EBC Exercise 10 Flashing an LED

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thumb‎ Embedded Linux Class by Mark A. Yoder

The "Hello World" program is the traditional first program for many classes. Flashing an LED is the embedded equivalent. Here we will explore a few ways to flash and LED on the Beagle and explore General Purpose I/O (gpio) along the way. This will call be done from the command line of the Beagle, so there is no need for the host computer.

gpio via the Shell Command Line and sysfs

(Note: Draw the new diagrams with this.)

The easiest way to do general purpose I/O (gpio) on the Beagle is through a terminal window and a shell prompt. In Linux most everything is treated as a file. Even things that aren't files. In our class we'll use a virtual file system called sysfs. sysfs exposes the drivers for the hardware so you get easily use them.

Try this, open a terminal and type:

beagle$ cd /sys
beagle$ ls -F
block/  bus/  class/  dev/  devices/  firmware/  fs/  kernel/  module/  power/

Here we see several directories that represent hardware we can control. Explore a bit and see what you find.

Now try:

beagle$ cd /sys/class
beagle$ ls -F
bccat/      hwmon/        mtd/             scsi_disk/     usb_device/
bdi/        i2c-adapter/  net/             scsi_generic/  usbmon/
block/      i2c-dev/      omap-previewer/  scsi_host/     vc/
bluetooth/  input/        omap-resizer/    sound/         video4linux/
bsg/        leds/         pvr/             spi_master/    vtconsole/
display/    mdio_bus/     regulator/       spidev/
firmware/   mem/          rfkill/          thermal/
gpio/       misc/         rtc/             tty/
graphics/   mmc_host/     scsi_device/     ubi/

Explore some. What do you find? In graphics you will see the 3 frame buffers supported by the processor. In sound you'll see the alsa sound devices.

Flashing the user LEDs

The Beagle xM has two user LEDS, user0 and user1, that you can control. The Bone has 4. Try this:

beagle$ cd /sys/class/leds
beagle$ ls -F
beaglebeagleboard::pmu_stat@  beagleboard::usr0@  beagleboard::usr1@

Here you see the directories for controlling each of the usr LEDs. By default, usr0 flashes a heartbeat pattern and usr1 flashes when the micro SD card is accessed. Let's control usr0.

beagle$ cd beagleboard\:\:usr0
beagle$ ls -F
brightness  device@  max_brightness  power/  subsystem@  trigger  uevent

See what's in brightness, max_brightness and trigger by using the cat command. For example:

beagle$ cat trigger
none nand-disk mmc0 [heartbeat]

This shows trigger can have 4 values. The present value is heartbeat. Check the LED, is the beating? You can stop the heartbeat via:

beagle$ echo none > trigger
beagle$ cat trigger
[none] nand-disk mmc0 heartbeat 

Did it stop beating? You can now turn it on and off with:

beagle$ echo 1 > brightness
beagle$ echo 0 > brightness

Is it responding correctly?

The Bone has more trigger options. Try:

beagle$ cat trigger
[none] mmc0 timer heartbeat backlight gpio default-on 
beagle$ echo timer > trigger
brightness  delay_on  max_brightness  subsystem  uevent
delay_off   device    power           trigger
beagle$ echo 100 > delay_on
beagle$ echo 900 > delay_off

What does this do?

Adding your own LED - bone

It's not hard to use the gpio pins to control your own LED. All you need is an LED and a 220Ω resistor. Here's a picture of how it's wired. We are just doing the LED at the top of the breadboard for now.

Bone gpio.JPG

So how do you know where to connect it? The BeagleBone System Reference Manual has the details. Figure 3 on page 18 of RevA5.01 shows:

Beagle Bone.jpg

There at two expansion headers, P8 and P9. Look at the bottom of the left header and you'll see it's labeled P9. Table 11 on page 59 shows the pinout for P9.

