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SDMMC Host Driver

Overview

ESP32's SDMMC host peripheral has two slots:

  • Slot 0 (:cSDMMC_HOST_SLOT_0) is an 8-bit slot. It uses HS1_* signals in the PIN MUX.
  • Slot 1 (:cSDMMC_HOST_SLOT_1) is a 4-bit slot. It uses HS2_* signals in the PIN MUX.

Pin mappings of these slots are given in the table below.

Signal Slot 0 Slot 1
CMD GPIO11 GPIO15
CLK GPIO6 GPIO14
D0 GPIO7 GPIO2
D1 GPIO8 GPIO4
D2 GPIO9 GPIO12
D3 GPIO10 GPIO13
D4 GPIO16
D5 GPIO17
D6 GPIO5
D7 GPIO18
CD any input via GPIO matrix
WP any input via GPIO matrix

The Card Detect and Write Protect signals can be routed to arbitrary pins using the GPIO matrix. To reserve the pins, set the gpio_cd and gpio_wp members of the :cppsdmmc_slot_config_t structure before calling :cppsdmmc_host_init_slot. Please note that it is not advised to specify a Card Detect pin when working with SDIO cards, because the card detect signal in ESP32 can also trigger SDIO slave interrupt.

Warning

Pins used by Slot 0 (HS1_*) are also used to connect the SPI flash chip in ESP32-WROOM and ESP32-WROVER modules. These pins cannot be shared between an SD card and SPI flash. If you need to use Slot 0, connect SPI flash to different pins and set eFuses accordingly.

Supported Speed Modes

SDMMC Host driver supports the following speed modes:

  • Default Speed (20 MHz), 4-line/1-line (with SD cards), and 8-line (with 3.3 V eMMC)
  • High Speed (40 MHz), 4-line/1-line (with SD cards), and 8-line (with 3.3 V eMMC)
  • High Speed DDR (40 MHz), 4-line (with 3.3 V eMMC)

Speed modes not supported at present:

  • High Speed DDR mode, 8-line eMMC
  • UHS-I 1.8 V modes, 4-line SD cards

Using the SDMMC Host Driver

Of all the functions listed below, only the following ones will be used directly by most applications:

  • :cppsdmmc_host_init
  • :cppsdmmc_host_init_slot
  • :cppsdmmc_host_deinit

Other functions, such as the ones given below, will be called by the SD/MMC protocol layer via function pointers in the :cppsdmmc_host_t structure:

  • :cppsdmmc_host_set_bus_width
  • :cppsdmmc_host_set_card_clk
  • :cppsdmmc_host_do_transaction

Configuring Bus Width and Frequency

With the default initializers for :cppsdmmc_host_t and :cppsdmmc_slot_config_t (:cSDMMC_HOST_DEFAULT and :cSDMMC_SLOT_CONFIG_DEFAULT), SDMMC Host driver will attempt to use the widest bus supported by the card (4 lines for SD, 8 lines for eMMC) and the frequency of 20 MHz.

In the designs where communication at 40 MHz frequency can be achieved, it is possible to increase the bus frequency by changing the max_freq_khz field of :cppsdmmc_host_t:

sdmmc_host_t host = SDMMC_HOST_DEFAULT();
host.max_freq_khz = SDMMC_FREQ_HIGHSPEED;

To configure the bus width, set the width field of :cppsdmmc_slot_config_t. For example, to set 1-line mode:

sdmmc_slot_config_t slot = SDMMC_SLOT_CONFIG_DEFAULT();
slot.width = 1;

See also

See SD/SDIO/MMC Driver <../storage/sdmmc> for the higher level driver which implements the protocol layer.

See SD SPI Host Driver <sdspi_host> for a similar driver which uses the SPI controller and is limited to SD protocol's SPI mode.

See sd_pullup_requirements for pullup support and compatibilities of modules and development kits.

API Reference