BBB Enabling PWM
These notes are based on the 3.8.13-bone70 complete installation as described over at BBB New Debian Image.
Contents
Selecting Overlays to use
Having being reading numerous pages about Device Tree Overlays I finally came up with a couple of solutions for how to enable the AM355 pinmux and setting the correct modes in order to enable PWM output.
If you just need one or a few PWM outputs you can generate overlay files over at this page.
If you want to enable a complete set of facilities on the BBB you can jump down to the [[ ]] section.
Single PWM Setup
Over at this page you can generate a device overlay file for each PWM channel you want to use.
For example in order to enable PWM on P9 pin 14. See the figure below how to setup the configurator
From the generator you will get an output like this:
/* * Copyright (C) 2013 CircuitCo * Copyright (C) 2013 Texas Instruments * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. * * This is a template-generated file from BoneScript */ /dts-v1/; /plugin/; / { compatible = "ti,beaglebone", "ti,beaglebone-black"; /* identification */ part-number = "BS_PWM_P9_14_0x16"; /* state the resources this cape uses */ exclusive-use = /* the pin header uses */ "P9.14", /* the hardware IP uses */ "ehrpwm1A"; fragment@0 { target = <&am33xx_pinmux>; __overlay__ { bs_pwm_P9_14_0x16: pinmux_bs_pwm_P9_14_0x16 { pinctrl-single,pins = <0x048 0x16>; }; }; }; fragment@1 { target = <&ocp>; __overlay__ { bs_pwm_test_P9_14 { compatible = "pwm_test"; pwms = <&ehrpwm1 0 500000 1>; pwm-names = "PWM_P9_14"; pinctrl-names = "default"; pinctrl-0 = <&bs_pwm_P9_14_0x16>; enabled = <1>; duty = <0>; status = "okay"; }; }; }; };
Save this in a file name as suggested: bspwm_P9_14_16-00A0.dts
Compile it using this command:
root@begalebone $] dtc -O dtb -o bspwm_P9_14_16-00A0.dtbo -b 0 -@ bspwm_P9_14_16-00A0.dts
Copy the .dtbo file to /lib/firmware
As instructed on the generator page you can enable this ovelay by issuing these commands:
root@beaglebone $] grep -q am33xx_pwm /sys/devices/bone_capemgr.?/slots || echo am33xx_pwm > /sys/devices/bone_capemgr.?/slots root@beaglebone $] echo bspwm_P9_14_16 > /sys/devices/bone_capemgr.?/slots
If this works and you can see the PWM device by issuing
root@beaglebone:~# ll /sys/devices/ocp.3/bs_pwm_test_P9_14.15/
and get this output
total 0 drwxr-xr-x 3 root root 0 May 31 09:34 . drwxr-xr-x 42 root root 0 Mar 1 21:19 .. lrwxrwxrwx 1 root root 0 May 31 09:37 driver -> ../../../bus/platform/drivers/pwm_test -rw------- 1 root root 4096 May 31 09:37 duty -r--r--r-- 1 root root 4096 May 31 09:37 modalias -rw------- 1 root root 4096 May 31 09:37 period -rw------- 1 root root 4096 May 31 09:37 polarity drwxr-xr-x 2 root root 0 May 31 09:37 power -rw------- 1 root root 4096 May 31 09:37 run lrwxrwxrwx 1 root root 0 May 31 09:34 subsystem -> ../../../bus/platform -rw-r--r-- 1 root root 4096 May 31 09:34 uevent
you can put the commands into your /etc/rc.local file for it to be executed at startup.
Complete Setup
This section describes a setup of the complete - almost - IO system on the BBB.
Preparing
In /opt/source/beaglebone-universal-io you'll find some general device drivers for the BBB.
Since BBB uses the FDT (Flattened Device Tree), where a general driver performs the most of the work, no special driver is needed for PWM.
