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User Accessible GPIOs (J12)

Helios4 provides 12 GPIOs on header J12 which can be used for user application. Those GPIOs are provided via an 16-bit IO Expander PCA9655E connected to I2C bus 0.

J12 Pinout

Pinout Table

Pin Port Remarks
1 - 3.3V supply
2 IO0_2
3 IO0_3
4 IO0_4
5 IO0_7
6 IO1_0
7 IO1_1
8 IO1_2
9 IO1_3
10 IO1_4
11 IO1_5
12 IO1_6
13 IO1_7
14 - GND


Ports IO0_0, IO0_1, IO0_5, and IO0_6 are reserved for system use.


It is not advisable to access the I2C IO Expander directly using I2C utilities.

Accessing GPIOs under Linux

If the kernel supports debugfs (CONFIG_DEBUG_FS=y), list of GPIOs can be retrieved with the following command

sudo cat /sys/kernel/debug/gpio

Look for the gpiochip2: GPIOs XXX-YYY section, whereas XXX is first GPIO number and YYY is last GPIO number of IO expander.

gpiochip2: GPIOs 496-511, parent: i2c/0-0020, pca9555, can sleep:
 gpio-496 (                    |board-rev-0         ) in  lo    
 gpio-497 (                    |board-rev-1         ) in  lo    
 gpio-498 (                    |(null)              ) out hi    
 gpio-499 (                    |(null)              ) in  hi    
 gpio-500 (                    |(null)              ) in  hi    
 gpio-501 (                    |usb-overcurrent-stat) in  hi    
 gpio-502 (                    |USB-PWR             ) out hi    
 gpio-503 (                    |(null)              ) in  hi    
 gpio-504 (                    |(null)              ) in  hi    
 gpio-505 (                    |(null)              ) in  hi    
 gpio-506 (                    |(null)              ) in  hi    
 gpio-507 (                    |(null)              ) in  hi    
 gpio-508 (                    |(null)              ) in  hi    
 gpio-509 (                    |(null)              ) in  hi    
 gpio-510 (                    |(null)              ) in  hi    
 gpio-511 (                    |(null)              ) in  hi    

Another way to get first GPIO number of the IO expander

cat /sys/bus/i2c/devices/0-0020/gpio/gpiochip*/base

Therefore the mapping between header J12 Pins and Sysfs GPIO numbers will be as described in the following table

GPIO Table

Pin Sysfs GPIO number Remarks
1 - 3.3V supply
2 498
3 499
4 500
5 503
6 504
7 505
8 506
9 507
10 508
11 509
12 510
13 511
14 - GND


The mapping table is unlikely to change between Kernel version.

GPIO Control

1. Export the GPIO number you want to use

echo N | sudo tee -a /sys/class/gpio/export

2. Set the direction, "out" for Output or "in" for Input

echo DIRECTION | sudo tee -a /sys/class/gpio/gpioN/direction

3. Now you can read or change the GPIO value

To read GPIO value

cat /sys/class/gpio/gpioN/value

To change GPIO value (only if GPIO set as Output)

echo VALUE | sudo tee -a /sys/class/gpio/gpioN/value


Pay attention to the path, /sys/class/gpio/gpioN/ where N is the GPIO number.


Set IO1_7 (pin 13) output as high

echo 511 | sudo tee -a /sys/class/gpio/export
echo "out" | sudo tee -a /sys/class/gpio/gpio511/direction
echo 1 | sudo tee -a /sys/class/gpio/gpio511/value

Use GPIO with Device Tree Overlay


Device Tree Compiler (dtc) from OS package manager usually is too old, use the one from kernel source or download binary version for Arm here.

Another way to use the GPIO is by using device tree. In device tree the user accessible GPIO is labelled as expander0.

Instead of directly modifying the Helios4 device tree source (armada-388-helios4.dts) and recompiling, Linux and U-Boot provide a mechanism called device tree overlay. With overlay, user just needs to create simple device tree that would be overlay'd on top of base device tree.

For example, to use IO0_2 as power off button input, create following device tree source and save it as power-button.dts


/ {
    fragment@0 {
        target-path = "/gpio-keys";
        __overlay__ {
            power-button {
                label = "Soft Power Off";
                linux,code = <116>;
                gpios = <&expander0 2 1>;

Download dtc and compile device tree with this command

chmod 755 dtc
./dtc -I dts -O dtb -o power-button.dtbo power-button.dts

Button Wiring

button wiring

The GPIO has internal pull up resistor, when the button is not pressed the input read as High and when the button is pressed the input read as Low, therefore we use active low flag.

In the above example you will find the 2 following lines

linux,code = <116>;
gpios = <&expander0 2 1>;

For linux,code property, you can use one of the following values. For complete even code list refer to input-event-codes.h.

