0 interface supports:

SDHC: 4 GB — 32 GB (FAT32)
SDXC: 64 GB — 2 TB (exFAT, with appropriate filesystem support)
Speed class: Class 10 or UHS-I recommended for sustained write performance

Hardware setup

Power off the board.
Insert a microSD card into the TF card slot on the board. The slot uses a push-push mechanism — press to insert, press again to eject.
Power the board back on.

SDMMC setup (ESP-IDF)

Include the SDMMC and VFS headers in your application:

#include "esp_vfs_fat.h"
#include "sdmmc_cmd.h"
#include "driver/sdmmc_host.h"

Mount the SD card using the SDMMC host driver:

#define MOUNT_POINT "/sdcard"

esp_vfs_fat_sdmmc_mount_config_t mount_config = {
    .format_if_mount_failed = false,
    .max_files = 5,
    .allocation_unit_size = 16 * 1024,
};

sdmmc_card_t *card;
sdmmc_host_t host = SDMMC_HOST_DEFAULT();
host.max_freq_khz = SDMMC_FREQ_HIGHSPEED;  // 40 MHz

sdmmc_slot_config_t slot_config = SDMMC_SLOT_CONFIG_DEFAULT();
slot_config.width = 4;  // 4-bit SDIO mode

esp_err_t ret = esp_vfs_fat_sdmmc_mount(
    MOUNT_POINT, &host, &slot_config, &mount_config, &card);

if (ret == ESP_OK) {
    sdmmc_card_print_info(stdout, card);
}

Once mounted, use standard C file I/O:

FILE *f = fopen("/sdcard/data.txt", "w");
if (f) {
    fprintf(f, "Sensor reading: %d\n", sensor_value);
    fclose(f);
}

f = fopen("/sdcard/data.txt", "r");
if (f) {
    char line[64];
    fgets(line, sizeof(line), f);
    printf("Read: %s", line);
    fclose(f);
}

Unmount when done (important before power-off to prevent corruption):
```
esp_vfs_fat_sdcard_unmount(MOUNT_POINT, card);
```

Onboard NOR flash (16 MB QSPI)

The board includes 16 MB of NOR flash connected via QSPI. Part of this flash is used for firmware storage. The remaining space can be partitioned for application data.

Flash partition layout

ESP-IDF uses a partition table to divide the flash into regions. A typical layout:

Partition	Type	Size	Purpose
`nvs`	data	24 KB	Non-volatile storage (WiFi credentials, settings)
`phy_init`	data	4 KB	PHY calibration data
`factory`	app	4 MB	Main application firmware
`storage`	data	~10 MB	Application data (SPIFFS, LittleFS, or FATFS)
`ota_0` / `ota_1`	app	variable	OTA update slots (if OTA is enabled)

Using SPIFFS for flash storage

SPIFFS (SPI Flash File System) is a lightweight filesystem designed for NOR flash:

#include "esp_spiffs.h"

esp_vfs_spiffs_conf_t conf = {
    .base_path = "/storage",
    .partition_label = "storage",
    .max_files = 5,
    .format_if_mount_failed = true,
};

esp_err_t ret = esp_vfs_spiffs_register(&conf);
if (ret == ESP_OK) {
    FILE *f = fopen("/storage/config.json", "w");
    fprintf(f, "{\"brightness\": 80}");
    fclose(f);
}

Using LittleFS (alternative)

LittleFS handles power-loss resilience better than SPIFFS and is recommended for applications where data integrity is critical:

idf.py add-dependency "joltwallet/littlefs"

#include "esp_littlefs.h"

esp_vfs_littlefs_conf_t conf = {
    .base_path = "/storage",
    .partition_label = "storage",
    .format_if_mount_failed = true,
};

esp_vfs_littlefs_register(&conf);

Choosing between SD card and flash

Factor	microSD card	Onboard NOR flash
Capacity	4 GB — 128+ GB	~10 MB usable
Removable	Yes	No
Speed (write)	~10—50 MB/s	~1—3 MB/s
Wear leveling	Handled by card controller	Must use wear-aware FS (LittleFS)
Power-loss safety	Risk of corruption without unmount	LittleFS handles gracefully
Best for	Logs, media, large datasets	Configuration, small assets, OTA

Troubleshooting

Symptom	Cause	Fix
SD card mount fails	Card not formatted as FAT32	Reformat the card (use SD Card Formatter tool)
“No card detected”	Card not fully inserted	Reseat the card in the slot
Slow write speeds	Using 1-bit SDIO mode	Set `slot_config.width = 4` for 4-bit mode
Flash partition not found	Partition label mismatch	Verify the label matches your `partitions.csv`
SPIFFS corruption after power loss	SPIFFS limitation	Migrate to LittleFS for better resilience