REV. A

–14–

AD8801/AD8803

PSHY

PSHA

*

* Enter Contents of SDI1 Data Register

*

LDAA

$0000

Hi-byte data loaded from memory

STAA

SDI1

SDI1 = data in location 0000H

*

* Enter Contents of SDI2 Data Register

*

LDAA

$0001

Low-byte data loaded from memory

STAA

SDI2

SDI2 = Data in location 0001H

*

LDX

#SDI1

Stack pointer at 1st byte to send via SDI

LDY

#$1000

Stack pointer at on-chip registers

*

* Reset AD8801 to one-half scale (AD8803 does not have a Reset input)

*

BCLR

PORTC,Y $02

Assert /RS

BSET

PORTC,Y $02

De-assert /RS

*

* Get AD8801/03 ready for data input

*

BCLR

PORTD,Y $20

Assert /CS

*

TFRLP

LDAA

0,X

Get a byte to transfer via SPI

STAA

SPDR

Write SDI data reg to start xfer

*

WAIT

LDAA

SPSR

Loop to wait for SPIF

BPL

WAIT

SPIF is the MSB of SPSR

*

(when SPIF is set, SPSR is negated)

INX

Increment counter to next byte for xfer

CPX

#SDI2+1

Are we done yet ?

BNE

TFRLP

If not, xfer the second byte

*

* Update AD8801 output

*

BSET

PORTD,Y $20

Latch register & update AD8801

*

PULA

When done, restore registers X, Y & A

PULY

PULX

RTS

** Return to Main Program **

Listing 3. AD8801/AD8803 to MC68HC11 Interface Program Source Code