PWM
On this page, you will find usefull informations to configure and use the Pulse Width Modulation (PWM) capabilities of your APF boards.
The i.MX processor is capable of driving 2 PWM signals:
- one is linked to the LCD controller and allows controling the LCD backlight (fixed frequency)
- the other can be used to generate a PWM signal in several mode:
- audio playback
- real PWM
Contents
[hide]Hardware
PWM pin detail for each APF boards can be found here (under 'BaseBoards' section).
Linux driver installation
i.MX PWM driver is installed by default in standard Armadeus rootfs.
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Note: On APF51Dev, as PWM outputs are multiplexed and used as user LED and button by default, you may have to define APF51DEV_USE_LED_AS_PWM or APF51DEV_USE_SWITCH_AS_PWM in Linux sources (arch/arm/mach-mx5/apf51dev-baseboard.c), rebuild and reflash the Linux kernel. |
Usage
# modprobe imx-pwm i.MX PWM driver v0.81 imx-pwm imx-pwm.0: initialized
- On APF28 with 2.6.35 kernel, the PWM driver is builtin so no need to load it before use.
- For the one who plan to use sound, /dev/pwm0 device file should be automatically created.
PWM Mode
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Note: The PWM has a resolution of 1uS |
After driver loading, you have access to /sys/class/pwm/... functionalities:
- to change frequency (for example 500 Hz):
# echo 500 > /sys/class/pwm/pwm0/frequency
- if you prefer working with periods (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 $2 %"
exit 0
Audio Mode
Audio playback mode is working since v0.5 of PWM driver (thanks to SebastienR). You can play 8, 16 & 32 KHz 8bit linear RAW sound. It's recommended to play 8kHz one, as they are using less CPU resources. To convert a sound to the right format we recommend to use SoX (on Linux). For example:
[host]$ 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.
