WiringPi/wiringPi/softPwm.c

/*
 * softPwm.c:
 *	Provide 2 channels of software driven PWM.
 *	Copyright (c) 2012 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 <pthread.h>

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

#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 uS, so a pulse time
//	of 100 and a range of 100 gives a period of 100 * 100 = 10,000 uS
//	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 100uS - this is because the Linux timer calls are just
//	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 int marks [MAX_PINS] ;
static int range [MAX_PINS] ;

int newPin = -1 ;


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

static PI_THREAD (softPwmThread)
{
  int pin, mark, space ;

  pin    = newPin ;
  newPin = -1 ;

  piHiPri (50) ;

  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)
{
  pin &= 63 ;

  /**/ if (value < 0)
    value = 0 ;
  else if (value > range [pin])
    value = range [pin] ;

  marks [pin] = value ;
}


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

int softPwmCreate (int pin, int initialValue, int pwmRange)
{
  int res ;

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

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

  newPin = pin ;
  res = piThreadCreate (softPwmThread) ;

  while (newPin != -1)
    delay (1) ;

  return res ;
}