;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 17-13 Mazidi & Causey HCS12 book. See Chapter 17 for PWM discussion
;Written and tested by M. Mazidi

   
        ABSENTRY Entry        ; for absolute assembly: mark this as application entry point
    
; Include derivative-specific definitions 
		INCLUDE 'mc9s12dp256.inc'     ;CPU used by Dragon12+ board
 


;code section
        ORG   $4000     ;Flash ROM address for Dragon12+
Entry:
	      LDS     #$4000    ;Stack
       
        MOVB #$04, PWMPRCLK	 ;ClockA=Fbus/2**4=24MHz/16=1.5MHz
	      MOVB #125, PWMSCLA	 ;ClockSA=1.5MHz/2x125=1.5Mhz/250=6000Hz
	      MOVB #$10, PWMCLK	   ;Use clockSA for chan 4 PWM
	      MOVB #$10,PWMPOL		 ;High then low for polarity
	      MOVB #$0,PWMCAE		   ;Left aligned
	      MOVB #$0, PWMCTL	   ;8-bit chan,PWM during feeze and wait
	      MOVB #100, PWMPER4	 ;PWM_Freq=ClockSA/100=6000Hz/100=60 Hz. CHANGE THIS  
	      MOVB #50, PWMDTY4		 ;50% duty cycle   AND THIS TO SEE THE EFFECT ON SERVO MOTOR (TRY LESS THAN 10%)
	      MOVB #0,PWMCNT4		   ;Start the counter with zero (optional)
	      BSET PWME,%00010000  ;Enable chan 4 PWM
	      BRA $

        
            
;**************************************************************
;*                 Interrupt Vectors                          *
;**************************************************************
            ORG   $FFFE
            DC.W  Entry     ;Reset Vector. CPU wakes here and it is sent to start of the code at $4000