SPEd 2013 Workshop

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thumb‎ Embedded Linux Class by Mark A. Yoder

Here are the labs for the afternoon Linux part of the SPEd2013 Workshop

Wiring Up

First wire up an LED, switch and a potentiometer. The table below shows what pins are available. The LED and switch can go on any GPIO (general purpose IO) pin. The pot goes on one of the AIN (analog in) pins.

                 P9                                       P8


Wiring an LED

Take an LED and attach the long lead (anode) into pin P9_11. That is, pin 11 on the P9 (left) header. Put the short lead on a column on the breadboard. Put one end of the 270Ω resistor in the same column as the LED and the other end in pin P9_1 (GND).

Wiring a switch

Attach one end of the switch on P9_2 (GND) and the other on P9_13. If you don't have a switch, put one end of a wire in P9_2 and touch the other end to P9_13 when you want to turn it on.

Wiring a potentiometer

Attach the middle pin of the pot to P9_36 (AIN5). Place one of the outer pins to P9_32 (Vdd_ADC) which is a 3.3V analog reference. Place the other on pin P9_34 (GNDA_ADC) the analog ground.

Linux Warm Up


Before we can interact with LEDs and switches we need to learn some simple Linux commands.

  1. On your host computer, running Windows, start up puTTY.
  2. If you get a Security Warning, click Run.
  3. Enter in the Host Name field and click Open
  4. Login as root with no password.
  5. Enter ls to list what files you have. You shouldn't see much.

At this point you need to learn a few simple Linux commands for creating and displaying files. Once you know these commands it's easy to turn an LED on and off.

First, let's edit a file using the nano editor. Nano is a simple editor that easy to learn. This will edit (and create) the file play.txt.

bone$ nano play.txt

Add a couple of lines of text to the file, it doesn't really matter what and then Exit. You can list the files in the current directory with ls and show the contents of a file with cat.

bone$ ls
Desktop  play.txt
bone$ cat play.txt
A couple of lines
of text.

Use echo to print a line of text.

bone$ echo This is a line of text
This is a line of text

Here's a powerful operator. You can take the output of any command and redirect it to a file with >.

bone$ echo This is a line of text > here.txt
bone$ cat here.txt
This is a line of text

We are almost there. Use cd to change directories. / is the top level directory.

bone$ cd /
bone$ ls
bin   dev  home  lost+found  mnt   run   sys  usr
boot  etc  lib   media       proc  sbin  tmp  var

If you ever get lost, cd alone takes you home.

bone$ cd
gone$ ls
Desktop  here.txt  play.txt

Now you are ready to flash an LED.

Blinking an LED

gpio via the Shell Command Line and sysfs

Another easy way to do general purpose I/O (gpio) on the Beagle is through a terminal window and a shell prompt. In Linux, almost everything is treated as a file, even things that aren't files. Here we'll use a virtual file system called sysfs. sysfs exposes the drivers for the hardware so you can easily use them.

Try this:

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

The "/" after the name means it's a directory. 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
backlight/  firmware/     lcd/       mtd/           scsi_disk/   ubi/
bdi/        gpio/         leds/      net/           scsi_host/   udc/
block/      graphics/     mbox/      power_supply/  sound/       uio/
bluetooth/  hwmon/        mdio_bus/  regulator/     spi_master/  usbmon/
bsg/        i2c-adapter/  mem/       rfkill/        spidev/      vc/
dma/        i2c-dev/      misc/      rtc/           thermal/     vtconsole/
drm/        input/        mmc_host/  scsi_device/   tty/         watchdog/

Explore some.

Blinking a USR LED

The Beagle Black has four user LEDS, user0 - user3, that you can control. Try this:

bone$ cd /sys/class/leds
bone$ ls -F
beaglebone:green:usr0  beaglebone:green:usr2
beaglebone:green:usr1  beaglebone:green:usr3

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.

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

(The "@" after the name means it's a link.) See what's in brightness, max_brightness and trigger by using the cat command. For example:

bone$ cat trigger
none nand-disk mmc0 timer oneshot [heartbeat] backlight gpio cpu0 default-on transient

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

bone$ echo none > trigger
bone$ cat trigger
[none] nand-disk mmc0 timer oneshot heartbeat backlight gpio cpu0 default-on transient 

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

bone$ echo 1 > brightness
bone$ echo 0 > brightness

Is it responding correctly?

The Bone has more trigger options. Try:

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

What does this do?

Blinking an External LED via gpio

Earlier we wired an LED to the P9_12 General Purpose IO (gpio) port and controlled it via BoneScript. Here we'll control it via a shell command. First we need to figure out which gpio pin P9_12 is attached to. The following figure shows it attached to gpio_60.


Here's how you turn it on

bone$ cd /sys/class/gpio
bone$ ls -F
export  gpiochip0@  gpiochip32@  gpiochip64@  gpiochip96@  unexport

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

bone$ echo 60 > export
bone$ ls -F
export  gpio60@  gpiochip0@  gpiochip32@  gpiochip64@  gpiochip96@  unexport

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

bone$ cd gpio60
bone$ ls
active_low  direction  edge  power  subsystem  uevent  value
bone$ echo out > direction
bone$ echo 1 > value

Your LED should be on!

bone$ echo 0 > value

Now it's off.

Reading a switch

Now that you have an LED working, wiring in a switch is easy. Earlier you wired a switch to P9_42, which from the table above is gpio_7.

Based on what you saw above.

bone$ cd /sys/class/gpio
bone$ echo 7 > export
bone$ cd gpio7
bone$ ls
bone$ echo in > direction
bone$ cat value

Now hold the button down and try again.

bone$ cat value

Once you have the switch and LED working use nano and put the following in a file.

bone$ cd  (Go back home)
bone$ nano button.sh
cd /sys/class/gpio
while [ 1 ]
   cat gpio7/value
   sleep 0.25

Quit nano and run

bone$ chmod +x button.sh  (This makes button.sh executable)
bone$ ./button.sh

What happens when you push the button? Hit Ctrl-C to quit button.sh.

Now experiment around. Can you flash the LED? How fast? Make the LED read the switch.

Other Languages

Out-of-the-box the bone can run

  • C
  • C++
  • bash
  • perl
  • python
  • Javascript

Here's a simple C example

bone$ cd
bone$ nano hello.c
#include <stdio.h>
main ()
        printf("hello, world\n");
bone$ cc hello.c
bone$ ./a.out
hello, world

Try your favorite language.

thumb‎ Embedded Linux Class by Mark A. Yoder