3.2.2.13. PWM
Introduction
Linux has support for Enhanced Pulse Width Modulator (ePWM) and Auxiliary Pulse Width Modulator (APWM) modules. APWM is Enhanced Capture (eCAP) module configured in PWM mode. These devices are part of the Pulse-Width Modulation Subsystem (PWMSS).
PWMSS software architecture
Kernel Configuration
Enable the eHRPWM driver
Device Drivers --->
<*> Pulse Width Modulation(PWM) Support --->
<*> eHRPWM PWM support
Enable the eCAP driver
Device Drivers --->
<*> Pulse Width Modulation(PWM) Support --->
<*> eCAP PWM support
Note
eHRPWM modules provide two PWM output channels per instance.
eCAP modules provide a single PWM output channel pre instance when configured in APWM mode.
Driver Usage
Using PWM with the sysfs interface
The PWMs can be used from the userspace with a simple sysfs interface exposed at /sys/class/pwm/. Each probed PWM controller/chip will be exported as pwmchipN, where N is the base of the PWM chip. The following attributes are available for each pwmchipN:
npwm
The number of PWM channels this chip supports.
export
Exports a PWM channel for use with sysfs.
unexport
Unexports a PWM channel from sysfs.
The PWM channels are numbered per-chip from 0 to npwm-1. When a PWM channel is exported a pwmX directory will be created in the pwmchipN directory, where X is the number of the channel that was exported. The following attributes will then be available in the pwmX directory:
period
The total period of the PWM signal in nanoseconds.
duty_cycle
The active time of the PWM signal in nanoseconds. Must be less than the period.
polarity
Changes the polarity of the PWM signal. The value written is the string “normal” or “inversed”.
enable
Enable/disable the PWM signal.
0 - disabled
1 - enabled
Example
List the available PWM controllers.
$ ls /sys/class/pwm/
pwmchip0 pwmchip1 pwmchip2 pwmchip3 pwmchip5 pwmchip7
The number of channels per controller is reported in the npwm attribute.
$ cat /sys/class/pwm/pwmchip5/npwm
2
The pwmchip5 controller is a 2 channel ePWM. Export the second channel (EPWM_B, channel 1).
$ echo 1 > /sys/class/pwm/pwmchip5/export
Verify pwm1 directory exists in the pwmchip5 directory.
$ ls /sys/class/pwm/pwmchip5/
device export npwm power pwm1 subsystem uevent unexport
Note
The PWM period and duty cycle must be configured before enabling any channel.
Set a period of 100 milliseconds with 30% duty cycle. The values are in nanoseconds.
$ echo 100000000 > /sys/class/pwm/pwmchip5/pwm1/period
$ echo 30000000 > /sys/class/pwm/pwmchip5/pwm1/duty_cycle
Set the PWM signal polarity to “inversed” (or “normal”) if required.
$ echo "inversed" > /sys/class/pwm/pwmchip5/pwm1/polarity
$ cat /sys/class/pwm/pwmchip5/pwm1/polarity
inversed
Enable the PWM channel.
$ echo 1 > /sys/class/pwm/pwmchip5/pwm1/enable
Using PWM with kernel PWM API
A few examples of kernel drivers using the kernel PWM API:
pwm-backlight: drivers/video/backligt/pwm_bl.c
pwm-beeper: drivers/input/misc/pwm-beeper.c
pwm-vibrator: drivers/input/misc/pwm-vibra.c
Troubleshooting the PWM setup
The current PWM settings for ePWM and eCAP(APWM) can be monitored from the debugfs pwm entry.
$ cat /sys/kernel/debug/pwm
platform/23020000.pwm, 2 PWM devices
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
pwm-1 ((null) ): period: 0 ns duty: 0 ns polarity: normal
platform/23010000.pwm, 2 PWM devices
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
pwm-1 (sysfs ): requested enabled period: 100000000 ns duty: 50000000 ns polarity: inverse
platform/23000000.pwm, 2 PWM devices
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
pwm-1 ((null) ): period: 0 ns duty: 0 ns polarity: normal
platform/23120000.pwm, 1 PWM device
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
platform/23110000.pwm, 1 PWM device
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
platform/23100000.pwm, 1 PWM device
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
Testing eHRPWM in TI PSDK
EHRPWM can be tested using the following overlays for each of the following devices:
AM62X SK: k3-am625-sk-rpi-hdr-ehrpwm.dtso
AM62AX SK: k3-am62a7-sk-rpi-hdr-ehrpwm.dtso
AM62PX SK: k3-am62p5-sk-rpi-hdr-ehrpwm.dtso