Write a program to echo the string, Computer Engineering

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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

2169_Write a program to echo the string.png

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

1494_Write a program to echo the string1.png

 

    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

2071_Write a program to echo the string2.png

 
    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

1310_Write a program to echo the string3.png

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


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