Bone P9 pinout.jpg

On the first photo above you can see that pin 1 (Ground) is wired to the - bus and pin 3 (3.3V) is wired to the + bus. The 220Ω resistor is wired to the - bus and the other end is attached to the negative lead of the LED. The positive lead is attached to pin 12 which, as shown in Table 11, is attached to gpio1_28. The gpio pins are in banks of 32 each, so to find the gpio number to use on the Beagle, use 1*32+28=60. Here's how you turn it on

beagle$ cd /sys/class/gpio
beagle$ ls
export  gpiochip0  gpiochip32  gpiochip64  gpiochip96  unexport

Presently no gpio pins are visible. You need to tell it which pin to export

beagle$ echo 60 > export
beagle$ ls
gpio60  gpiochip0  gpiochip32  gpiochip64  gpiochip96

Notice gpio60 has appeared. All we need to do is tell it which direction and then turn it on.

beagle$ echo out > direction
beagle$ echo 1 > value

Your LED should be on! When you are done you can unexport the pin and it will disappear.

beagle$ cd ..
beagle$ echo 60 > unexport

Reading a switch - bone

Now that you have an LED working, wiring in a switch is easy. The picture above shows a push button switch wired in at the bottom of the P9 header. A 4.7kΩ resistor is attached to the - bus and the switch. The other end of the switch is attached to pin 42 which is gpio0_7.

Based on what you saw above, show how to read the switch.

Once you have the switch and LED working you can use the following scripts to play with them.

beagle$ cd ~/exercises/gpio
beagle$ ./togglegpio.sh 60 0.1

The LED should be blinking on and off.

beagle$ ./readgpio.sh 7

Push the pushbutton and see what happens.


Can you modify the scripts above to read the switch and turn the LED on and off?

Analog in - bone

(This is based on this.)

The bone has eight Analog Inputs. Several are exposed on P9. They are labeled AIN in table 11 above. How many to you find?

The AIN pins are sampled at 12 bits and 100k samples per second. The input voltage is between 0 and 1.8V. Fortunately, both voltages are available on P9.

The photo above shows a small potentiometer wired to the bone. One end goes to the analog ground (pin 34), the other analog 1.8V (pin 32). The wiper is attached to AIN5 which is pin 36.

You interact with the analog in much like the gpio, but it appears in a different location. How do you find that location? One approach is the find command.

beagle$ find / -name ain5

I used Ctrl-C to interrupt since it already found what I wanted.

beagle$ cd /sys/devices/platform/omap/tsc
beagle$ ls
ain1  ain3  ain5  ain7  driver    power      uevent
ain2  ain4  ain6  ain8  modalias  subsystem

There are the various analog inputs, but watch out. This interface starts numbering at 1 and Table 11 starts at 0, so to read AIN5 you need to look at ain6!

beagle$ cat ain6

(Unfortunately the interface doesn't print a RETURN after the value.) Change the pot and rerun the cat. What's the min and max value you get? Is it 12 bits?


Rewrite the scripts from before to read an AIN pin and continuously display it's value.

Reading a gpio pin with an Oscilloscope - xM

You can easily access many of the gpio pins via the Main Expansion Header. Page 107 of the BeagleBoard-xM System Reference Manual has this figure. MainHeader.png

Unfortunately the gpio pins don't appear here. It turns out the processor has more internal I/O lines than it has physical pins. Each physical pin can can be connected to up to 8 internal lines. BeagleBoardPinMux does a nice job of explaining it all. The big clue is here BeagleBoardPinMux#Beagle which references Table 22 on page 108 of the -xM System Reference Manual.


Note that gpio130 appears on pin 21 of the Expansion Header. Also note that pins 27 and 28 are ground. Attach your scope probe to these. Now, let's put a signal on the pin.

beagle$ cd /sys/class/gpio
beagle$ ls -F
export      gpiochip128@  gpiochip192@  gpiochip64@  unexport
gpiochip0@  gpiochip160@  gpiochip32@   gpiochip96@

Notice there is no folder for gpio130. Create it with:

beagle$ echo 130 > export
beagle$ ls -F
export    gpiochip0@    gpiochip160@  gpiochip32@  gpiochip96@
gpio130@  gpiochip128@  gpiochip192@  gpiochip64@  unexport

Update your exercises folder and cd to gpio directory.

 beagle$ cd
 beagle$ cd exercises/gpio
 beagle$ git pull
 beagle$ gedit togglegpio.sh

Can you tell what the program is doing? Try running it:

beagle$ togglegpio 130 0.05

Note that if you are the root user (which is the default case for Angstrom), you will have to type the following command:

beagle$ ./togglegpio 130 0.05

The first argument tells which gpio port to toggle, the second tells how long to delay between toggling. In this example 0.05 s is 50 ms, which should give a period around 100ms. Measure the signal on an oscilloscope.

