AD573

–8–

It is also possible to write a faster-executing assembly-language

routine to control the AD573. Such a routine will require a de-

lay between starting and reading the converter. This can be eas-

ily implemented by calling the Apple’s WAIT subroutine (which

resides at location $FCA8) after loading the accumulator with a

number greater than or equal to two.

8085-Series Microprocessor Interface

The AD573 can also be used with 8085-series microprocessors.

These processors use separate control signals for RD and WR,

as opposed to the single R/W control signal used in the 6800/

6500 series processors.

There are two constraints related to operation of the AD573

with 8085-series processors. The first problem is the width of

the CONVERT pulse. The circuit shown in Figure 17 (essen-

tially the same as that shown in Figure 13) will produce a wide

enough CONVERT pulse when the 8085 is running at 5 MHz.

For 8085 systems running at slower clock rates (3 MHz), the

flip-flop-based circuit can be eliminated since the WR pulse will

be approximately 500 ns wide.

The other consideration is the access time of the AD573’s three-

state output data buffers, which is 250 ns maximum. It may be

necessary to insert wait states during RD operations from the

AD573. This will not be a problem in systems using memories

with comparable access times, since wait states will have already

been provided in the basic system design.

Figure 17. AD573–8085A Interface Connections

The following assembly-language subroutine can be used to

DE register pair.

ADC:

LXI H, 3000 ; LOAD HL WITH AD573 ADDRESS

MOV M, A

; START CONVERSION

MVI B, 06

; LOAD DELAY PERIOD

LOOP: DCR B

; DELAY LOOP

JNZ LOOP

;

MOV A, M

; READ LOW BYTE

ANI C0

; MASK LOWER 6 BITS

MOV E, A

; STORE CLEAN LOW BYTE IN E

INR L

; LOAD HIGH BYTE ADDRESS

MOV D, M

; MOVE HIGH BYTE TO D

RET

; EXIT

REV. B