Blink an LED#
The “Hello World!” of the embedded world is to blink an LED. Here we’ll show you how to do just that in three simple steps.
Plug in the Beagle
Log into the Beagle
Blink the LED
These steps will work for any of the Beagles.
Plug in the Beagle#
For this step you need to get a USB cable and attach your Beagle to your host computer with it. Where you attached the cable depends on which Beagle you have. Click on the tab for your board.
Fig. 1 Tethered Configuration#
Fig. 2 Mini USB Connection to the Board as seen from the bottom.#
For more details see: Connecting Up Your BeagleBone Black
Fig. 3 Micro USB Connection to the Blue#
Fig. 4 Tethered Configuration#
Fig. 5 USB-c Connection to the Board#
Fig. 6 USB Connection to the PC/Laptop#
Fig. 7 Board Power LED#
For more details see: Quick Start Guide
Fig. 8 Tethering BeaglePlay to PC#
For more details see: Quick Start Guide
Fig. 9 Micro USB Connection#
For more details see: Connecting Up PocketBeagle
Once attached you will see some LEDs blinking. Wait a bit and the blinking will settle down to a steady heart beat.
The Beagle is now up and running, but you didn’t have to load up Linux. This is because all Beagles (except PocketBeagle, see Update board with latest software to install an image on the Pocket) have built-in flash memory that has the Debian distribution of Linux preinstalled.
Using VS Code#
Important
If VS code is not installed on your board please skip this section and refer to next section on how to login and run the code via command line.
Recent Beagles come with the IDE Visual Studio Code (https://code.visualstudio.com/) installed and
running. To access it, open a web browser on your host computer and browse to: 192.168.7.2:3000
(use 192.168.6.2:3000 for the Mac) and you will see something like:
At this point you can either run the scripts via a command line within VS Code, or
run them by clicking the RUN Code button.
Running via the command line#
Open a terminal window in VS Code by dropping down the Terminal menu and selecting New Terminal
(or entering Ctrl+`). The terminal window appears at the bottom of the screen as shown below.
You can now enter commands and see them run as shown below.
Running via the RUN button#
Use the file navigator on the left to navigate to examples/BeagleBone/Black/blinkInternalLED.sh and you will see:
This code blinks one of the USR LEDs built into the board. Click on the RUN Code triangle on the upper right of the
screen (see red arrow) to run the code. (You could also enter Ctrl+Alt+N) The USR3 LED should now be blinking.
Click on the Stop Code Run (Ctrl+Alt+M) square to the right of the Run Code button.
Time to play! Try changing the LED number (on line 10) from 3 to something else. Click the Run Code
button (no need to save the file, autosave is on by default).
Try running seqLEDs.py.
Using command line#
To access the command line and your host is a Mac, take the ssh (Mac) tab. If you
are running Linux on your host, take the ssh (Linux) tab. Finally take the
putty (Windows) tab for command line from Windows.
If you are running a Mac host, open a terminal widow and run
host:~$ ssh debian@192.168.6.2
Use the password temppwd.
If you are running a Linux host, open a terminal widow and run
host:~$ ssh debian@192.168.7.2
Use the password temppwd.
If you are running Window you need to run an ssh client
to connect to the Beagle. I suggest you use putty.
You can download it here: https://www.putty.org/.
Once installed, launch it and connect to your Beagle
by sshing to 192.168.7.2.
Login with user debian
and password temppwd.
Blink an LED#
Once logged in the rest is easy. First:
bone:~$ cd ~/examples/BeagleBone/Black
bone:~$ ls
README.md blinkInternalLED.sh blinkLED2.py input2.js
analogIn.py blinkLED.bs.js blinkLEDold.py seqLEDs.py
analogInCallback.js blinkLED.c fadeLED.js swipeLED.js
analogInContinuous.py blinkLED.js fadeLED.py
analogInOut.js blinkLED.py gpiod
analogInSync.js blinkLED.sh input.js
Here you see a list of many scripts that demo simple input/output on the Beagle. Try one that works on the internal LEDs.
bone:~$ cat blinkInternalLED.py
LED="3"
LEDPATH='/sys/class/leds/beaglebone:green:usr'
while true ; do
echo "1" > ${LEDPATH}${LED}/brightness
sleep 0.5
echo "0" > ${LEDPATH}${LED}/brightness
sleep 0.5
done
bone:~$ ./blinkInternalLED.py
^c
Here you see a simple bash script that turns an LED on and off. Enter Ctrl+c to stop the script.
