Difference between revisions of "EBC Exercise 13 Pulse Width Modulation"

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The [http://processors.wiki.ti.com/index.php/AM335x_PWM_Driver's_Guide AM335x PWM Driver's Guide] details what eCAP and eHRPWM are and gives some examples.  Before you can use the PWM's, you need to make sure the pin MUXes are set correctly. There are two ways to do this, the slick way (which may not work) and the sure way.
 
The [http://processors.wiki.ti.com/index.php/AM335x_PWM_Driver's_Guide AM335x PWM Driver's Guide] details what eCAP and eHRPWM are and gives some examples.  Before you can use the PWM's, you need to make sure the pin MUXes are set correctly. There are two ways to do this, the slick way (which may not work) and the sure way.
 
=== The Slick Way ===
 
 
On your Bone
 
 
beagle$ '''cd exercises'''
 
beagle$ '''git pull'''
 
beagle$ '''cd pinMux'''
 
beagle$ '''ln -s $PWD/pinMux.html /var/lib/cloud9/bone101'''
 
beagle$ '''ls -ls /var/lib/cloud9/bone101/pinMux.html'''
 
0 lrwxrwxrwx 1 root root 39 Sep  6 14:19 /var/lib/cloud9/bone101/pinMux.html -> /home/root/exercises/pinMux/pinMux.html
 
beagle$ '''cd /var/lib/cloud9'''
 
beagle$ '''git pull'''
 
beagle$ '''shutdown -r now'''
 
 
Yup, after updating /var/lib/cloud9 you have to reboot.
 
 
Now, check the settings by browsing to [http://beagle/pinMux.html http://''bone''/pinMux.html] (where ''bone'' is the IP address of your Bone.) to see how the pins are set.  You'll see something like:
 
 
[[File:PimMux1.png]]
 
  
 
=== The Sure Way ===
 
=== The Sure Way ===

Revision as of 14:45, 31 May 2013

thumb‎ Embedded Linux Class by Mark A. Yoder


3.8 Kernel

This is for the 3.8 kernel. EBC Exercise 13 Pulse Width Modulation 3.2 is for the 3.2 kernel.

In a previous exercise (EBC Exercise 11 gpio Polling and Interrupts) you saw how to use the gpio to produce a square wave out using a C program and sysfs. I was able to get a 1.5kHz square wave out; however we can do much better using some built in hardware on the Beagle.

In this exercise you will learn how to use the Beagle's pulse width modulation (pwm) hardware using the sysfs interface and also learn about pin multiplexing (pin mux) on the way.

PWM on the Bone

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

beagle$ cd /sys/class/pwm
beagle$ ls -F
ecap.0@  ecap.2@      ehrpwm.0:1@  ehrpwm.1:1@  ehrpwm.2:1@
ecap.1@  ehrpwm.0:0@  ehrpwm.1:0@  ehrpwm.2:0@

The AM335x PWM Driver's Guide details what eCAP and eHRPWM are and gives some examples. Before you can use the PWM's, you need to make sure the pin MUXes are set correctly. There are two ways to do this, the slick way (which may not work) and the sure way.

The Sure Way

beagle$ cd ~/exercises/pinMux
beagle$ ./pinMux.sh

This just lists all the files in the mux directory and their contents.

Either Way

This just lists every find in the mux directory.

You can control the pin MUXing this way:

beagle$ cd /sys/kernel/debug/omap_mux
beagle$ ls
ain0               gpmc_ad2        lcd_data3      mii1_txd2
ain1               gpmc_ad3        lcd_data4      mii1_txd3
ain2               gpmc_ad4        lcd_data5      mii1_txen
ain3               gpmc_ad5        lcd_data6      mmc0_clk
ain4               gpmc_ad6        lcd_data7      mmc0_cmd
ain5               gpmc_ad7        lcd_data8      mmc0_dat0
ain6               gpmc_ad8        lcd_data9      mmc0_dat1
ain7               gpmc_ad9        lcd_hsync      mmc0_dat2
...

There are some 126 pins that you can control what they output. How do you know which one to change? Let's use ehrpwm.1:0. This will show up at ehrpwm1A (the 0 maps to A). Try:

beagle$ grep ehrpwm *
gpmc_a0:signals: gpmc_a0 | gmii2_txen | rgmii2_tctl | rmii2_txen | gpmc_a16 | pr1_mii_mt1_clk | ehrpwm1_tripzone_input | gpio1_16
gpmc_a1:signals: gpmc_a1 | gmii2_rxdv | rgmii2_rctl | mmc2_dat0 | gpmc_a17 | pr1_mii1_txd3 | ehrpwm0_synco | gpio1_17
gpmc_a2:signals: gpmc_a2 | gmii2_txd3 | rgmii2_td3 | mmc2_dat1 | gpmc_a18 | pr1_mii1_txd2 | ehrpwm1A | gpio1_18
gpmc_a3:signals: gpmc_a3 | gmii2_txd2 | rgmii2_td2 | mmc2_dat2 | gpmc_a19 | pr1_mii1_txd1 | ehrpwm1B | gpio1_19
...

