chap1.asm

Motorola 6808嵌入式接口设计开发原程序

; Chapter 1 6808 assembly language programs
; Jonathan W. Valvano
; This software accompanies the book,
; Embedded Microcomputer Systems: Real Time Interfacing
; published by Brooks Cole, 1999
;

; Program 1.10. Assembly definitions of the MC68HC708XL36 I/O ports.
PORTA equ  $0000  8-bit bidirectional I/O port
PORTB equ  $0001  8-bit bidirectional I/O port
PORTC equ  $0002  8-bit bidirectional I/O port
PORTD equ  $0003  8-bit bidirectional I/O port
DDRA  equ  $0004  specifies whether each bit  is input(0) or output(1)
DDRB  equ  $0005  specifies whether each bit is input(0) or output(1)
DDRC  equ  $0006  specifies whether each bit is input(0) or output(1)
DDRD  equ  $0007  specifies whether each bit is input(0) or output(1)
PORTE equ  $0008  8-bit bidirectional I/O port
PORTF equ  $0009  6-bit bidirectional I/O port
PORTG equ  $000A  4-bit bidirectional I/O port
PORTH equ  $000B  4-bit bidirectional I/O port
DDRE  equ  $000C  specifies whether each bit is input(0) or output(1)
DDRF  equ  $000D  specifies whether each bit is input(0) or output(1)
DDRG  equ  $000E  specifies whether each bit is input(0) or output(1)
DDRH  equ  $000F  specifies whether each bit is input(0) or output(1)
TSC   equ  $0021  Timer status and control
TCNT  equ  $0022  16-bit up counter incremented every clock period

; Program 1.14. Assembly software that reads from port A and writes to port B.
; Program to implement a NOT gate
; Port A are the 8 digital inputs and Port B are the 8 digital outputs
; Software continuously repeats the following
; 1) Read value from Port A
; 2) Calculate the logical complement
; 3) Write the result out to Port B
NotGate lda  PORTA   Read from Port A into Register A
        coma         Logical complement
        sta  PORTB   Write from Register A to Port B
        bra  NotGate

; Program 1.16. Assembly software that initializes an I/O port to input.
; MC68HC708XL36
    clr  DDRA

; Program 1.18. Assembly software that reads from an I/O port input.
; MC68HC708XL36
    lda  PORTA
    sta  Happiness

; Program 1.20. Assembly software that initializes an I/O port to output.
; MC68HC708XL36
    lda  #$0F
    sta  DDRA

;Program 1.22. Assembly software that outputs an I/O port output.
; MC68HC708XL36
    lda  #$FF
    sta  PORTA

; Program 1.24. Assembly subroutines that provide initialization, read and write access an I/O port.
; MC68HC708XL36
Init sta  DDRA
     rts
Set  sta  PORTA
     rts
Read lda  PORTA
     rts

; Program 1.26. Assembly subroutine that initializes the I/O port.
; MC68HC708XL36
PORTC equ $0002 ;I/O port
DDRC  equ $0006  
init  mov #$F0,DDRC
      rts

; Program 1.27. Assembly program that implements the 4 bit NOT gate.
; MC68HC708XL36
     org  $6E00 ;EEPROM
main rsp        ;SP=$00FF
     bsr  init  ;ritual
loop lda  PORTC ;input
     coma       ;logical not
     nsa        ;swap 7-4, 3-0
     sta  PORTC ;output
     bra  loop  ;repeat
     org  $FFFE
     fdb  main  ;reset vector