The MultiMediaCard (MMC)/Secure Digital (SD)/Secure Digital Input Output (SDIO) host driver implements a standard Linux driver interface to the ultra MMC/SD host controller (uSDHC). The host driver is part of the Linux kernel MMC framework.

The NXP i.MX6UL CPU has two uSDHC controllers.

On the ConnectCore 6UL system-on-module:

CAUTION! The ConnectCore 6UL system on module can only boot from USDHC1, so MMC can not be used for booting in wireless variant.

On the ConnectCore 6UL SBC Express:

On the ConnectCore 6UL SBC Pro:

Kernel configuration

You can manage the uSDHC driver support through the following kernel configuration option:

This option is enabled as built-in on the default ConnectCore 6UL kernel configuration file.

Platform driver mapping

The driver for the uSDHC is located at:




uSDHC driver

Device tree bindings and customization

The uSDHC device tree binding is documented at Documentation/devicetree/bindings/mmc/fsl-imx-esdhc.txt.

Common MMC device tree bindings are documented at Documentation/devicetree/bindings/mmc/mmc.txt.

The common i.MX6UL CPU device tree defines all the uSDHC ports. The platform device tree must:

If you are using the wireless variant of the ConnectCore 6UL SOM, do not modify the uSDHC1 because it is used in the imx6ul-ccimx6ul-wb.dtsi include file:

/* USDHC1 (Wireless) */
&usdhc1 {
    pinctrl-names = "default", "sleep";
    pinctrl-0 = <&pinctrl_usdhc1 &pinctrl_wifibt_ctrl>;
    pinctrl-1 = <&pinctrl_usdhc1_sleep &pinctrl_wifibt_ctrl_sleep>;
    bus-width = <4>;
    status = "okay";

If you are using the non-wireless variant, you may use usdhc1 interface on your carrier board.

Select microSD card or eMMC on ConnectCore 6UL SBC Pro

The ConnectCore 6UL SBC Pro carrier board connects uSDHC2 interface to a microSD card holder (using four data lines) and also to a 4 GB eMMC (using four or eight data lines). You cannot use both interfaces simultaneously since they are multiplexed with hardware controlled by a GPIO.

The default device tree enables the microSD card interface.

/* USDHC2 (microSD, conflicts with eMMC) */
&usdhc2 {
    pinctrl-assert-gpios = <&gpio5 1 GPIO_ACTIVE_LOW>;
    broken-cd;    /* no carrier detect line (use polling) */
    status = "okay";
/* USDHC2 (eMMC, conflicts with microSD) */
//&usdhc2 {
//    pinctrl-assert-gpios = <&gpio5 1 GPIO_ACTIVE_HIGH>;
//    non-removable;
//    /*
//     * Comment these two lines for 4-bit data bus or leave uncommented
//     * for 8-bit data bus
//     */
//    pinctrl-0 = <&pinctrl_usdhc2_8databits>;
//    bus-width = <8>;
//    status = "okay";

To enable the eMMC:

Note The interface is the same, but each block uses different configuration options.

User space usage

The MMC block driver handles the file system read/write calls and uses the low-level MMC host controller interface driver to send the commands to the uSDHC controller.

The MMC device driver exposes the device through the file system at /dev/mmcblkX where X is a number, starting at zero, that indicates the device index. The MMC device provided in the ConnectCore 6UL SBC Pro has four hardware partitions:

The MMC mapping will look like this:





If the block device is partitioned, the partitions will appear as /dev/mmcblkXpY where Y is a number, starting at one, that indicates the partition index.

By default, formatted partitions are auto-mounted upon detection if they are block devices. You can also mount a partition's file system using the mount command with the partition node, the file system type, and the mount point:

mkdir -p /media/mmcblk1p1 && mount -t vfat /dev/mmcblk1p1 /media/mmcblk1p1

Detect microSD card

The microSD card holder on the ConnectCore 6UL SBC Pro does not have a card detection line. However, the Linux driver supports card detection by polling for the presence of a card.

Formatted partitions are auto-mounted upon card insertion.

Note If the device (microSD or MMC) is not partitioned, you can used fdisk to create one partition of type vfat and then give it format with mkfs, for example:

echo -e 'o\nn\np\n1\n\n\nt\nb\nw\n' | fdisk /dev/mmcblk1 > /dev/null
mkfs.vfat /dev/mmcblk1p1

If the device is partitioned but you still want to re-partition or re-format it, you must first umount all the mounted partitions.