STM32 SD card with FatFs

Micro-SD Adaptor with jumper leads soldered on

In this article we will be interfacing a SD-card using STM32.

For this purpose we have modified a SD to Micro-SD adaptor by soldering 9 jumper leads on it (see photo on the right).

We'll be using two different SD cards throughout these exercises:

SD-Card Pinout

The pinout of a SD-card and a MicroSD-card are as follows:

It is important to notice that all SD-cards can be interfaced in two different ways:

SDIO
SPI

DIY SD-card Adaptor

A quick way to hook up a SD-card to a STM32 is to use one of those SD to Micro-SD adaptors that come with most Micro-SD's when you buy them. I made one looking like this:

Combining that with the pinout from the previous section, we can make the following table:

Pin	Color	SDIO	SPI
1	White	CD/Dat3	CS
2	Grey	CMD	DI
3	Purple	VSS1	VSS1
4	Blue	VDD	VDD
5	Green	CLK	SCLK
6	Yellow	VSS2	VSS2
7	Orange	DAT0	DO
8	Red	DAT1
9	Black	DAT2

As per the SD-card specification in both the case of SPI and SDIO, all signals except the clock should be pulled up. Our DIY adaptor does not include pull-up resistors, so we will have to rely on the internal ones, which are really quite weak but as long as the leads are not too long it should work ok.

SDIO

Some MCUs - for example the STM32F405 - includes a SDIO peripheral which can be used to drive a SD-card directly. Schematics will look like this:

Notice the pull-up resistors on all lines except the CLK.

On a PCB that could look like this:

The SDIO can be configured using STM32CubeMX like this:

1-bit

4-bit

For some reason that I am not entirely clear about, SD cards must be initialized with 1-bit mode only and then switched to 4-bit. The easiest way to do this is to configure it as 4-bit in STM32CubeMX and then modify the 'MX_SDIO_SD_Init' function like this:

/**
 * @brief SDIO Initialization Function
 * @param None
 * @retval None
 */
static void MX_SDIO_SD_Init(void)
{

    /* USER CODE BEGIN SDIO_Init 0 */

    /* USER CODE END SDIO_Init 0 */

    /* USER CODE BEGIN SDIO_Init 1 */

    /* USER CODE END SDIO_Init 1 */
    hsd.Instance = SDIO;
    hsd.Init.ClockEdge = SDIO_CLOCK_EDGE_RISING;
    hsd.Init.ClockBypass = SDIO_CLOCK_BYPASS_ENABLE;
    hsd.Init.ClockPowerSave = SDIO_CLOCK_POWER_SAVE_DISABLE;
    hsd.Init.BusWide = SDIO_BUS_WIDE_4B;
    hsd.Init.HardwareFlowControl = SDIO_HARDWARE_FLOW_CONTROL_DISABLE;
    hsd.Init.ClockDiv = 0;
    /* USER CODE BEGIN SDIO_Init 2 */

    // First init with 1B bus - SD card will not initialize with 4 bits
    hsd.Init.BusWide = SDIO_BUS_WIDE_1B;
    if (HAL_SD_Init(&hsd) != HAL_OK) {
        Error_Handler();
    }

    // Now we can switch to 4 bit mode
    if (HAL_SD_ConfigWideBusOperation(&hsd, SDIO_BUS_WIDE_4B) != HAL_OK) {
        Error_Handler();
    }

    /* USER CODE END SDIO_Init 2 */

}

SPI

NOTICE! While it is supported by most, some SD-cards do not support SPI access. If this fails for you, try with another card. Older cards are more likely to work.