Accessing I2C Devices in Linux

The Linux kernel provides a device driver for the I2C controller of the i.MX RT1060, enabled in the kernel with the CONFIG_I2C_IMX_LPI2C build-time option. Another kernel configuration option that you will require is CONFIG_I2C_CHARDEV. This option enables the kernel API that allows accessing I2C devices from user-space application code. Both these kernel configuration options are enabled by default in the rootfs project.

The i.MX RT1060 includes four I2C bus controllers. The parameters of each bus controller (interrupt request numbers, DMA channels, etc.) are specified in the lpi2c* child nodes properties in the soc node in the projects/rootfs/rootfs.dts.IMXRT106X_NXPEVK file:

soc {
    aips0: aips-bus@40000000 {
        ...
        lpi2c1: lpci2c@403f0000 {
            compatible = "fsl,imx7ulp-lpi2c";
            reg = <0x403f0000 0x4000>;
            interrupts = <28>;
            clocks = <&lpi2c_clk>;
            status = "disabled";
        };
        ...
    };
};

The bus number assignment per I2C controller is made in the aliases node:

aliases {
    ...
    i2c0 = &lpi2c1;
    ...
};

Assignment of the I2C signals to specific pins is described in the pinctrl_lpi2c* child nodes of the iomuxc node:

&iomuxc {
    ...
    pinctrl_lpi2c1: lpi2cgrp {
        fsl,pins = <
            MXRT105X_PAD_AD_B1_00_LPI2C1_SCL        MXRT105X_PAD_CFG_I2C
            MXRT105X_PAD_AD_B1_01_LPI2C1_SDA        MXRT105X_PAD_CFG_I2C
        >;
    };
    ...
};

Finally the rootfs.dts.IMXRT106X_NXPEVK file defines the I2C interface reference, with additional properties which enable the appropriate I2C bus controller, connect it to the pins specified, and select the bus speed:

&lpi2c1 {
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_lpi2c1>;
    status = "okay";

    imx-ccm-init = "fsl,imx6sx-ccm";

    #address-cells = <1>;
    #size-cells = <0>;

    ...
};

The Emcraft uClinux distribution includes the Linux run-time tools that can be used to access I2C devices from user space. The following lines in the rootfs.initramfs file add these tools to the target file system:

file /usr/sbin/i2cdump ${INSTALL_ROOT}/A2F/root/usr/sbin/i2cdump 755 0 0
file /usr/sbin/i2cdetect ${INSTALL_ROOT}/A2F/root/usr/sbin/i2cdetect 755 0 0
file /usr/sbin/i2cget ${INSTALL_ROOT}/A2F/root/usr/sbin/i2cget 755 0 0
file /usr/sbin/i2cset ${INSTALL_ROOT}/A2F/root/usr/sbin/i2cset 755 0 0

The functionality described above is enabled by default in the rootfs project. So, just run the rootfs.uImage binary on your target and observe the kernel messages about the I2C bus being enabled:

i2c i2c-0: LPI2C adapter registered

Run the Linux I2C tools to examine I2C devices on your target. For instance, the following command scans the I2C1 bus interface and reports any devices it detects on the bus:

/ # i2cdetect -y 0 0 1 2 3 4 5 6 7 8 9 a b c d e f 00: -- -- -- -- -- -- -- -- -- -- -- -- -- 10: -- -- -- -- -- -- -- -- -- -- 1a -- -- -- -- 1f 20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 30: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- -- 40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 70: -- -- -- -- -- -- -- --

Refer to the documentation available from the Internet for detailed manuals on the Linux I2C tools, for instance: http://www.lm-sensors.org/wiki/i2cToolsDocumentation. Sometimes your I2C device is already supported in the Linux kernel, and you want to utilize it in some standard way. To provide such access to the I2C device you need:

Enable the appropriate I2C device driver in your Linux kernel configuration;
Add information about your I2C device into the appropriate i2c node reference in the rootfs.dts.IMXRT106X_NXPEVK file.

As an example you may see the NXP IMX RT1060 EVK board touchscreen support in the rootfs project:

CONFIG_TOUCHSCREEN_FT5X46 in rootfs.kernel.IMXRT106X_NXPEVK
ft node in rootfs.dts.IMXRT106X_NXPEVK

As another example let's connect an AT24 EEPROM with address 0x56 to the I2C1 bus, and provide user with a simple read/write interface to it:

Enable the EEPROM driver in the Linux kernel configuration (Device Drivers -> Misc devices -> EEPROM support -> I2C EEPROMs / RAMs / ROMs from most vendors):

[yur@ubuntu ~/projects/rootfs] $ make kmenuconfig

Add information about the AT24 EEPROM device into rootfs.dts.IMXRT106X_NXPEVK:

    &lpi2c1 {
        ...
        eeprom0: eeprom0@56 {
            compatible = "at,24c256";
            reg = <0x56>;
        };
        ...
    };

Build rootfs.uImage:

[yur@ubuntu ~/projects/rootfs] $ make

Run the resultant image on the target, and validate access to the EEPROM device via the standard interfaces defined by the kernel drivers:

/ # echo -e "Some text written to EEPROM\n" > \ /sys/bus/i2c/devices/0-0056/eeprom / # cat /sys/bus/i2c/devices/0-0056/eeprom Some text written to EEPROM

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