How to Control Blue Status LED

We can control the blue LED with editing the trigger mode in sysfs.

The red LED is hard-wired to the power input rail so that cannot be changed.

Simply you can turn on/off that with putting an option using echo command.

Do echo none to turn off.

root@odroid:~# echo none > /sys/class/leds/blue\:heartbeat/trigger

Do echo MODE to turn on with MODE.

# Turn on that solidly.
root@odroid:~# echo default-on > /sys/class/leds/blue\:heartbeat/trigger
 
# Heartbeat mode. This is the original state.
root@odroid:~# echo heartbeat > /sys/class/leds/blue\:heartbeat/trigger

There are many other modes you can select. But some of them may now work. This is an example for N2.

root@odroid:~# cat /sys/class/leds/blue\:heartbeat/trigger
none kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock timer oneshot [heartbeat] backlight gpio cpu0 cpu1 cpu2 cpu3 cpu4 cpu5 default-on transient panic rc_feedback emmc sd

netdev trigger will blink the blue led if there's network traffic.

• Requirement for N2: Ubuntu 18.04 kernel 4.9.230-76, Ubuntu 20.04 kernel 4.9.230-95
• Requirement for C4: Ubuntu 20.04 kernel 4.9.230-29
• Requirement for XU4: Kernel 5.4

Enable netdev trigger:

echo netdev > /sys/class/leds/blue\:heartbeat/trigger

Set the interface to be monitored:

echo eth0 > /sys/class/leds/blue\:heartbeat/device_name

Interval in mS for blinking, lower is faster blinking, higher is slower, minimum is around 40

echo 40 > /sys/class/leds/blue\:heartbeat/interval

Link: Should we monitor if the link is up or down? (Link = Physical cable).

1 we monitor link status, the led behaviour is: on when link is up, flashing when there's activity.
0 we don't monitor link status, the led behaviour is: always off, flashing when there's activity.

echo 0 > /sys/class/leds/blue\:heartbeat/link

RX/TX Blinking

Enable RX Blinking

echo 1 > /sys/class/leds/blue\:heartbeat/rx

Enable TX Blinking

echo 1 > /sys/class/leds/blue\:heartbeat/tx

The Red LED on the HC4 board is not hard-wired and you can turn it off by software.
The GPIO number of the red LED on the HC4 board is 503.

# Export GPIO
root@odroid:~# echo 503 > /sys/class/gpio/export
 
# To set as Output 
root@odroid:~# echo out > /sys/class/gpio/gpio503/direction
 
# Output as low.
root@odroid:~# echo 0 > /sys/class/gpio/gpio503/value