ASSIGNMENT 3: PLAYING WITH THE 68HC11

Due Wednesday, Oct 9, 2002

Introduction

In this assignment, you will (1) construct a minimal 68HC11 system on your breadboard, and (2) use the serial port of a computer to load programs into the HC11’s internal 512 bytes of RAM (making use of the serial bootstrap feature of the CPU).

Initially, you will load a known-working program into the HC11 to verify that your wiring setup and serial line configuration is working. Afterward, you will modify and write new programs for the HC11 and use your setup to load them into the HC11 for testing and debugging.

Hardware Setup: HC11 Wiring

The drawing above shows how the 68HC11 chip should be wired. Here are a few notes of interest:

• Power is applied via Vdd (pin 48); ground is applied at Vss (pin 23).
  
• The pins ~RESET, ~XIRQ, and ~IRQ must all be tied high.
  
• The analog input references should be connected: Vrl to ground, and Vrh to +5v.
  
• The MODA (pin 25) and MODB (pin 24) lines must be tied to ground. This put the HC11 into the serial bootstrap mode when it “wakes up.”
  
• There is a 1K pullup resistor on the HC11’s TxD output. This is necessary because when the HC11 starts up in bootstrap, the pin can only assert ground, and must be pulled high to produce a logic one (“wired OR mode”).
  
• The ceramic resonator is connected to the XTAL and EXTAL lines. The outer two pins of the resonator go to XTAL and EXTAL (either way—there’s no polarity). The center pin of the resonator goes to ground.
  
• You must add a 10K resistor in parallel with the outer two pins of the resonator to get the oscillation frequency to stabilize.
  

When wiring up the power busses, make sure to insert one 0.1µF capacitor (they’re marked “104”) into each power-ground bus. The diagram above illustrates this. The purpose of these capacitors is to smooth out fluctuations in the power supply, making sure the HC11 operates properly.

Software Setup

Loading the boot program into the HC11 consists of:

1. Turn on the HC11 with MODA and MODB held low. Your circuit (if wired properly) does this. The chip automatically powers on in serial boot mode.
   
2. Open a serial connection with parameters 1200 baud, no parity, 8 data bits, 1 stop bit (1200-N-8-1).
   
3. Write a 255 out the serial port. When received by the HC11, this is used to detect the baud rate.
   
4. Write 512 bytes of data out the serial port. These data are loaded into the HC11’s internal RAM, and then execution automatically jumps to address 0.
   

        ldaa 0x10
        staa 0x1000
loop:   jmp loop

;;; beep.s
;;; toggles Port A4 with delay loops between each toggle,
;;; creating an oscillation in the audible range.

        ldx #0x1000             ; point at register base
loop:   bset 0,x,#0x10          ; set bit 4 in PORTA register
declp1: deca                    ; decrement reg A
        bne declp1              ;   and loop till it hits zero.
        bclr 0,x,#0x10          ; clear bit 4 of PORTA
declp2: deca                    ; decrement reg A
        bne declp2              ;   till it hits zero.
        bra loop                ; now recycle at the beginning.

;;; serialxmit.s
;;; counts from 0x21 to 0x5a on 68HC11 serial port and then repeats

base=   0x1000
scsr=   0x102e                  ; serial comms status reg
scdr=   0x102f                  ; serial comms data reg

        ldx #base               ; pointer to register base
loop:   ldaa #0x21              ; start with ASCII 0x21 to transmit
xmit:   staa <scdr,x            ; transmit reg a
        brclr <scsr,x,#0x40,.   ; loop here until byte transmitted
        inca                    ; inc char to be transmitted
        cmpa #0x5B              ; see if it's beyond ASCII 0x5a
        bne xmit                ; no, send the next one
        bra loop                ; reset char to 0x21

!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVW
XYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRST
UVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGHIJKLMNOPQ
RSTUVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGHIJKLMN
OPQRSTUVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGHIJK
LMNOPQRSTUVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDEFGH
IJKLMNOPQRSTUVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@ABCDE
FGHIJKLMNOPQRSTUVWXYZ!-#$%&'()*+,–./0123456789:;<=>?@AB
CDEFGHIJKLMNOPQRSTUVWXYZ

• Connect the eight Port C pins to your switch bank (pushbuttons and slide switches), so you can readily generate different values on Port C.
  
• The Port C register defaults as inputs in the HC11’s bootstrap mode, so you can simply read the PORTC register (no other set-up is required) to read the pins.
  
• There is a sample ASCII table on page 58 of the Motorola M68HC11ERG document.
  
• Converting the Port C value to hexadecimal is relatively straightforward. You convert the high “nybble” (upper four bits) into the correct ASCII digit from 0 to 9 or A to F, send it out, and then convert the lower nybble and set it out.
  
• Masking operations (e.g., “ANDA #0xF0”) and bit shift operations (e.g., “LSRA”) should be helpful.
  
• Don’t read the Port C register twice, first transforming the high nybble and then reading it again and transforming the low nybble! This could introduce errors if the Port C value changed between processing of the high nybble and the low nybble. Read it once and cache it between these two operations.
  
• You may wish to solve this problem incrementally rather than writing the whole entire code and then trying to get it to work! E.g., get the high-nybble conversion working and printing, then do the low nybble.