|
|
(17 intermediate revisions by 3 users not shown) |
Line 2: |
Line 2: |
| [[Category:ECE497]] | | [[Category:ECE497]] |
| {{YoderHead}} | | {{YoderHead}} |
| + | {{EBC3.8}}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 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 we will learn how to use the Beagle's pulse width modulation (pwm) hardware by writing directly to the registers that control it and also learn about pin multiplexing (pin mux) on the way. | + | In this exercise you will learn how to use the Beagle's pulse width modulation (pwm) hardware using the sysfs interface. |
| | | |
| == PWM on the Bone == | | == PWM on the Bone == |
| + | |
| + | (Note: The pwm interface seems to changing. Some of this may not apply in the future.) |
| | | |
| The Bone has a PWM interface at <code>/sys/class/pwm/</code>. You can see what's there by: | | The Bone has a PWM interface at <code>/sys/class/pwm/</code>. You can see what's there by: |
Line 13: |
Line 16: |
| beagle$ '''cd /sys/class/pwm''' | | beagle$ '''cd /sys/class/pwm''' |
| beagle$ '''ls -F''' | | beagle$ '''ls -F''' |
− | ecap.0@ ecap.2@ ehrpwm.0:1@ ehrpwm.1:1@ ehrpwm.2:1@ | + | export unexport |
− | ecap.1@ ehrpwm.0:0@ ehrpwm.1:0@ ehrpwm.2:0@ | + | Hmmm, there isn't much there. We have to run a command to make something appear. Try |
− | | + | beagle$ '''SLOTS=/sys/devices/bone_capemgr.*/slots''' |
− | 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. Check the setting by browsing to [http://beagle/pinMux.html http://''beagle''/pinMux.html] (where ''beagle'' is the IP address of your Beagle.) to see how the pins are set. You'll see something like:
| + | beagle$ '''PINS=/sys/kernel/debug/pinctrl/44e10800.pinmux/pins''' |
− | | + | beagle$ '''echo am33xx_pwm > $SLOTS''' |
− | [[File:PimMux1.png]]
| + | beagle$ '''ls -F''' |
− | | + | export pwmchip0@ pwmchip2@ pwmchip3@ pwmchip5@ pwmchip7@ unexport |
− | You can control the pin MUXing this way:
| + | Now we need to run another command to say which pwm pin we want to use. I'm using P9_21. |
− | | + | beagle$ '''echo bone_pwm_P9_21 > $SLOTS |
− | beagle$ '''cd /sys/kernel/debug/omap_mux''' | + | Now you can export a pwm much list you export a gpio port |
− | beagle$ '''ls''' | + | beagle$ '''echo 1 > export''' |
− | ain0 gpmc_ad2 lcd_data3 mii1_txd2 | + | beagle$ '''cd pwm1''' |
− | ain1 gpmc_ad3 lcd_data4 mii1_txd3 | + | beagle$ '''ls -F''' |
− | ain2 gpmc_ad4 lcd_data5 mii1_txen | + | device@ duty_ns period_ns polarity power/ run subsystem@ uevent |
− | ain3 gpmc_ad5 lcd_data6 mmc0_clk | + | Try a 1kHz frequency with a 25% duty cycle |
− | ain4 gpmc_ad6 lcd_data7 mmc0_cmd | + | beagle$ '''echo 1000000 > period_ns''' |
− | ain5 gpmc_ad7 lcd_data8 mmc0_dat0 | + | beagle$ '''echo 250000 > duty_ns''' |
− | 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.
| |
− | | |
− | [[File: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 1 > run''' |
− | beagle$ '''echo 100 > period_freq''' | + | If you have an oscilloscope try probing pin P9_21. I'm getting a nice clean 1kHz signal, with no variation. Let's try a higher frequency, like 10 MHz. |
− | beagle$ '''echo 10 > duty_percent''' | + | beagle$ '''echo 50 > duty_ns''' |
− | | + | beagle$ '''echo 100 > period_ns''' |
− | Stick a scope on the pin and you should see a 100Hz, 10% duty cycle square wave.
