5962-8850501RA Datasheet, PDF (8/10 Pages) Analog Devices – 10-Bit A/D Converter

AD573
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
control an AD573 residing at memory locations 3000_H and
3001_H. The 10 bits of data are returned (left-justified) in the
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
–8–
REV. B