Using a PCA9685 I2C 16 PWM lines generator with Roadrunner board

The PCA9685 is an I2C-bus controlled 16-channel LED controller optimized for Red/Green/Blue/Amber (RGBA) color backlighting applications. Each LED output has its own 12-bit resolution (4096 steps) fixed frequency individual PWM controller that operates at a programmable frequency from a typical of 24 Hz to 1526 Hz with a duty cycle that is adjustable from 0 % to 100 % to allow the LED to be set to a specific brightness value. All outputs are set to the same PWM frequency.

Linux config

-> Device Drivers                                                                                          │  
    -> Pulse-Width Modulation (PWM) Support (PWM [=y])
        <*>   NXP PCA9685 PWM driver 

Device tree

0x70 is the default address of the Mikroe click board

i2c0: i2c@f8028000 {
    dmas = <0>, <0>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_i2c0_default>;
    i2c-sda-hold-time-ns = <350>;
    status = "okay";

    pca9685-0@70 {
        compatible = "nxp,pca9685-pwm";
        reg = <0x70>;
    };

};

User space sysfs interface

If the chip is probed this directory will be created

/sys/class/pwm/pwmchip0

For any PWM line you want to use you have to export it.

echo 0 > /sys/class/pwm/pwmchip0/export
echo 1 > /sys/class/pwm/pwmchip0/export
echo 2 > /sys/class/pwm/pwmchip0/export
echo 3 > /sys/class/pwm/pwmchip0/export

To use again that line ad generic GPIO you can unexport it:

echo 0 > /sys/class/pwm/pwmchip0/unexport
echo 1 > /sys/class/pwm/pwmchip0/unexport
echo 2 > /sys/class/pwm/pwmchip0/unexport
echo 3 > /sys/class/pwm/pwmchip0/unexport

For any exported channel a directory called pwmX wil be created with the following structure:

/sys/class/pwm/pwmchip0/pwmX/
      |-- duty_cycle (r/w) duty cycle (in nanoseconds)
      |-- enable     (r/w) enable/disable PWM
      |-- period     (r/w) period (in nanoseconds)
      |-- polarity   (r/w) polarity of PWM
      |-- power
      `-- uevent

The follow example illustrate how enable a PWM signale with a period of 1mS with a 0.5mS of duty cycle:

echo 1000000 > /sys/class/pwm/pwmchip0/pwm0/period
echo 500000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable

Pin used on Roadrunner

The I2C port used is i2c-0

PIN_PD21  TWD0    Data
PIN_PD22  TWCK0   Clock

Links

Sergio Tanzilli
System designer and Acme co-founder
Personal email: tanzilli@acmesystems.it
Webpages: https://www.acmesystems.it - https://www.tanzolab.it - http://www.tanzilli.com
Github repositories: https://github.com/tanzilli and https://github.com/acmesystems