;Testing HCS12 MCU Module (from Freescale or Axiom) by Toggling PORTB in Assembly Language 
 
;The HCS12 MCU Modules can be used with Freescale Student Project Board and they are made by Axiom. 
;They can also be used as stand-alone without the Project Board. This Assembly tests the MCU Module without the Project Board
;The HCs12 MCU Modules are either C128 (HCSxxx128), DT256 (HCSxxx256), or XDT512 (HCSxxx512). See the designation on your Module.
;These HCS12 MCU Module comes with BDM and ready to work with CodeWarrior(only). Connect the module to the x86 PC USB port via a USB cable.
;The MCU module has 4-bit green LEDs and are located between Brown POT and  Blue DIP SW. 
;The LEDs are connected to PORTB7-PORTB4 via row of jumpers right in front of LEDs.
;Make sure the Jumpers are set for LEDs. They are designateed as LED1-LED4  
;Make sure the VB jumper for PWRr_SEL (next to power jack) is ALSO set. VB is the middle Jumper and allows the power come from USB cable (500 mA Maximum)

;WHEN USB CABLE IS THE EXCLEUSIVE PROVIDER OF POWER TO CPU MODULE, NEVER USE IT TO DRIVE POWER HUNGRY DEVICES SUCH AS DC OR STEPPER MOTORS

;In HCS12 MCU module RAM address is from $1000-3FFF 
;We use RAM addresses starting at $1000 for scratch pad (variables) and $3FFF for Stack

;Modified from an example from Chapter 4 of HCS12 textbook by Mazidi & Causey
;Written and tested by Mazidi 
    
 
;Make sure you are in TBDML Mode before downloading 
;Press F7 (to Make), then F5(Debug) to downLOAD,and F5 once more to start the program execution
   
        ABSENTRY Entry        ; for absolute assembly: mark this as application entry point
    
; Include derivative-specific definitions 
		INCLUDE 'mc9s12dt256.inc'     ;CPU used by DT256 Axiom MCU Module, Modify it for C128 or XDT512 Module
 
 
;----------------------USE $1000-$2FFF for Scratch Pad 
R1      EQU     $1001
R2      EQU     $1002
R3      EQU     $1003

;code section
        ORG   $4000     ;Flash ROM address 
Entry:
	      LDS     #$4000    ;Stack
        LDAA    #$F0
        STAA    DDRB		;PB7-PB4 output
  
;-------Toggling LEDs on PB7-PB4 of PORTB

BACK   LDAA #%01010000
    	 STAA PORTB		;Store A into PORTB
	     JSR DELAY
	     LDAA #%10100000
	     STAA PORTB
	     JSR DELAY

	      BRA BACK    
  
;----------DELAY
DELAY

        PSHA		;Save Reg A on Stack
        LDAA    #10		;Change this value to see  
        STAA    R3		;how fast LEDs Toggle
;--10 msec delay. The Axiom board works with XTAL=4MHz 
;Freq. for Instruction Clock Cycle is 2MHz (1/2 of 4MHz). 
;(1/2MHz) x 10 Clk x 100 x 20 = 10 msec. Overheads are excluded in this calculation.         
L3      LDAA    #10
        STAA    R2
L2      LDAA    #200
        STAA    R1
L1      NOP         ;1 Intruction Clk Cycle
        NOP         ;1
        NOP         ;1
        DEC     R1  ;4
        BNE     L1  ;3
        DEC     R2  ;Total Instr.Clk=10
        BNE     L2
        DEC     R3
        BNE     L3
;--------------        
        PULA			;Restore Reg A
        RTS
;-------------------

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