Assignment: gpio from the shell

Measuring a gpio pin on an Oscilloscope

Answer the following questions about gpio measurements.

  1. What's the min and max voltage?
  2. What period is it?
  3. How close is it to 100ms?
  4. Why do they differ?
  5. Run htop and see how much processor you are using.
  6. Try different values for the sleep time (2nd argument). What's the shortest period you can get? Make a table of the values you try and the corresponding period and processor usage.
  7. How stable is the period?
  8. Try launching something like mplayer. How stable is the period?
  9. Try cleaning up togglegpio and removing unneeded lines. Does it impact the period?
  10. togglegpio uses bash (first line in file). Try using sh. Is the period shorter?
  11. What's the shortest period you can get?

Toggling the LEDs

Modify togglegpio (call it toggleLED) to toggle the LEDs. Can you get the LED to appear to dim by changing the duty cycle of the toggling?

User Button to gpio 130

Write a shell script that reads the User Button and outputs it value on gpio pin 130.

Count the User Button Presses

Write a shell script that displays a count of the number of times the User Button has been pressed.

Copy gpio 130 to gpio 131

Write a shell script that copies the value of gpio pin 130 to gpio pin 131. How much CPU time does it take? What's the delay from the time the input changes until the output changes? How constant is the delay?

Additional Exercises

Reading the Keyboard and Mouse

Try this:

beagle$ cd /sys/class/input
beagle$ ls -F
beagle$ evtest event2
Hit ctrl-C to stop

Now move your mouse around, or try you keyboard. My mouse is plugged into the bottom left USB port and event2 responds to it. Where do your keyboard and mouse appear?

Reading the User Button

(Note: This may not work on some versions of the OS. In these versions another program is controlling the gpio port that the User button is attached to.)

The Beagle has a couple of push buttons. One reboots the whole board. Use with care. One is for you to use, it's to the right of the Reset button, between the two stacks of USB ports. BeagleUserButton.png

It's attached to gpio port 4. You can read it via:

beagle$ cd /sys/class/gpio
beagle$ ls -F
export    gpio133@  gpio137@  gpio141@      gpiochip160@  gpiochip96@
gpio130@  gpio134@  gpio138@  gpio162@      gpiochip192@  unexport
gpio131@  gpio135@  gpio139@  gpiochip0@    gpiochip32@
gpio132@  gpio136@  gpio140@  gpiochip128@  gpiochip64@

Notice there is no gpio4. Here's how you can create it, set it to an input port and read its value:

beagle$ echo 4 > export
beagle$ ls
beagle$ cd gpio4
beagle$ echo in > direction
beagle$ cat value

Try holding down the switch and doing cat value again. Does the value change? There's a shell script called readgpio that repeatedly reads the switch.

beagle$ readgpio 4

Try pushing the switch. Does it work? Hit ctrl-C to stop. Look at readgpio. How does it work?

beagle$ which readgpio
beagle$ cp /usr/bin/readgpio ~
beagle$ gedit ~/readgpio


  1. Here is wh1ts article on flashing an LED. It is referenced in the readgpio file that comes on the Beagle.
  2. This Make magazine article has a few more details.
  3. Here in a gpio reference for Linux in general. It includes sample 'C' code for flashing at 1 Hz.
  4. Here is a posting in the Beagle Google group about gpio.
  5. Here is some information about gpio from the kernel point of view.
  6. Here is some info on a GPIO Event Driver
  7. Here is info on how to set edge to falling and poll() the pin.
  8. Here is the kernel Documentation on gpio.

thumb‎ Embedded Linux Class by Mark A. Yoder