Difference between revisions of "PWM"

Revision as of 18:38, 20 December 2008

On this page, you will find usefull informations to configure and use the Pulse Width Modulation (PWM) capabilities of your APF9328 board.

Your board is capable of driving 2 PWM signals:

• one is linked to the LCD controller and allow to control the LCD backlight
• the other can be used to generate a PWM signal in several mode:
  • audio playback
  • real PWM

Linux driver installation

i.MXL PWM driver is now installed by default in standard Armadeus rootfs. Following instructions are only given as reference (to uninstall it at rootfs generation for example):

• Launch Linux configuration:

$ make linux26-menuconfig

And in Device Drivers ---> Armadeus specific drivers choose:

• Then rebuild your system:

$ make

Installation

Either copy generated module target/linux/modules/pwm/pwm.ko to your system with TFTP or NFS or reflash the generated rootfs. For more informations.

Usage

• Load the module:

# insmod /lib/modules/2.6.23.1/extra/pwm/pwm.ko

or (if you have reflashed your rootfs):

# modprobe pwm

• Create device file if not existing (Major is dynamically allocated)

# DEV=`cat /proc/devices | grep imx-pwm | cut -d " " -f 1`; mknod /dev/pwm c $DEV 0

PWM Mode

Remark:

• The pwm has a resolution of 1uS

Now you have access to /sys/class/pwm/ functionnalities:

• to change frequency (for example 500 Hz):

# echo 500 > /sys/class/pwm/pwm0/frequency

• to change period (for example 100us):

# echo 100 > /sys/class/pwm/pwm0/period

• to change duty cycle (for example 50.0%)

# echo 500 > /sys/class/pwm/pwm0/duty

• to activate PWM:

# echo 1 > /sys/class/pwm/pwm0/active

• to de-activate PWM:

# echo 0 > /sys/class/pwm/pwm0/active

Audio Mode

Audio playback mode is working since v0.5 of PWM driver (thanks to User:SebastienR). You can play 8, 16 & 32 KHz 8bit linear RAW sound. It's recommended to play 8kHz one, as they use less CPU ressources. To convert a sound to the right format we recommend to use SoX (on Linux). For exemple:

[host]$ sox /usr/share/sounds/KDE_Chimes_1.ogg -r 8192 -b -u -c 1 Chimes.raw
-r -> sampling rate
-b -> byte/8bit signal
-u -> linear /not compressed
-c 1 -> Mono / 1 channel

Then you can test it using target/linux/modules/pwm/test/testplaymode:

[host:/.../armadeus]$ make -C target/linux/modules/pwm/test/
[host:/.../armadeus]$ cp target/linux/modules/pwm/test/testplaymode $NFS_DIR
[host:/.../armadeus]$ cp target/linux/modules/pwm/test/KDE_Startup_2.raw $NFS_DIR

on target do:

# testplaymode

or

# testplaymode KDE_Startup_2.raw

Links

• SoX, sound format converter
• http://en.wikipedia.org/wiki/Pulse-width_modulation