Write a program to echo the string 'Hello' to the serial channel (SCI) using the protocol of baud rate 9600,8 bits , no parity and 1 stop bit 

Consider the baud register as shown below

Using a combination of bits5,4,2,1,0 we can set up the required baud rate .On the target system provided the input clock crystal is a 8Mhz (2Mhz internal E) version , therefore for a baud rate of 9600 is the easiest one

      Bit 5= 1 Bit 4 = 1 Bit 2 = 0 Bit 1 = 0 Bit 0 = 0

i.e. (2Mhz / 13) = 9600 *8 bits per second i.e. 9600 Baud

          Baud = 0x30

The bit length mode is set by bit 4 of the SCCR1 as shown below

    Bit = 0 'Start bit 8 data bits 1 stop bit'
    Bit = 1 'Start bit 9 data bits 1 stop bit '

Therefore bit 4 of sccr1 = 0    Sccr1 = 0x00

The SCI control 2 enables us to set interrupts if data is received or can be transmitted; in this case we simply want to enable the transmitter and receiver  
      
Therefore

 
    TE Bit 3, 2= 1
 
    Sccr2 = 0x0c 

This completes the SCI set-up, the remaining register is useful in determining whether data is received or can be transmitted

We can simply check bit7  if this is '1' we can send more data ,Since the configuration is actually set-up on the development boards we do not need to re-program the baud rates but simply use the comms channel. Therefore the a simple hello display program follows

  name ex6  ;Use of the uart to echo Hello ten times
baud  equ $2b ;Baud rate  register
sccr1  equ $2c  ;SCI control 1
sccr2  equ $2d ;SCI control 2
scsr  equ $2e  ;SCI status
scdr  equ $2f  ;Serial data register
  org $500  ;Start on external ram
;Note that the uart is configured by the monitor
;the following codes are used for eprom use
; sccr1 = 0 hex (No 9th bit)
;baud = 30 Hex (9600 using  8Mhz xtal )
; sccr2 = 0c hex (Enable tx)
  ldx #$0a  ;load up Counter with 10
loop  ldaa #'H'  ;load up ASCII H
  jsr txd    ;Send data
  ldaa #'e'  ;Load up ASCII e
  jsr txd    ;Send data
  ldaa #'l' ;Load up ASCII l
  jsr txd    ;Send data
  ldaa #'l' ;Load up ASCII l
  jsr txd    ;Send data
  ldaa #'o'  ;Load up ASCII o
  jsr txd    ;Send data
  ldaa #$0a  ;Load up ASCII newline
  jsr txd    ;Send data
  ldaa #$0d  ;Load up ASCII return
  jsr txd    ;Send data
  dex    ;X=X-1
  bne loop  ;repeat till zero
  swi    ;return back to monitor
txd  ldab scsr  ;Get sci status
  andb #$80  ;Mask off bit 7 TDRE
  beq txd  ;Wait till ready to transmit
  staa scdr  ;send data from A out
  ldy #$ff  ;Software time delay
loop1  dey    ;instead of XON/XOFF protocol
  bne loop1  ;wait till y = 0
  rts
  end