Blinking via Python#
Here’s a script that sequences the LEDs on and off.
bone:~$ cat seqLEDs.py
import time
import os
LEDs=4
LEDPATH='/sys/class/leds/beaglebone:green:usr'
# Open a file for each LED
f = []
for i in range(LEDs):
f.append(open(LEDPATH+str(i)+"/brightness", "w"))
# Sequence
while True:
for i in range(LEDs):
f[i].seek(0)
f[i].write("1")
time.sleep(0.25)
for i in range(LEDs):
f[i].seek(0)
f[i].write("0")
time.sleep(0.25)
bone:~$ ./seqLEDs.py
^c
Again, hit Ctrl+c to stop the script.
Blinking from Command Line#
You can control the LEDs from the command line.
bone:~$ cd /sys/class/leds
bone:~$ ls
beaglebone:green:usr0 beaglebone:green:usr2 mmc0::
beaglebone:green:usr1 beaglebone:green:usr3 mmc1::
Here you see a list of LEDs. Your list may be slightly
different depending on which Beagle you are running.
You can blink any of them. Let’s try usr1.
bone:~$ cd beaglebone\:green\:usr1/
bone:~$ ls
brightness device max_brightness power subsystem trigger uevent
bone:~$ echo 1 > brightness
bone:~$ echo 0 > brightness
When you echo 1 into brightness the LED turns on.
Echoing a 0 turns it off.
Blinking other LEDs#
You can blink the other LEDs by changing in to thier directories and doing the same. Let’s blink the USR0 LED.
bone:~$ cd ../beaglebone\:green\:usr0/
bone:~$ echo 1 > brightness
bone:~$ echo 0 > brightness
Did you notice that LED usr0 blinks on it’s own in a
heartbeat pattern? You can set an LED trigger. Here’s
what triggers you can set:
bone:~$ cat trigger
none usb-gadget usb-host rfkill-any rfkill-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 disk-activity disk-read disk-write i
de-disk mtd nand-disk [heartbeat] backlight gpio c
pu cpu0 cpu1 cpu2 cpu3 activity default-on panic
netdev mmc0 mmc1 mmc2 phy0rx phy0tx phy0assoc phy0radio
rfkill0 gpio-0:00:link gpio-0:00:1Gbps gpio-0:00:100Mbps
gpio-0:00:10Mbps gpio-0:01:link gpio-0:01:10Mbps
bone:~$ echo none > trigger
Notice [heartbeat] is in brackets. This shows it’s the
current trigger. The echo changes the trigger to none.
Try experimenting with some of the other triggers and see if you can figure them out.
Another way to Blink an LED#
An interesting thing about Linux is there are often many ways
to do the same thing. For example, I can think of at least five ways to blink
an LED. Here’s another way using the gpiod system.
First see where the LEDs are attached.
bone:~$ gpioinfo | grep -e chip -ie usr
gpiochip0 - 32 lines:
gpiochip1 - 32 lines:
line 21: "[usr0 led]" "beaglebone:green:usr0" output active-high [used]
line 22: "[usr1 led]" "beaglebone:green:usr1" output active-high [used]
line 23: "[usr2 led]" "beaglebone:green:usr2" output active-high [used]
line 24: "[usr3 led]" "beaglebone:green:usr3" output active-high [used]
gpiochip2 - 32 lines:
gpiochip3 - 32 lines:
Here we asked how the LEDs are attached to the General Purpose
IO (gpio) system. The answer is, (yours will be different for a
different Beagle)
there are four interface chips and the LEDs are attached to
chip 1. You can control the gpios (and thus the LEDs) using
the gpioset command.
bone:~$ gpioset --mode=time --sec=2 1 22=1
bone:~$ gpioset --mode=time --sec=2 1 22=0
The first command sets chip 1, line 22 (the usr1 LED) to 1 (on) for 2 seconds. The second command turns it off for 2 seconds.
Try it for the other LEDs.
Note
This may not work on all Beagles since it depends on which version of Debian you are running.