Note 18: The stack pointer must be dimensioned because a RST 7 instruction pushes the PC onto the stack.

Note 19: All address used were arbitrarily chosen.

The standard control bus signals of the 8080 CS, RD and

WR) can be directly wired to the digital control inputs of the

A/D and the bus timing requirements are met to allow both

starting the converter and outputting the data onto the data

bus. A bus driver should be used for larger microprocessor

systems where the data bus leaves the PC board and/or

must drive capacitive loads larger than 100 pF.

4.1.1 Sample 8080A CPU Interfacing Circuitry and

Program

The following sample program and associated hardware

shown in

Figure 12 may be used to input data from the

converter to the INS8080A CPU chip set (comprised of the

INS8080A microprocessor, the INS8228 system controller

and the INS8224 clock generator). For simplicity, the A/D is

controlled as an I/O device, specifically an 8-bit bi-directional

port located at an arbitrarily chosen port address, E0. The

TRI-STATE output capability of the A/D eliminates the need

for a peripheral interface device, however address decoding

is still required to generate the appropriate CS for the con-

verter.

It is important to note that in systems where the A/D con-

verter is 1-of-8 or less I/O mapped devices, no address

decoding circuitry is necessary. Each of the 8 address bits

(A0 to A7) can be directly used as CS inputs — one for each

I/O device.

4.1.2 INS8048 Interface

The INS8048 interface technique with the ADC0801 series

(see

Figure 13) is simpler than the 8080A CPU interface.

There are 24 I/O lines and three test input lines in the 8048.

With these extra I/O lines available, one of the I/O lines (bit

0 of port 1) is used as the chip select signal to the A/D, thus

eliminating the use of an external address decoder. Bus

control signals RD, WR and INT of the 8048 are tied directly

to the A/D. The 16 converted data words are stored at

on-chip RAM locations from 20 to 2F (Hex). The RD and WR

signals are generated by reading from and writing into a

dummy address, respectively. A sample interface program is

shown below.

SAMPLE PROGRAM FOR

Figure 12 ADC0801–INS8080A CPU INTERFACE

DS005671-99

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