Added software PWM module into wiringPi - library code

and an example.

wiringPi/softPwm.c

@@ -0,0 +1,116 @@
+/*
+ * 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 ;
+}
+
+void softPwmWrite (int pin, int value)
+{
+  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 ;
+}