Interface a suitable DAC using 8255 PPI to a 8085 microprocessors to generate a square wave oscillator between 0V and -5V having a frequency of 1 KHz. Show the interfacing circuit and the necessary program.

The necessary interfacing circuit is drawn below using a 8255 PPI and a 1408 DAC. The circuit outputs 0V for 10000000=10H while -5v for 00000000=00H DAC output.

The program is as follows:
 PORT A ADDRESS  0000H
CONTROL REGISTER ADDRESS  0003H
CONTROL WORD   10010000=90H
ASSUMPTION  8085 CLOCK FREQUENCY OF 1 MHz
8085 CODE:

MVI B,46H        ;     counter initialization(46h=70)  

MVI A, 90H

STA 0003H

MVI A,OOH

STA 0000H

CALL DELAY

MVI A,10H

STA,0000H

CALL DELAY

HLT

DELAY  :      DCR B                              ;     a 1ms time delay subroutine

                         JNZ DELAY

                        RET