Kode Dot

microSD card

The card slot on a Kode Dot: 4-bit SDIO straight to the P4, a power switch that lives on the C5, and the code to mount a card and write a file.

What it is

A microSD slot on the edge of the board, for files: recordings, logs, images, anything bigger than the flash should hold. The card talks to the P4 over SDIO with four data lines, the fast interface, not the SPI fallback, and mounts as a FAT filesystem you read and write with ordinary fopen calls.

One thing about it is unlike any other board you have used: the card's power is switched by the C5, not by the P4. Until the C5 closes that switch the slot is unpowered, and a card in it is invisible. Kode's C5 firmware closes it at boot.

How it is connected

flowchart LR
p4["ESP32-P4"] -- "SDIO 4-bit, slot 0" --> sd["microSD"]
c5["ESP32-C5"] -. "IO26 SD_PWR, active low" .-> sw["P-MOSFET"] -. "SD1_VDD" .-> sd
sd -. "card detect" .-> c5

The slot is on the P4's SD/MMC slot 0, GPIO39 to GPIO44, with 47 kΩ pull-ups to the card's rail. That rail comes through a P-channel MOSFET whose gate is IO26 on the C5, pulled high so the card is off until the C5 drives it low. Card detect is IO25 on the C5. The P4's own SDIO pins live in an IO bank fed by its internal LDO channel 4, which the SD driver has to raise to 3.3 V.

Specifications

Gotchas

Apps that use it

On the marketplace, apps that put this part to work. Install one from start/first-app and see it before you write a line.

Example code

Power the card through the C5, mount it, write a line to a file and read it back. The log shows the line you wrote.

/*
 * sd_roundtrip.c - 2026-09-18 - from Kode with love
 *
 * Mounts the Kode Dot's microSD card, writes one line to a file and reads it
 * back. Asks the C5 to power the slot first, brings the SD/MMC host up with
 * the IO bank at 3.3 V, mounts FAT at /sdcard, and logs the line it wrote.
 * The C5 has to be running Kode's firmware for the power request to land.
 */
#include <stdio.h>
#include "driver/sdmmc_host.h"
#include "esp_hosted.h"
#include "esp_log.h"
#include "esp_vfs_fat.h"
#include "sd_pwr_ctrl_by_on_chip_ldo.h"
 
/* Power from the C5, host with the LDO, mount, write, read. */
void app_main(void)
{
    ESP_ERROR_CHECK(esp_hosted_init());
    ESP_ERROR_CHECK(esp_hosted_cp_gpio_set_level(26, 0));   /* card power on, active low */
 
    sdmmc_host_t host = SDMMC_HOST_DEFAULT();
    sd_pwr_ctrl_ldo_config_t ldo = { .ldo_chan_id = 4 };     /* the SDIO pins' IO bank */
    ESP_ERROR_CHECK(sd_pwr_ctrl_new_on_chip_ldo(&ldo, &host.pwr_ctrl_handle));
    sdmmc_slot_config_t slot = SDMMC_SLOT_CONFIG_DEFAULT();
    slot.width = 4;
    esp_vfs_fat_sdmmc_mount_config_t mount = { .max_files = 4 };
    sdmmc_card_t *card;
    ESP_ERROR_CHECK(esp_vfs_fat_sdmmc_mount("/sdcard", &host, &slot, &mount, &card));
 
    FILE *f = fopen("/sdcard/hello.txt", "w");
    fprintf(f, "hello from the Kode Dot\n");
    fclose(f);
 
    char line[64];
    f = fopen("/sdcard/hello.txt", "r");
    fgets(line, sizeof line, f);
    fclose(f);
    ESP_LOGI("sd", "read back: %s", line);
}

The calls are the ones 06-STORAGE/microsd runs on a board. TBD: a compile of this box; it needs the esp_hosted component, which is not in the kode component yet.

In the SDK

• kdrv_storage: the card in kode-sdk.