ASEE 2013 Workshop

Here are the labs for the afternoon Linux part of the ASEE 2013 Workshop

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 192.167.7.2 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.

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   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
In the AM lab 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. In the AM lab 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 0 Now hold the button down and try again. bone$ cat value 1 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 ] do   cat gpio7/value sleep 0.25 done Quit nano and run bone$ chmod +x button.sh (This makes button.sh executable) bone$ ./button.sh What happens when you push the button?
 * 1) !/bin/bash

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

Analog In
The bone has eight Analog Inputs. Several are exposed on P9. They are labeled AIN in table 11 below. How many do 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.

You've already wired these up for the AM lab. You interact with the analog in much like the gpio, but it appears in a different. We have to run a command before the AIN interface appears. Just run them now, later I'll explain what you did if you are interested.

bone$ SLOTS=/sys/devices/bone_capemgr.*/slots bone$ echo cape-bone-iio > $SLOTS

You can now access the analog interface, let's explore.

bone$ cd /sys/devices/ocp.2/helper.14 bone$ ls -F AIN0 AIN2  AIN4  AIN6  driver@   power/      uevent AIN1 AIN3  AIN5  AIN7  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!

bone$ cat AIN6 1185

Change the pot and rerun cat. What's the min and max value you get? Is it 12 bits?

You can use the following script to continuously read the input. while [ 1 ] do   tr '\n' '\r' < AIN6 done

Pulse Width Modulation
(Note: The pwm interface seems to changing. Some of this may not apply in the future.)

The Bone has a PWM interface at. You can see what's there by:

bone$ cd /sys/class/pwm bone$ ls -F export unexport Hmmm, there isn't much there. We have to run a command to make something appear. Try bone$ SLOTS=/sys/devices/bone_capemgr.*/slots bone$ echo am33xx_pwm > $SLOTS bone$ ls -F export pwmchip0@  pwmchip2@  pwmchip3@  pwmchip5@  pwmchip7@  unexport Now we need to run another command to say which pwm pin we want to use. I'm using P9_21. bone$ '''echo bone_pwm_P9_21 > $SLOTS Now you can export a pwm much list you export a gpio port bone$ echo 1 > export bone$ cd pwm1 bone$ ls -F device@ duty_ns  period_ns  polarity  power/  run  subsystem@  uevent Try a 1Hz frequency with a 25% duty cycle bone$ echo 1000000000 > period_ns bone$ echo 250000000 > duty_ns bone$ echo 1 > run

Connect the LED from and watch it flash. Try changing the frequency and duty cycle. You may have to set the duty cycle to 0 to change the frequency. Can you guess why?

Challenge
Combine the analog in and the PWM by having the pot control the frequency or the duty cycle of the LED.