WiringPi/wiringPi/softPwm.c

/*
 * softPwm.c:
 *	Provide many channels of software driven PWM.
 *	Copyright (c) 2012-2017 Gordon Henderson
 ***********************************************************************
 * This file is part of wiringPi:
 *	https://projects.drogon.net/raspberry-pi/wiringpi/
 *
 *    wiringPi is free software: you can redistribute it and/or modify
 *    it under the terms of the GNU Lesser General Public License as
 *    published by the Free Software Foundation, either version 3 of the
 *    License, or (at your option) any later version.
 *
 *    wiringPi is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Lesser General Public License for more details.
 *
 *    You should have received a copy of the GNU Lesser General Public
 *    License along with wiringPi.
 *    If not, see <http://www.gnu.org/licenses/>.
 ***********************************************************************
 */

#include <stdio.h>
#include <malloc.h>
#include <pthread.h>

#include "wiringPi.h"
#include "softPwm.h"

// MAX_PINS:
//	This is more than the number of Pi pins because we can actually softPwm.
//	Once upon a time I let pins on gpio expanders be softPwm'd, but it's really
//	really not a good thing.

#define	MAX_PINS	64

// The PWM Frequency is derived from the "pulse time" below. Essentially,
//	the frequency is a function of the range and this pulse time.
//	The total period will be range * pulse time in µS, so a pulse time
//	of 100 and a range of 100 gives a period of 100 * 100 = 10,000 µS
//	which is a frequency of 100Hz.
//
//	It's possible to get a higher frequency by lowering the pulse time,
//	however CPU uage will skyrocket as wiringPi uses a hard-loop to time
//	periods under 100µS - this is because the Linux timer calls are just
//	not accurate at all, and have an overhead.
//
//	Another way to increase the frequency is to reduce the range - however
//	that reduces the overall output accuracy...

#define	PULSE_TIME	100

static volatile int marks         [MAX_PINS] ;
static volatile int range         [MAX_PINS] ;
static volatile pthread_t threads [MAX_PINS] ;
static volatile int newPin = -1 ;


/*
 * softPwmThread:
 *	Thread to do the actual PWM output
 *********************************************************************************
 */

static void *softPwmThread (void *arg)
{
  int pin, mark, space ;
  struct sched_param param ;

  param.sched_priority = sched_get_priority_max (SCHED_RR) ;
  pthread_setschedparam (pthread_self (), SCHED_RR, &param) ;

  pin = *((int *)arg) ;
  free (arg) ;

  pin    = newPin ;
  newPin = -1 ;

  piHiPri (90) ;

  for (;;)
  {
    mark  = marks [pin] ;
    space = range [pin] - mark ;

    if (mark != 0)
      digitalWrite (pin, HIGH) ;
    delayMicroseconds (mark * 100) ;

    if (space != 0)
      digitalWrite (pin, LOW) ;
    delayMicroseconds (space * 100) ;
  }

  return NULL ;
}


/*
 * softPwmWrite:
 *	Write a PWM value to the given pin
 *********************************************************************************
 */

void softPwmWrite (int pin, int value)
{
  if (pin < MAX_PINS)
  {
    /**/ if (value < 0)
      value = 0 ;
    else if (value > range [pin])
      value = range [pin] ;

    marks [pin] = value ;
  }
}


/*
 * softPwmCreate:
 *	Create a new softPWM thread.
 *********************************************************************************
 */

int softPwmCreate (int pin, int initialValue, int pwmRange)
{
  int res ;
  pthread_t myThread ;
  int *passPin ;

  if (pin >= MAX_PINS)
    return -1 ;

  if (range [pin] != 0)	// Already running on this pin
    return -1 ;

  if (pwmRange <= 0)
    return -1 ;

  passPin = malloc (sizeof (*passPin)) ;
  if (passPin == NULL)
    return -1 ;

  digitalWrite (pin, LOW) ;
  pinMode      (pin, OUTPUT) ;

  marks [pin] = initialValue ;
  range [pin] = pwmRange ;

  *passPin = pin ;
  newPin   = pin ;
  res      = pthread_create (&myThread, NULL, softPwmThread, (void *)passPin) ;

  if (res != 0)
    return res ;
  
  while (newPin != -1)
    delay (1) ;

  threads [pin] = myThread ;

  return res ;
}


/*
 * softPwmStop:
 *	Stop an existing softPWM thread
 *********************************************************************************
 */

void softPwmStop (int pin)
{
  if (pin < MAX_PINS)
  {
    if (range [pin] != 0)
    {
      pthread_cancel (threads [pin]) ;
      pthread_join   (threads [pin], NULL) ;
      range [pin] = 0 ;
      digitalWrite (pin, LOW) ;
    }
  }
}