AD420

Rev. H | Page 12 of 16

MICROPROCESSOR INTERFACE

AD420-TO-MC68HC11 (SPI BUS) INTERFACE

The AD420 interface to the Motorola serial peripheral interface

(SPI) is shown in Figure 11. The MOSI, SCK, and SS pins of the

HC11 are respectively connected to the DATA IN, CLOCK, and

LATCH pins of the AD420. The majority of the interfacing

issues are done in the software initialization. A typical routine,

such as the one shown below, begins by initializing the state of

the various SPI data and control registers.

INIT

LDAA

#$2F

; SS = 1; SCK = 0;

MOSI = 1

STAA

PORTD

;SEND TO SPI OUTPUTS

LDAA

#$38

; SS, SCK, MOSI =

OUTPUTS

STAA

DDRD

;SEND DATA DIRECTION

INFO

LDAA

#$50

;DABL INTRPTS, SPI

IS MASTER & ON

STAA

SPCR

;CPOL = 0, CPHA = 0,

1MHZ BAUDRATE

NEXTPT

LDAA

MSBY

;LOAD ACCUM W/UPPER

8 BITS

BSR

SENDAT

;JUMP TO DAC OUTPUT

ROUTINE

JMP

NEXTPT

;INFINITE LOOP

SENDAT

LDY

#$1000

;POINT AT ON-CHIP

REGISTERS

BCLR

$08,Y,$20

;DRIVE SS (LATCH)

LOW

STAA

SPDR

;SEND MS-BYTE TO SPI

DATA REG

WAIT1

LDAA

SPSR

;CHECK STATUS OF

SPIE

BPL

WAIT1

;POLL FOR END OF X-

MISSION

LDAA

LSBY

;GET LOW 8 BITS FROM

MEMORY

STAA

SPDR

;SEND LS-BYTE TO SPI

DATA REG

WAIT2

LDAA

SPSR

;CHECK STATUS OF

SPIE

BPL

WAIT2;

;POLL FOR END OF X-

MISSION

BSET

$08,Y,$20

;DRIVE SS HIGH TO

LATCH DATA

RTS

The SPI data port is configured to process data in 8-bit bytes.

The most significant data byte (MSBY) is retrieved from

memory and processed by the SENDAT routine. The SS pin is

driven low by indexing into the PORTD data register and clear

Bit 5. The MSBY is then sent to the SPI data register where it is

automatically transferred to the AD420 internal shift resister.

The HC11 generates the requisite eight clock pulses with data

valid on the rising edges. After the MSBY is transmitted, the

least significant byte (LSBY) is loaded from memory and

transmitted in a similar fashion. To complete the transfer, the

LATCH pin is driven high when loading the complete 16-bit

word into the AD420.

DATA IN

CLOCK

LATCH

MOSI

SCK

SS

68HC11

AD420

Figure 11. AD420-to-68HC11 (SPI) Interface

AD420 TO MICROWIRE INTERFACE

The flexible serial interface of the AD420 is also compatible

with the National Semiconductor MICROWIRE interface. The

MICROWIRE interface is used in microcontrollers such as the

COP400 and COP800 series of processors. A generic interface

to use the MICROWIRE interface is shown in Figure 12. The

G1, SK, and SO pins of the MICROWIRE interface are

respectively connected to the LATCH, CLOCK, and DATA IN

pins of the AD420.

DATA IN

CLOCK

LATCH

SO

SK

G1

MICROWIRE

AD420

Figure 12. AD420-to-MICROWIRE Interface