DESK-MX6-L/Pheripherals/GPIOs

History
Version	Issue Date	Notes
1.0.0	Oct 2020	First DESK release

Peripheral GPIOs

i.MX6 can handle external pins in many different ways and most of them can be configured as GPIOs. When a pin is set as a GPIO, it is possible to read its value, change its direction or change output value directly from the shell.

Device tree configuration

Here below an example of device tree configuration for using the RS-485 GPIOs used on standard DAVE's kit for the AXEL Lite SOM:

From imx6q-sbcx-cb0012.dts:

    leds {
        compatible = "gpio-leds";
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_rs232_485_422_1>;

        rs232_485_422 {
            gpios = <&gpio7 13 GPIO_ACTIVE_HIGH>;
            linux,default-trigger = "default-on";
        };

        rs232_on {
            gpios = <&gpio5 29 GPIO_ACTIVE_LOW>;
            linux,default-trigger = "default-on";
        };
    };
};

From mx6qdl-sbcx-revb-common.dtsi:

    rs232_485_422 {
        pinctrl_rs232_485_422_1: pinctrl_rs232_485_422_grp-1 {
            fsl,pins = <
                MX6QDL_PAD_GPIO_18__GPIO7_IO13      0x1b0b1 /* UART5_485/232 */
                MX6QDL_PAD_CSI0_DAT11__GPIO5_IO29   0x1b0b1 /* UART5_ON */
            >;
        };
    };

Accessing the peripheral

Linux messages at boot time

With the previous leds configuration for the GPIOs, it is possible to find them on sysfs under /sys/class/leds sub-directory:

root@desk-mx6:~# ls -la /sys/class/leds/
total 0
drwxr-xr-x  2 root root 0 Apr  2  2020 .
drwxr-xr-x 54 root root 0 Apr  2  2020 ..
lrwxrwxrwx  1 root root 0 Apr  2  2020 mmc0:: -> ../../devices/soc0/soc/2100000.aips-bus/2190000.usdhc/leds/mmc0::
lrwxrwxrwx  1 root root 0 Apr  2  2020 mmc1:: -> ../../devices/soc0/soc/2100000.aips-bus/2194000.usdhc/leds/mmc1::
lrwxrwxrwx  1 root root 0 Apr  2  2020 rs232_485_422 -> ../../devices/soc0/leds/leds/rs232_485_422
lrwxrwxrwx  1 root root 0 Apr  2  2020 rs232_on -> ../../devices/soc0/leds/leds/rs232_on
root@desk-mx6:~#

Usage with sysfs

• set EIM_D26 (MX6QDL_PAD_EIM_D26__GPIO3_IO26) as output GPIO
  • GPIO3_IO26 => (n-1)*32 + IO = (3-1)*32+26 = 90

root@desk-mx6:~# echo 90 > /sys/class/gpio/export
root@desk-mx6:~# echo out > /sys/class/gpio/gpio90/direction
root@desk-mx6:~# echo 1 > /sys/class/gpio/gpio90/value
root@desk-mx6:~#

• set EIM_D27 (MX6QDL_PAD_EIM_D27__GPIO3_IO27) as input GPIO

root@desk-mx6:~# echo 91 > /sys/class/gpio/export
root@desk-mx6:~# echo in > /sys/class/gpio/gpio91/direction
root@desk-mx6:~# cat /sys/class/gpio/gpio91/value
1
root@desk-mx6:~#

Additional information

Information about GPIOs usage under sysfs directory https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

sysfs GPIO ABI has been deprecated. See more inofrmation here about it. A character device access has to be used, more information here

Information about GPIOs library libgpiod - C library and tools - can be found on git.kernel.org

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