Software PWM Library

WiringPi includes a software-driven PWM handler capable of outputting a PWM signal on any of the Raspberry Pi’s GPIO pins.

There are some limitations… To maintain a low CPU usage, the minimum pulse width is 100μS. That combined with the default suggested range of 100 gives a PWM frequency of 100Hz. You can lower the range to get a higher frequency, at the expense of resolution, or increase to get more resolution, but that will lower the frequency. If you change the pulse-width in the driver code, then be aware that at delays of less than 100μS wiringPi does it in a software loop, which means that CPU usage will rise dramatically, and controlling more than one pin will be almost impossible.

Also note that while the routines run themselves at a higher and real-time priority, Linux can still affect the accuracy of the generated signal.

However, within these limitations, control of a light/LED or a motor is very achievable.

To use:

#include <wiringPi.h>
#include <softPwm.h>

When compiling your program you must include the pthread library as well as the wiringPi library:

cc -o myprog myprog.c -lwiringPi -lpthread

You must initialise wiringPi with one of wiringPiSetup(),  wiringPiSetupGpio() or wiringPiSetupPhys() functions. wiringPiSetupSys() is not fast enough, so you must run your programs with sudo.

Some expansion modules may also be fast enough to handle software PWM – it has been tested with the MCP23S17 GPIO expander on the PiFace for example.

The following two functions are available:

  • int softPwmCreate (int pin, int initialValue, int pwmRange) ;

This creates a software controlled PWM pin. You can use any GPIO pin and the pin numbering will be that of the wiringPiSetup() function you used. Use 100 for the pwmRange, then the value can be anything from 0 (off) to 100 (fully on) for the given pin.

The return value is 0 for success. Anything else and you should check the global errno variable to see what went wrong.

  • void softPwmWrite (int pin, int value) ;

This updates the PWM value on the given pin. The value is checked to be in-range and pins that haven’t previously been initialised via softPwmCreate will be silently ignored.

Notes

  • Each “cycle” of PWM output takes 10mS with the default range value of 100, so trying to change the PWM value more than 100 times a second will be futile.
  • Each pin activated in softPWM mode uses approximately 0.5% of the CPU.
  • There is currently no way to disable softPWM on a pin while the program in running.
  • You need to keep your program running to maintain the PWM output!