There are two steps in enabling a device:
1. Setup the multiplexing of the pins (i.e. connect the internal device to a pin accessible on the outside)
2. Setup the peripheral (the internal device such as PWM, GPIO, etc.) to the pin assigned above
Ensure that your general pinmux is setup for defaults by ensuring that the /sys/devices/ocp.N (with N currently showing 3) exists. If not then perform:
root@beaglebone$] cd /opt/source/beaglebone-universal-io root@beaglebone$] echo cape-universaln > /sys/devices/bone_capemgr.*/slots
expect an output with this included:
drwxr-xr-x 3 root root 0 May 24 09:24 P8_07_pinmux.15 drwxr-xr-x 3 root root 0 May 24 09:24 P8_08_pinmux.16 drwxr-xr-x 3 root root 0 May 24 09:24 P8_09_pinmux.17 drwxr-xr-x 3 root root 0 May 24 09:24 P8_10_pinmux.18 drwxr-xr-x 3 root root 0 May 24 09:24 P8_11_pinmux.19 drwxr-xr-x 3 root root 0 May 24 09:24 P8_12_pinmux.20 drwxr-xr-x 3 root root 0 May 24 09:24 P8_13_pinmux.21 drwxr-xr-x 3 root root 0 May 24 09:24 P8_14_pinmux.22 drwxr-xr-x 3 root root 0 May 24 09:24 P8_15_pinmux.23 drwxr-xr-x 3 root root 0 May 24 09:24 P8_16_pinmux.24 drwxr-xr-x 3 root root 0 May 24 09:24 P8_17_pinmux.25 drwxr-xr-x 3 root root 0 May 24 09:24 P8_18_pinmux.26 drwxr-xr-x 3 root root 0 May 24 09:24 P8_19_pinmux.27 drwxr-xr-x 3 root root 0 May 24 09:24 P8_26_pinmux.28 drwxr-xr-x 3 root root 0 May 24 09:24 P9_11_pinmux.29 drwxr-xr-x 3 root root 0 May 24 09:24 P9_12_pinmux.30 drwxr-xr-x 3 root root 0 May 24 09:24 P9_13_pinmux.31 drwxr-xr-x 3 root root 0 May 24 09:24 P9_14_pinmux.32 drwxr-xr-x 3 root root 0 May 24 09:24 P9_15_pinmux.33 drwxr-xr-x 3 root root 0 May 24 09:24 P9_16_pinmux.34 drwxr-xr-x 3 root root 0 May 24 09:24 P9_17_pinmux.35 drwxr-xr-x 3 root root 0 May 24 09:24 P9_18_pinmux.36 drwxr-xr-x 3 root root 0 May 24 09:24 P9_21_pinmux.37 drwxr-xr-x 3 root root 0 May 24 09:24 P9_22_pinmux.38 drwxr-xr-x 3 root root 0 May 24 09:24 P9_23_pinmux.39 drwxr-xr-x 3 root root 0 May 24 09:24 P9_24_pinmux.40 drwxr-xr-x 3 root root 0 May 24 09:24 P9_26_pinmux.41 drwxr-xr-x 3 root root 0 May 24 09:24 P9_27_pinmux.42 drwxr-xr-x 3 root root 0 May 24 09:24 P9_30_pinmux.43 drwxr-xr-x 3 root root 0 May 24 09:24 P9_41_pinmux.44 drwxr-xr-x 3 root root 0 May 24 09:24 P9_42_pinmux.46 drwxr-xr-x 3 root root 0 May 24 09:24 P9_91_pinmux.45 drwxr-xr-x 3 root root 0 May 24 09:24 P9_92_pinmux.47
Connecting pins to the PWM Chip
Now in order to connect the PWM to a pin look up what pin PWM can be output to in the cape-universaln-00A0.dts file. You'll find that you can connect them to for instance P9.14, P9.16, P9.21, and P9.22.
To convince yourself on your choice issue this command:
root@beagelbone$] config-pin -l P9.14
expect this output:
default gpio gpio_pu gpio_pd pwm
Now in order to enable the pwm to the P9.14 pin issue:
root@begalebone$] config-pin P9.14 pwm
To ensure yourself that you've got the PWM connected to the pin issue
root@beaglebone$] cat /sys/devices/ocp.3/P9_14_pinmux.32/state
expect this output
pwm
Locate the PWM control files
Now where are the PWM?
Issue
root@beaglebone$] ll /sys/class/pwm/
and expect this
total 0 drwxr-xr-x 2 root root 0 Jan 1 2000 . drwxr-xr-x 59 root root 0 Jan 1 2000 .. --w------- 1 root root 4096 Jan 1 2000 export lrwxrwxrwx 1 root root 0 May 24 09:30 pwmchip0 -> ../../devices/ocp.3/48300000.epwmss/48300200.ehrpwm/pwm/pwmchip0 lrwxrwxrwx 1 root root 0 May 24 09:30 pwmchip2 -> ../../devices/ocp.3/48300000.epwmss/48300100.ecap/pwm/pwmchip2 lrwxrwxrwx 1 root root 0 May 24 09:30 pwmchip3 -> ../../devices/ocp.3/48302000.epwmss/48302200.ehrpwm/pwm/pwmchip3 lrwxrwxrwx 1 root root 0 May 24 09:30 pwmchip5 -> ../../devices/ocp.3/48304000.epwmss/48304200.ehrpwm/pwm/pwmchip5 lrwxrwxrwx 1 root root 0 May 24 09:30 pwmchip7 -> ../../devices/ocp.3/48304000.epwmss/48304100.ecap/pwm/pwmchip7 --w------- 1 root root 4096 Jan 1 2000 unexport
How to interpret this output. First we'll have to look into this technical document over the AM335x Sitara Processors. (If the document is not available search for AM335 Sitara Processor Technical Reference Manual - it's a ~22 MB document).
