Troubles when adding a LED to Device Tree

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1

Hi. First of all, thanks in advance for those who try to help me.

I’m studying about Device Trees, and I’ve chosen to use Beaglebone Green Wireless as a board to practice.
I’ve done the following steps:

  1. From u-boot serial, I’ve seen that Beaglebone green Wireless is using uses am335x-bonegreen-wireless-uboot-univ.dtb as DTB file
  2. Cloned / downloaded beaglebone Kernel (from GitHub - beagleboard/linux: The official Read Only BeagleBoard and BeagleBone kernel repository https://git.beagleboard.org/beagleboard/linux)
  3. I want to add LED at GPIO 47 ( = GPIO1_16, and accordingly to board schematic, AM33xx GPMC_A0 pin). I’ve edited am335x-bone-common.dtsi file, adding led (led6) in leds node as follows:

`

leds {
pinctrl-names = “default”;
pinctrl-0 = <&user_leds_s0>;
compatible = “gpio-leds”;
led2 {
label = “beaglebone:green:usr0”;
gpios = <&gpio1 21 GPIO_ACTIVE_HIGH>;
linux,default-trigger = “heartbeat”;
default-state = “off”;
};
led3 {
label = “beaglebone:green:usr1”;
gpios = <&gpio1 22 GPIO_ACTIVE_HIGH>;
linux,default-trigger = “mmc0”;
default-state = “off”;
};
led4 {
label = “beaglebone:green:usr2”;
gpios = <&gpio1 23 GPIO_ACTIVE_HIGH>;
linux,default-trigger = “cpu0”;
default-state = “off”;
};
led5 {
label = “beaglebone:green:usr3”;
gpios = <&gpio1 24 GPIO_ACTIVE_HIGH>;
linux,default-trigger = “mmc1”;
default-state = “off”;
};

led6 {
label = “beaglebone:green:extLed”;
gpios = <&gpio1 16 GPIO_ACTIVE_HIGH>;
linux,default-trigger = “heartbeat”;
default-state = “off”;
};

};

`

And I’ve added gpio2_16 in pinmux of user leds, as follows:

`

&am33xx_pinmux {
user_leds_s0: user_leds_s0 {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_GPMC_A5, PIN_OUTPUT_PULLDOWN, MUX_MODE7) /* gpmc_a5.gpio1_21 /
AM33XX_PADCONF(AM335X_PIN_GPMC_A6, PIN_OUTPUT_PULLUP, MUX_MODE7) / gpmc_a6.gpio1_22 /
AM33XX_PADCONF(AM335X_PIN_GPMC_A7, PIN_OUTPUT_PULLDOWN, MUX_MODE7) / gpmc_a7.gpio1_23 /
AM33XX_PADCONF(AM335X_PIN_GPMC_A8, PIN_OUTPUT_PULLUP, MUX_MODE7) / gpmc_a8.gpio1_24 /
AM33XX_PADCONF(AM335X_PIN_GPMC_A0, PIN_OUTPUT_PULLUP, MUX_MODE7) / gpmc_a0.gpio1_16 */

;
};

`

  1. I’ve successfully compiled all dtbs by doing:

`
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- dtbs

`

  1. After that, I’ve updated the recently compiled am335x-bonegreen-wireless-uboot-univ.dtb file to /boot/dtbs/{uname-r} folder in Beaglebone Green

  2. And, finally, I rebooted the board. However, board has entered in a “strange state”, with all user leds powered on. Using U-Boot serial, I’ve noticed that boot process have stopped at “starting kernel” message

In this case, what have I done wrong? Are my modifications in DTS file right?

Again, thanks in advance.

2

Hi!

Forget about all that device tree trouble, use libpruio instead. You can configure the pin and set the desired output state by a single function call in your programm running at user space (no sudo commands, no rebooting, …)

gpio_setValue(Io, P9_15, 1) // configure and set high

gpio_setValue(Io, P9_15, 0) // set low

You’ll have faster boot, less memory consumption, and your code executes much faster (compared with sysfs GPIO control).

You may want to check out some examples.

Regards

3

Your steps look right, but there is a simpler way to do them. Check out:
https://elinux.org/EBC_Exercise_17_Switching_a_GPIO_to_an_LED

No need to cross compile etc.

–Mark

4

Hi TJF.

I’ve never heard about libpruio before. It seems to be very nice and fast, once it uses PRUs for writing and reading I/Os, per my understanding. Thanks for the suggestion.

Best Regards,
Pedro Bertoleti

5

Hi Mark. Great, thanks!

Per my understanding, the Device Tree approach of https://elinux.org/EBC_Exercise_17_Switching_a_GPIO_to_an_LED doesn’t include modifying user_leds_s0 (pinmux for user leds). Do you know why isn’t this needed?

Best Regards,
Pedro Bertoleti

6

Pedro:
I think I got lucky and the mux was already set to gpio.

–Mark