AM335x PWM Driver's Guide

Introduction
The Pulse-Width Modulation Subsystem (PWMSS) consists of an instance of Enhanced High Resolution Pulse Width Modulator (eHRPWM), Enhanced Capture (eCAP), and Enhanced Quadrature Encoded Pulse (eQEP) modules. AM335x has 3-instances of PWMSS.

eHRPWM
The eHRPWM is an effective PWM peripheral, able to generate complex pulse width waveforms with minimal CPU overhead or intervention. The ePWM module represents one complete PWM channel composed of two PWM outputs: EPWMA and EPWMB.


 * 1) Dedicated 16 bit time-base with Period / Frequency control
 * 2) Two PWM outputs (EPWMA and EPWMB) that can be used in the following configurations
 * 3) Two independent PWM outputs with single-edge operation
 * 4) Two independent PWM outputs with dual-edge symmetric operation
 * 5) One independent PWM output with dual-edge asymmetric operation
 * 6) Programmable event prescaling minimizes CPU overhead on interrupts.

eCAP
The eCAP module can function either in PWM mode or Capture mode.


 * 1) Dedicated external pin
 * 2) 32 bit Time Base counter
 * 3) 4 x 32 bit Time-stamp Capture registers (CAP1-CAP4)
 * 4) Input Capture signal pre-scaling
 * 5) Interrupt capabilities on any of the 4 capture events

Current PWM Driver only supports below Features


 * 1) only eHRPWM &amp; eCAP is supported
 * 2) only PWM mode is supported for eCAP.

Procedure to build eHRPWM driver
Device Drivers ---&gt; &lt;*&gt; PWM Support ---&gt; &lt;*&gt; Davinci eHRPWM support

Procedure to build eCAP driver
Device Drivers ---&gt; &lt;*&gt; PWM Support ---&gt; &lt;*&gt; eCAP PWM support

eHRPWM
eHRPWM can be controlled from the user space through SYSFS interface. SYSFS interface for eHRPWM is available at /sys/class/pwm/ehrpwm.i:j Where, ‘i’ is the eHRPWM instance ‘j’ is the eHRPWM ouptut 0 or 1[Each instance of eHRPWM has 2 channel outputs] Various SYSFS Attributes

2 types of SYSFS attributes are available

 Note
 * 1) Request and Control attributes
 * 2) Configuration attributes


 * Below examples uses eHRPWM instance 2 and output channel 0 (i = 2, j = 0).

Type 1 attributes 


 * request Attribute.

Request PWM-SS for permission to use the module. Writing 1 to the request attribute Acquires the device and writing 0 to the request attribute Frees/Releases the device. Before performing any operations, device has to be requested first.

Example

target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:0/request target$ echo 0 &gt; /sys/class/pwm/ehrpwm.2:0/request
 * Request the Device:
 * free the device:


 * run Attribute

Start or Stop the PWM Module. 0 – Stop Module, 1 – Start module.

Example

target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:0/run target$ echo 0 &gt; /sys/class/pwm/ehrpwm.2:0/run CAUTION Before enabling the module, the module needs to be configured using below configuration attributes. Else proper operation is not assured.
 * Start the PWM
 * Stop the PWM

Type 2 attributes 

i.Setting the Period Following attributes set the period of the PWM waveform.


 * frequency Attribute

Enter the period in HZ. Once the frequency is configured, make sure to set the duty cycle value.

Example Setup PWM to generate a pwm signal of 100 HZ with 50% duty cycle. target$ echo 100 &gt; /sys/class/pwm/ehrpwm.2:0/period_freq target$ echo 50 &gt; /sys/class/pwm/ehrpwm.2:0/duty_percent


 * period_ns Attribute

Enter the period in nano seconds value.

Example if the period is 1 sec, enter target$ echo 1000000000 &gt; /sys /class/pwm/ehrpwm.2:0/period_ns

ii.Setting the Duty Following attributes set the duty of the PWM waveform.


 * duty_percent Attribute

Enter the Duty cycle value in percentage.

Example To configure for 50% duty cycle, enter target$ echo 50 &gt; /sys/class/pwm/ehrpwm.2:0/duty_percent


 * duty_ns Attribute

duty_ns takes the duty cycle value in nano seconds.

Example To configure for 0.5s, enter target$ echo 500000000 &gt; /sys /class/pwm/ehrpwm.2:0/duty_ns

iii.Setting the Polarity


 * Polarity Attribute.

