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== Pulse Width Modulation ==
== Pulse Width Modulation ==
Revision as of 13:38, 10 June 2013
Embedded Linux Class by Mark A. Yoder
Here are the labs for the afternoon Linux part of the ASEE 2013 Workshop
Before we can interact with LEDs and switches we need to learn some simple Linux commands.
- On your host computer, running Windows, start up puTTY.
- If you get a Security Warning, click Run.
- Enter 188.8.131.52 in the Host Name field and click Open
- Login as root with no password.
- 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.
bone$ nano play
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 bone$ cat play 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 bone$ cat here 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 play
Now you are ready to flash an LED.
gpio via the Shell Command Line and sysfs
The easiest 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. 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.
bone$ cd /sys bone$ 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.
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/
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
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?
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$ echo out > direction bone$ echo 1 > value
Your LED should be on! When you are done you can unexport the pin and it will disappear.
bone$ cd .. bone$ echo 60 > unexport
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$ nano flash.sh #!/bin/bash cd /sys/class/gpio while [ 1 ] do cat gpio7/value > gpio60/value sleep 0.25 done
Quit nano and run
bone$ chmod +x flash.sh (This makes flash.sh executable) bone$ ./flash.sh
What happens when you push the button?
Now experiment around. How fast can you flash the LED?
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' < AIN5 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
/sys/class/pwm/. 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?
Combine the analog in and the PWM by having the pot control the frequency or the duty cycle of the LED.
Embedded Linux Class by Mark A. Yoder