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README.md
SD Card example
This example demonstrates how to use an SD card with ESP32. Example does the following steps:
- Use an "all-in-one"
esp_vfs_fat_sdmmc_mount
function to:- initialize SDMMC peripheral,
- probe and initialize the card connected to SD/MMC slot 1 (HS2_CMD, HS2_CLK, HS2_D0, HS2_D1, HS2_D2, HS2_D3 lines),
- mount FAT filesystem using FATFS library (and format card, if the filesystem can not be mounted),
- register FAT filesystem in VFS, enabling C standard library and POSIX functions to be used.
- Print information about the card, such as name, type, capacity, and maximum supported frequency.
- Create a file using
fopen
and write to it usingfprintf
. - Rename the file. Before renaming, check if destination file already exists using
stat
function, and remove it usingunlink
function. - Open renamed file for reading, read back the line, and print it to the terminal.
Note: despite the name, sdmmc
component doesn't support MMC/eMMC cards yet. It is also possible to extend sdmmc
component to support SPI mode with SD cards via SPI peripheral.
Hardware
To run this example, ESP32 development board needs to be connected to SD card as follows:
ESP32 pin | SD card pin | Notes |
---|---|---|
GPIO14 (MTMS) | CLK | 10k pullup |
GPIO15 (MTDO) | CMD | 10k pullup |
GPIO2 | D0 | 10k pullup, pull low to go into download mode |
GPIO4 | D1 | 10k pullup; not used in 1-line mode |
GPIO12 (MTDI) | D2 | otherwise 10k pullup (see note below!); not used in 1-line mode |
GPIO13 (MTCK) | D3 | 10k pullup needed at card side, even in 1-line mode |
N/C | CD | |
N/C | WP |
This example doesn't utilize card detect (CD) and write protect (WP) signals from SD card slot.
Note about GPIO2
GPIO2 pin is used as a bootstrapping pin, and should be low to enter UART download mode. One way to do this is to connect GPIO0 and GPIO2 using a jumper, and then the auto-reset circuit on most development boards will pull GPIO2 low along with GPIO2, when entering download mode.
Note about GPIO12
GPIO12 is used as a bootstrapping pin to select output voltage of an internal regulator which powers the flash chip.
- For boards which don't use the internal regulator, GPIO12 can be pulled high.
- On boards which use the internal regulator and a 3.3V flash chip, GPIO12 should be pulled up high, which is compatible with SD card operation.
- For boards which use 1.8V flash chip, GPIO12 needs to be low at reset.
- In this case, internal pullup can be enabled using a
gpio_pullup_en(GPIO_NUM_12);
call. Most SD cards work fine when an internal pullup on GPIO12 line is enabled. Note that if ESP32 experiences a power-on reset while the SD card is sending data, high level on GPIO12 can be latched into the bootstrapping register, and ESP32 will enter a boot loop until external reset with correct GPIO12 level is applied. - Another option is to program flash voltage selection efuses: set
SDIO_TIEH=0
andSDIO_FORCE=1
. This will permanently select 1.8v output voltage for the internal regulator, and GPIO12 will not be used as a bootstrapping pin anymore. Then it is safe to connect a pullup resistor to GPIO12. This option is suggested for production use.
- In this case, internal pullup can be enabled using a
4-line and 1-line modes
By default, example code uses the following initializer for SDMMC host peripheral configuration:
sdmmc_host_t host = SDMMC_HOST_DEFAULT();
Among other things, this sets host.flags
to SDMMC_HOST_FLAG_4BIT
, which means that SD/MMC driver will switch to 4-line mode when initializing the card (initial communication always happens in 1-line mode). If some of D1, D2, D3 pins are not connected to the card, set host.flags
to SDMMC_HOST_FLAG_1BIT
— then the SD/MMC driver will not attempt to switch to 4-line mode.
Note that even if D3 line is not connected to the ESP32, it still has to be pulled up at card side, otherwise the card will go into SPI protocol mode.
Example output
Here is an example console output. In this case a 128MB SDSC card was connected, and format_if_mount_failed
parameter was set to true
in the source code. Card was unformatted, so the initial mount has failed. Card was then partitioned, formatted, and mounted again.
I (1776) example: Initializing SD card
W (1856) vfs_fat_sdmmc: failed to mount card (13)
W (1856) vfs_fat_sdmmc: partitioning card
W (1856) vfs_fat_sdmmc: formatting card
W (2726) vfs_fat_sdmmc: mounting again
I (2736) example: Card info:
I (2736) example: Name: SU128
I (2736) example: Type: SDSC
I (2736) example: Capacity: 120 MB
I (2736) example: Max clock speed: 25 MHz
I (2736) example: Opening file
I (2756) example: File written
I (2756) example: Renaming file
I (2756) example: Reading file
I (2756) example: Read from file: 'Hello SU128!'
I (2756) example: Card unmounted