//Controlling Servo Motor with PWM (pulse width modulation) on Dragon12 Plus Trainer Board 
//On Dragon12 board we have connections for four servo motors. They are located at the top middle of the board
//They are designated as PP4, PP5, PP6, and PP7. Next to them is the blue terminal block (T7) for external power connection for servo motors
//Next to the blue terminal block (T7) is the jumper to select power source for servo motor (SERVO_PWR J35)
//We can use on-board power or bring in an external power source for the servo motor. 
//Depending on the size of the servo motor you might have to connect the blue T7 terminal block to external power (5-6V) .
//and set the SERVO_PWR jumper J35 to right (EXT). If you use on-board power set the jumper J35 to left.    

//The servo motros have 3 leads. They are Black (ground), Red(power) and White for PWM control pulse.
//See http://www.parallax.com/Store/Robots/AllRobots/tabid/755/CategoryID/2/List/0/SortField/0/Level/a/ProductID/488/Default.aspx
//On Dragon12 board the PP4 has 3 pins. The left one is for PWM motor control, the middle one is the power, and the right one is for ground.
//The same is true for the other three servo motro connections of PP5,PP6 and PP7
//So when you connect a servo motor to PP4 (or PP5, or PP6,..) make sure the white lead of the servo motor is on the left connection.

//This program uses the PWM of chan 4 (PP4) to control servo motor. 
//Modified from Example 17-16 Mazidi & Causey HCS12 book. See Chapter 17 for PWM discussion
//Written and tested by M. Mazidi
 
#include <hidef.h>      /* common defines and macros */
#include "derivative.h"      /* derivative-specific definitions */


void main(void) 
{
  /* put your own code here */
  
  PWMPRCLK=0x04; //ClockA=Fbus/2**4=24MHz/16=1.5MHz	
	PWMSCLA=125; 	 //ClockSA=1.5MHz/2x125=6000 Hz
	PWMCLK=0b00010000; 	 //ClockSA for chan 4
	PWMPOL=0x10; 		     //high then low for polarity
	PWMCAE=0x0; 		     //left aligned
	PWMCTL=0x0;	         //8-bit chan, PWM during freeze and wait
	PWMPER4=100; 	 //PWM_Freq=ClockSA/100=6000Hz/100=60Hz. CHANGE THIS
	PWMDTY4=50; 	 //50% duty cycle           AND THIS TO SEE THE EFFECT ON MOTOR. TRY LESS THAN 10%
	PWMCNT4=10;		 //clear initial counter. This is optional
	PWME=0x10; 	   //Enable chan 4 PWM
  while(1);      //stay here forever
  
 
}