Event Code Name Event Code Description
KEY_POWER 116 Power Button
KEY_SLEEP 142 Sleep Button
KEY_WAKEUP 116 Power Button
BTN_0 0x100 User Button 0
BTN_1 0x101 User Button 1
BTN_2 0x102 User Button 2
BTN_3 0x103 User Button 3
BTN_4 0x104 User Button 4
BTN_5 0x105 User Button 5
BTN_6 0x106 User Button 6
BTN_7 0x107 User Button 7
BTN_8 0x108 User Button 8
BTN_9 0x109 User Button 9

For gpios properties, the syntax is as follow

<&expander0 index flag>

Where index is one of the following values

Port Number Index
IO0_2 2
IO0_3 3
IO0_4 4
IO0_7 7
IO1_0 8
IO1_1 9
IO1_2 10
IO1_3 11
IO1_4 12
IO1_5 13
IO1_6 14
IO1_7 15

And flag is one of the following values

Flag Property
0 GPIO line is active high
1 GPIO line is active low

For more info please refer to gpio-keys binding.

Device Tree Overlay under Armbian


Armbian older than version 5.98 is still not compiled with overlay support. Refer to instruction to Compile Helios4 DTB with Symbol Support or use precompiled binary.

Armbian Default (Stretch, Linux Kernel 4.14):


sudo cp lk4.14_armada-388-helios4.dtb /boot/dtb/armada-388-helios4.dtb

Armbian Next (Buster, Linux Kernel 4.19):


sudo cp lk4.19_armada-388-helios4.dtb /boot/dtb/armada-388-helios4.dtb

Create /boot/overlay-user/ to store the overlay and copy the overlay to the folder

sudo mkdir -p /boot/overlay-user
sudo cp power-button.dtbo /boot/overlay-user/

Then edit /boot/armbianEnv.txt and append the overlay filename (without dtbo extension) to user_overlays


Reboot the system to load the overlay.


If there is more than one overlay file, separate it by space. For example

user_overlays=power-button sleep-button

Additional Steps for U-Boot 2018.11 (Armbian Default)

Bootscript (boot.scr) used in Armbian Default does not have routine to automatically load overlay from /boot/overlay-user therefore /boot/boot.cmd need to be modified.

Append the following block

fdt addr ${fdt_addr}
fdt resize 65536

for overlay_file in ${user_overlays}; do
    if load ${boot_interface} 0:1 ${loadaddr} ${prefix}overlay-user/${overlay_file}.dtbo; then
        echo "Applying user provided DT overlay ${overlay_file}.dtbo"
        fdt apply ${loadaddr} || setenv overlay_error "true"

if test "${overlay_error}" = "true"; then
    echo "Error applying DT overlays, restoring original DT"
    load ${boot_interface} 0:1 ${fdt_addr} ${prefix}dtb/${fdtfile}


bootz ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr}

so it become

fdt addr ${fdt_addr}
fdt resize 65536

for overlay_file in ${user_overlays}; do
    if load ${boot_interface} 0:1 ${loadaddr} ${prefix}overlay-user/${overlay_file}.dtbo; then
        echo "Applying user provided DT overlay ${overlay_file}.dtbo"
        fdt apply ${loadaddr} || setenv overlay_error "true"

if test "${overlay_error}" = "true"; then
    echo "Error applying DT overlays, restoring original DT"
    load ${boot_interface} 0:1 ${fdt_addr} ${prefix}dtb/${fdtfile}

bootz ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr}

Recompile with

mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr

Device Tree Overlay under Other Distro

Compile Helios4 DTB with Symbol Support

Download Linux Kernel source code and extract it to ~/src/linux. Change directory to ~/src/linux

Download and apply kernel patch for

  • Linux Kernel 4.14
git apply --apply compile-dtb-lk-4.14.patch
  • Linux Kernel 4.19
git apply --apply compile-dtb-lk-4.19.patch

Compile Helios4 device tree

make armada-388-helios4.dtb

Copy the dtb to boot folder (eg. /boot/dtb/)

sudo cp arch/arm/boot/dts/armada-388-helios4.dtb /boot/dtb/

Copy the overlay also to the same folder.

Apply overlay on U-Boot

To apply overlay to base dtb, the procedure is

  1. Load Helios4 dtb to memory
  2. Set fdt address to dtb address
  3. Resize the fdt
  4. Load overlay to memory
  5. Apply from overlay address
  6. Boot the kernel

Example command

load mmc 0:1 ${ramdisk_addr_r} /boot/uInitrd
load mmc 0:1 ${kernel_addr_r} /boot/zImage

load mmc 0:1 ${fdt_addr_r} /boot/dtb/${fdtfile}
fdt addr ${fdt_addr_r}
fdt resize 65536
load mmc 0:1 0x300000 /boot/dtb/power-button.dtbo
fdt apply 0x300000

bootz ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr_r}
Updated on 2019-12-13