DESK-MX6-L/Pheripherals/GPIOs

History
ID#	Issue Date	Notes
14083	16/07/2021	First DESK release
17005	03/03/2022	DESK 3.0.0 release
17064	04/10/2022	Add libgpiod example

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:~#

Userspace gpio access

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

Usage with libgpiod

sysfs GPIO ABI has been deprecated. See more information 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

libgpiod example

It is possible yo cross-compile libgpiod in the target (if the related Yocto package is not available for installation). Once the autoconf-archive package has been installed

dnf install autoconf-archive

it is possible to clone the libgpiod git repository and compile the related sources:

git clone https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git
cd libgpiod 
./autogen.sh --enable-tools=yes --prefix=/usr/local 
make
make install

Once installed, the gpio utilities are availabel in the target:

root@desk-mx6:~# gpio
gpiodetect  gpiofind    gpioget     gpioinfo    gpiomon     gpioset
root@desk-mx6:~# gpio

gpio interrupt example

If a push button, with proper pull-up, is connected to a GPIO line available - for example using the J33.34 pin available on SBC AXEL Lite WIDE connector - the related GPIO can be used as an interrupt:

• J33.40 is connected to J10.40 SOM pin
  • J33 connector is the WIDE connector in the AXEL Lite EVK
  • J10 connector is the SO-DIMM connector in the AXEL Lite EVK
  • ALT-5 alternate function for this pin is GPIO1_IO06
• the corresponding pin number is 0 6
• gpiomon utility can be used for monitor the pin status and triggering, for example, the falling-edge event:

root@desk-mx6:~# gpiomon --falling-edge 0 6
event: FALLING EDGE offset: 6 timestamp: [     696.463539802]
event: FALLING EDGE offset: 6 timestamp: [     696.643661944]
^Croot@desk-mx6:~#

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