Setup Signal Polarity

Example To set the polarity to Active High, Enter target$ echo 1 &gt; /sys /class/pwm/ehrpwm.2:0/polarity

Example To set the polarity to Active Low, Enter target$ echo 0 &gt; /sys /class/pwm/ehrpwm.2:0/polarity

Example Configuration: Following example shows steps to configure the PWM for 100 HZ with 50% duty cycle. target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:0/request target$ echo 100 &gt; /sys/class/pwm/ehrpwm.2:0/period_freq target$ echo 50 &gt; /sys/class/pwm/ehrpwm.2:0/duty_percent target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:0/run Issuing above commands will generate PWM wave. This can be verified by probing to eHRPWM pin. IMPORTANT


 * 1) To change the frequency, first stop the PWM waveform, reset the duty to zero and then follow the last 3 steps in the above Example Configuration.
 * 2) eHRPWM 2 instance channel 1 pwm pin is hooked to Haptics AM335x EVM. Changing the duty percentage/frequency will affect the speed of the Haptics.

Controlling haptics
Haptics on Evaluation Module can be controlled through eHRPWM sysfs interface.

target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:1/request target$ echo 250 &gt; /sys/class/pwm/ehrpwm.2:1/period_freq target$ echo 50 &gt; /sys/class/pwm/ehrpwm.2:1/duty_percent target$ echo 1 &gt; /sys/class/pwm/ehrpwm.2:1/run

eCAP
The current release of the driver supports only PWM mode. eCAP can be controlled from the user space through SYSFS interface. SYSFS interface for eCAP is available at target$ cat /sys/class/pwm/ecap.i Where, ‘i’ is the eCAP instance. Various SYSFS Attributes

2 types of SYSFS attributes are available


 * 1) Request and Control attributes
 * 2) Configuration attributes

 Note


 * Below examples uses eCAP instance 0 (i = 0).

Type 1 attributes 
 * request Attribute.

Request PWM-SS for permission to use the module. Writing 1 to the request attribute Acquires the device and writing 0 to the request attribute Frees/Releases the device. Before performing any operations, device has to be requested first.

Example

target$ echo 1 &gt; /sys/class/pwm/ecap.0/request target$ echo 0 &gt; /sys/class/pwm/ecap.0/request
 * Request the Device:
 * free the device:


 * run Attribute

Start or Stop the PWM Module. 0 – Stop Module, 1 – Start module.

Example

target$ echo 1 &gt; /sys/class/pwm/ecap.0/run target$ echo 0 &gt; /sys/class/pwm/ecap.0/run CAUTION Before enabling the module, the module needs to be configured using below configuration attributes. Else proper operation is not assured.
 * Start the PWM
 * Stop the PWM

Type 2 attributes 

i.Setting the Period Following attributes set the period of the PWM waveform.


 * frequency Attribute

Enter the period in HZ. Once the frequency is configured, make sure to set the duty cycle value.

Example Setup PWM to generate a pwm signal of 100 HZ with 50% duty cycle. target$ echo 100 &gt; /sys/class/pwm/ecap.0/period_freq target$ echo 50 &gt; /sys/class/pwm/ecap.0/duty_percent


 * period_ns Attribute

Enter the period in nano seconds value.

Example if the period is 1 sec, enter target$ echo 1000000000 &gt; /sys /class/pwm/ecap.0/period_ns

ii.Setting the Duty Following attributes set the duty of the PWM waveform.


 * duty_percent Attribute

Enter the Duty cycle value in percentage.

Example To configure for 50% duty cycle, enter target$ echo 50 &gt; /sys/class/pwm/ecap.0/duty_percent


 * duty_ns Attribute

duty_ns takes the duty cycle value in nano seconds.

Example To configure for 0.5s, enter target$ echo 500000000 &gt; /sys /class/pwm/ecap.0/duty_ns

iii.Setting the Polarity


 * Polarity Attribute.

Setup Signal Polarity

Example To set the polarity to Active High, Enter target$ echo 1 &gt; /sys /class/pwm/ecap.0/polarity

Example To set the polarity to Active Low, Enter target$ echo 0 &gt; /sys /class/pwm/ecap.0/polarity

Example Configuration: Following example shows steps to configure the PWM for 100 HZ with 50% duty cycle. target$ echo 1 &gt; /sys/class/pwm/ecap.0/request target$ echo 100 &gt; /sys/class/pwm/ecap.0/period_freq target$ echo 50 &gt; /sys/class/pwm/ecap.0/duty_percent target$ echo 1 &gt; /sys/class/pwm/ecap.0/run Issuing above commands will generate PWM wave. This can be verified by probing to eCAP pin. IMPORTANT
 * 1) To change the frequency, first stop the PWM waveform, reset the duty to zero and then follow the last 3 steps in the above Example Configuration.
 * 2) In this EVM, eCAP instance 0 pin is hooked to backlight of the LCD. Changing the duty percentage will affect the brightness of the LCD screen.

Controlling backlight
Following are the 2 procedures to vary brightness of the LCD screen.

i. Setting duty percentage of pwm wave from eCAP sysfs files target$ echo val &gt; /sys/class/pwm/ecap.0/duty_percent 'val' can range from 0 to 100. ii. Setting brightness from backlight sysfs files target$ echo val > /sys/class/backlight/pwm-backlight/brightness 'val' can range from 0 to 100.