Looking for address 48300000 in the memory map, you'll find this is in the L4_PER block, and this is listed as the “PWM subsystem 0″, and 48300200 is the EHR0 PWM channel and 48300100 the eCAP0 PWM. Also, address 4830200 is the “PWM subsystem 1″, and address 4830400 is the “PWM subsystem 2″.
Now, using the pin names found from the BeagleBoard.org – bone101 page served from the board, and deductive reasoning you'll come up with the following maping of pin to export number:
export number pin name pins 0 EHRPWM0A P9.22,P9.31 1 EHRPWM0B P9.21,P9.29 2 ECAPPWM0 P9.42 3 EHRPWM1A P9.14,P8.36 4 EHRPWM1B P9.16,P8.34 5 EHRPWM2A P8.19,P8.45 6 EHRPWM2B P8.13,P8.46 7 ECAPPWM2 P9.28
Enable the pin for the PWM
Now we're ready to enable a pwm for instance P9_14. This is done by looking up P9.14 in the table above and see that it is export number 3. So perform
root@beaglebone$] echo 3 > /sys/class/pwm/export
NOTE here there needs to be a space between 3 and >!
root@beaglebone$] ls /sys/class/pwm/
will give you this
export pwm3 pwmchip0 pwmchip2 pwmchip3 pwmchip5 pwmchip7 unexport
No issuing
root@begalebone$] ll /sys/class/pwm/pwm3/
will output
total 0 drwxr-xr-x 3 root root 0 May 24 09:52 . drwxr-xr-x 4 root root 0 May 24 09:24 .. lrwxrwxrwx 1 root root 0 May 24 09:57 device -> ../../../48302200.ehrpwm -rw-r--r-- 1 root root 4096 May 24 09:57 duty_ns -rw-r--r-- 1 root root 4096 May 24 09:57 period_ns -rw-r--r-- 1 root root 4096 May 24 09:57 polarity drwxr-xr-x 2 root root 0 May 24 09:57 power -rw-r--r-- 1 root root 4096 May 24 09:57 run lrwxrwxrwx 1 root root 0 May 24 09:52 subsystem -> ../../../../../../class/pwm -rw-r--r-- 1 root root 4096 May 24 09:52 uevent
The files of interest here are the duty_ns, period_ns and polarity.
The polarity sets the polarity of the output signal. Typically setup once at initialisation.
The period_ns sets the period time active for the PWM signal. In nano seconds. Typically setup once at initialisation.
The duty_ns sets the duty cycle for the PWM signal - i.e. the length of the period time the PWM signal is active. In nano seconds. This one is constantly updated with the desired duty cycle.
Control the PWM
No you can control the PWM from the command line or you can write a C/C++ program that will open, write and close the above mentioned files in order to control the PWM.
Set the Pinmux and Enable at boot
If you want to enable the PWM in a quick way create a script in /root/bin call it enablePWM.sh. Enter this text into the script
#!/bin/bash # # This script will connect P9_14 to the PWM chip and enable the chip # # Author: Klaus Kolle # License: Public Domain, you may use it freely as desired, # but there is no warranty at all # Date: 2015 05 25 # # Add the BeagleBone overlay to the default setup echo cape-universaln > /sys/devices/bone_capemgr.9/slots # Connect the P9.14 to the PWM config-pin P9.14 pwm # Now we are ready to enable the PWM chip echo 3 > /sys/class/pwm/export # Everything done
If you want this script to run at every boot you need to instruct systemd to run the script at boot time.
In /etc/init.d create a new file called enable-pwm and put in this text into the file
#! /bin/sh ### BEGIN INIT INFO # Provides: enable-pwm # Required-Start: $all # Required-Stop: $all # Default-Start: 2 3 4 5 # Default-Stop: 0 1 6 # Short-Description: Enables the PWM chips and connects it through the pinmux # Description: Connecting the pwm output through the pinmux and enables the PWM chip on board ### END INIT INFO case “$1″ in start) /root/bin/enablePWM.sh ;; stop) #no-op ;; *) #no-op ;; esac exit 0
The first part after the #!/bin/sh tells the systemd information about your service.
Next comes the actual code part, where the case switches on the first parameter to the script. It can be start or stop. The start command will be handled in the section start). Likewise goes for the stop command.
In order to insert this as a service in the systemd execute these instructions:
root@beaglebone$] chmod +x enable-pwm ## Give execute rights to the script root@beaglebone$] insserv -n enable-pwm ## Checks the script root@beaglebone$] insserv enable-pwm ## Actually inserts the script
Now reboot the system and check if your PWM has be enables and routed to P9_14.
Acknowledgements
Thanks goes to this
for inspiration.