| + | I'm getting a 9 MHz signal that has lots of ringing. The timer in the bone must be off a bit. |
− | | |
− | | |
− | [https://groups.google.com/forum/#!searchin/beagleboard/bone$20pwm/beagleboard/alKf67dwMHI/t_tIQpJyM8wJ Good discussion]
| |
− | | |
− | Look in cloud9/bonescript/index.js to see how to do pinMuxing via /sys/kernel/debug/omap_mux/
| |
− | | |
− | == PWM on the xM ==
| |
− | | |
− | === PWM and pin MUXing ===
| |
− | | |
− | 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 ([http://www.ti.com/product/dm3730#technicaldocuments 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 [[BeagleBoardPinMux#Setting_Mux_Through_u-boot | 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 ([https://groups.google.com/d/topic/beagleboard/RI3qTxn68bY/discussion], [http://git.billgatliff.com/pwm.git/?p=pwm.git;a=commit;h=a49cbfff0fa09bff40d328f8985a0a7a7b951d6f] and [http://git.pengutronix.de/?p=imx/linux-2.6.git;a=commit;h=137654cde98a2ffe548f47f02e7fde512bc2091c]). [[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 [http://esdw.wordpress.com/2010/03/25/a-useful-tool-devmem2/ devmem2] to write to the memory mapped registers from a command line.
| |
− | | |
− | [http://www.jumpnowtek.com/index.php?option=com_content&view=article&id=56&Itemid=63 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 [[EBC_Exercise_01a_Getting_Exercise_Support_Materials | here]] if you aren't set up for git.)
| |
| | | |
− | # Look at the files to see what they are doing.
| + | 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. |
− | # Run '''make''', then '''pwd-demo'''.
| |
− | # Hook up a oscilloscope. (See Table 22 of the [http://beagleboard.org/static/BBxMSRM_latest.pdf Beagle System Reference manual] to see where to probe.) Are the pwd outputs doing what you expected?
| |
− | # What's the highest frequency you can generate? What's the lowest?
| |
− | # Create a new C program, based on pwm-demo, that takes 3 parameters, the <pwm to use>, <frequency> and <duty cycle>.
| |
− | # 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.
| |
− | # Write a shell file that will do the pin MUXing using '''devmem2'''.
| |
− | # Rewrite '''pwm-demo''' as a shell file that uses '''devmem2'''.
| |
| | | |
− | == Resources ==
| + | 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? |
| | | |
− | # [[BeagleBoardPWM]] from ECE597
| + | 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? |
− | # [[BeagleBoard/GSoC/2010_Projects/Pulse_Width_Modulation]] Google SoC project
| |
− | # [[BeagleBoardPinMux]], how to set the pin mux.
| |
− | # [http://www.gigamegablog.com/ Buttons and PWM]
| |
− | # [http://veter-project.blogspot.com/2011/09/real-time-enough-about-pwms-and-shaky.html Shaky PWMs]
| |
− | # [https://groups.google.com/forum/#!searchin/beagleboard/bone$20pwm/beagleboard/alKf67dwMHI/t_tIQpJyM8wJ PWM on the bone]
| |
| | | |
| {{YoderFoot}} | | {{YoderFoot}} |
Embedded Linux Class by Mark A. Yoder
This is for the 3.8 kernel. EBC Exercise 13 Pulse Width Modulation 3.2 is for the 3.2 kernel.
In a previous exercise (EBC Exercise 11 gpio Polling and Interrupts) you saw how to use the gpio to produce a square wave out using a C program and sysfs. I was able to get a 1.5kHz square wave out; however we can do much better using some built in hardware on the Beagle.
In this exercise you will learn how to use the Beagle's pulse width modulation (pwm) hardware using the sysfs interface.
PWM on the Bone
(Note: The pwm interface seems to changing. Some of this may not apply in the future.)
The Bone has a PWM interface at /sys/class/pwm/
. You can see what's there by:
beagle$ cd /sys/class/pwm
beagle$ ls -F
export unexport
Hmmm, there isn't much there. We have to run a command to make something appear. Try
beagle$ SLOTS=/sys/devices/bone_capemgr.*/slots
beagle$ PINS=/sys/kernel/debug/pinctrl/44e10800.pinmux/pins
beagle$ echo am33xx_pwm > $SLOTS
beagle$ 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.
beagle$ echo bone_pwm_P9_21 > $SLOTS
Now you can export a pwm much list you export a gpio port
beagle$ echo 1 > export
beagle$ cd pwm1
beagle$ ls -F
device@ duty_ns period_ns polarity power/ run subsystem@ uevent
Try a 1kHz frequency with a 25% duty cycle
beagle$ echo 1000000 > period_ns
beagle$ echo 250000 > duty_ns
beagle$ echo 1 > run
If you have an oscilloscope try probing pin P9_21. I'm getting a nice clean 1kHz signal, with no variation. Let's try a higher frequency, like 10 MHz.
beagle$ echo 50 > duty_ns
beagle$ echo 100 > period_ns
I'm getting a 9 MHz signal that has lots of ringing. The timer in the bone must be off a bit.
The AM335x PWM Driver's Guide details what eCAP and eHRPWM are and gives some examples.
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?
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?
Embedded Linux Class by Mark A. Yoder