This shows that ehrpwm1A shows up in the file gpmc_a2. Look in the file

beagle$ cat gpmc_a2
name: gpmc_a2.gpio1_18 (0x44e10848/0x848 = 0x0007), b NA, t NA
mode: OMAP_PIN_OUTPUT | OMAP_MUX_MODE7
signals: gpmc_a2 | gmii2_txd3 | rgmii2_td3 | mmc2_dat1 | gpmc_a18 | pr1_mii1_txd2 | ehrpwm1A | gpio1_18

This says the MUX is presently set on pin 7. Counting starts on the left with 0. We want pin 6. So:

beagle$ echo 6 > gpmc_a2
beagle$ cat gpmc_a2
name: gpmc_a2.ehrpwm1A (0x44e10848/0x848 = 0x0006), b NA, t NA
mode: OMAP_PIN_OUTPUT | OMAP_MUX_MODE6
signals: gpmc_a2 | gmii2_txd3 | rgmii2_td3 | mmc2_dat1 | gpmc_a18 | pr1_mii1_txd2 | ehrpwm1A | gpio1_18

Now it's mode 6, which is the PWM output. Refresh you pin MUX web page to see.

PinMux2.png

Notice header P9, pin 14 has changed! Now let's turn on the PWM.

beagle$ cd /sys/class/pwm/ehrpwm.1\:0
beagle$ echo 1 > run
beagle$ echo 10 > period_freq
beagle$ echo 25 > duty_percent

Note: If these command cause the SSH connection to close, you may have to get a newer version of Angstrom. Follow the instructions at EBC_Exercise_03_Installing_a_Beagle_OS and try it again.

Connect the LED from EBC_Exercise_10_Flashing_an_LED#Adding_your_own_LED_-_bone 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?

Stick a scope on the pin and see if the frequency and duty cycle are right. What's the highest frequency you can get? What's the lowest?

Good discussion

Assignment

If your git repository is set up just:

beagle$ cd exercises
beagle$ git pull
beagle$ cd pwm

(Follow the instructions here if you aren't set up for git.)

  1. Attach an LED and verify that it's blinking correctly.
  2. Hook up a oscilloscope. Are the PWM outputs doing what you expected?
  3. What's the highest frequency you can generate? What's the lowest?
  4. Verify your understanding of pin MUXing by generating a PWM signal that appears on pin 45 of P8. Document how you did it.

PWM on the xM

This section needs updating to use /sys/kernel/debug/omap_mux to get the pin MUXes.

The DM3730 has 11 general purpose timers, 4 of which (gpt8-gpt11) can be brought out of the chip and used for pulse width modulation (DM3730 TRM page 2689). The problem is the DM3730 has more internal lines than hardware I/O pins. The solution is that I/O pins run though a MUX that selects which internal lines appear on I/O pins. A given pin can have one from as many as eight lines assigned to it.

These MUXes are set at boot time, and must be set when the kernel boots, or in u-boot. I couldn't set them during kernel boot with the 2.6.32 kernel, so I used u-boot. BeagleBoardPinMux is a good place to learn about the pin MUXing. The u-boot details are here.

BeagleBoardPWM is a nice overview of how to do PWM on the Beagle. The version of the kernel and u-boot that I've given you should already be configured to access the PWM pins. If it isn't you'll have to recompile the Kernel and u-boot.

The standard way to interface with the outside world in Linux is through Kernel Drivers. Currently there are no standard PWM driver for the Beagle, though a couple have been proposed ([1], [2] and [3]). BeagleBoardPWM takes a more traditional MCU approach by accessing the memory mapped PWD registers directly using mmap in a C program. Although this approach works, it is really transitional until a standard can be established.

You could even do PWM from a shell command by using devmem2 to write to the memory mapped registers from a command line.

Here's another PWM lead.

Assignment

If your git repository is set up just:

beagle$ cd exercises
beagle$ git pull
beagle$ cd pwm

(Follow the instructions here if you aren't set up for git.)

  1. Look at the files to see what they are doing.
  2. Run make, then pwm-demo.
  3. Hook up a oscilloscope. (See Table 22 of the Beagle System Reference manual to see where to probe.) Are the pwm outputs doing what you expected?
  4. What's the highest frequency you can generate? What's the lowest?
  5. Create a new C program, based on pwm-demo, that takes 3 parameters, the <pwm to use>, <frequency> and <duty cycle>.
  6. Create a shell file that will call your new program and set up the three pwm's that appear on the expansion header and program them to do something interesting.
  7. Write a shell file that will do the pin MUXing using devmem2.
  8. Rewrite pwm-demo as a shell file that uses devmem2.

Resources

  1. BeagleBoardPWM from ECE597
  2. BeagleBoard/GSoC/2010_Projects/Pulse_Width_Modulation Google SoC project
  3. BeagleBoardPinMux, how to set the pin mux.
  4. Buttons and PWM
  5. Shaky PWMs
  6. PWM on the bone




thumb‎ Embedded Linux Class by Mark A. Yoder