Difference between revisions of "PWM"

Revision as of 19:10, 16 March 2010

On this page, you will find usefull informations to configure and use the Pulse Width Modulation (PWM) capabilities of your APF9328/APF27 boards.

The i.MX processor is capable of driving 2 PWM signals:

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

Hardware

i.MX PWM module is using PWMO (pin 2 of PortA on APF9328 and pin 5 of PortE on the APF27) as ouput.

• On APF9328DevLight this pin is accessible from TIMER zone under the board (See datasheets for more infos)
• On APF9328DevFull development boards this pin is connected to pin 4 (TIM2) of "Timers" connector (X21)
• On APF27Dev development boards this pin is connected to pin 4 of J22

Linux driver installation

i.MX PWM driver is installed by default in standard Armadeus rootfs.

Usage

• Load the module:

 # modprobe imx-pwm
 Initializing PWM class.
  Initializing PWM#0...PWM major number = 251
 PWM driver v0.7
 done.

• If you plan to use sound, create /dev/pwm device file if not already 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

After driver loading, 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

Bash test script

• A Bash test script is available given below. Just enter the desired frequency and duty cycle.

#!/bin/sh
#
# script to command imx pwm 
# usage : pwm_imx frequency duty 
#

if [ $# -lt 2 ] 
then
echo "Provide two arguments to this script !"
echo "pwm_imx frequency dutycycle"
exit 1
fi

SYS_DIR="/sys/class/pwm/pwm0/"
FREQUENCY=$1
DUTY=$2

DUTY=`(echo $DUTY | awk '{ print $1*10}')`

if [ ! -d "$SYS_DIR" ]; then
echo "Can't find /sys/ interface for PWM"
exit 1
fi

echo "Starting PWM"
echo 1 > $SYS_DIR/active

#SET FREQUENCY
echo $FREQUENCY > $SYS_DIR/frequency
echo "Setting PWM to $FREQUENCY Hz"

#SET DUTY CYCLE
echo $DUTY > $SYS_DIR/duty
echo "Setting DUTY to $2 %"

exit 0

Audio Mode

Audio playback mode is working since v0.5 of PWM driver (thanks to 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, which can be compiled that way:

 $ make shell_env
 $ . armadeus_env.sh
 $ export PATH=$PATH:$ARMADEUS_TOOLCHAIN_PATH
 $ make -C target/linux/modules/pwm/test/
 $ cp target/linux/modules/pwm/test/testplaymode /tftpboot/
 $ cp target/linux/modules/pwm/test/KDE_Startup_2.raw /tftpboot/

on target do:

 # testplaymode

or

 # testplaymode KDE_Startup_2.raw

Going further

If you need more than one PWM at a time, you can use the APF's FPGA with the following project.

Links

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