Interfacing of ADC 0808 with 8085

Interfacing of ADC 0808 with 8085

Fig. 13.6.1 shows interfacing of ADC 0808 with 8085 using 8255. ADC 0808 has eight input channels, so to select desired input channel, it is necessary to send 3-bit address on ADC, ADB and ADA inputs. The address of the desired channel is sent to the multiplexer address inputs through PB₀ to PB₂ lines.

After at least 50 ns, this address must be latched. This is achieved by sending ALE signal using port line PB3. After another 2.5 ,us the start of conversion signal (SOC) must be sent high and then low. Port line PB4 is used for this purpose. The end of conversion (EOC) is detected by port line PC0. The Fig. 13.6.2 shows timing waveforms for the ADC 0808. Schmitt-trigger inverter circuitry generates 300 kHz clock which is required to operate ADC 0808. The zener diode and LM 308 amplifier circuitry is used to produce a V_CC and and + V_REF of 5.12 V for the A/D converter. With this reference voltage the A/D converter will have 256 steps of 20 mV each. (See Fig. 13.6.2 )

I/O Map

Control Word

Routine does the conversion and reads the digital data

MVI A, 99H ; initialize 8255

OUT CR

MVI A, 00H

OUT PB ; Send address to select channel 0

MVI A, 08H

OUT PB ; Latch the given address by sending ALE high

Call Delay ; Wait for at least 2.5 µs

MVI A, 18H

OUT PB ; Make SOC high

NOP  ; Wait for at least 05 µs

MVI A, 08H

OUT PB ; Make SOC low

CHECK : IN PC ; Check for EOC

ANI 01

JZ CHECK ; If no, repeat

IN PA ; Read digital input

RET ; Return

Review Questions

1. Explain the interfacing of A/D converter with microprocessor.

2. Interface a ADC chip with 8085 processor through 8255 ports and write and ALP to use BSR mode to START conversion and STATUS CHECK mode to read output data. Explain the complete circuit and programs. Use I/O mapped I/O configuration. AU : May-04, Marks 16

3. With neat diagram explain the ADC interface to microprocessor.

AU : Dec.-04,10,11, May-08,11, Marks 16

4. Using peripheral mapped I/O, design an interface circuit to connect an ADC 0808 to a microprocessor. AU : May-05, Marks 6

5. Explain how 8085 can be connected to an A/D convertor. Describe the signals involved in the process of conversion.

AU: Dec.-05, Marks 12

6. What are the steps involved in interfacing an ADC with 8085.

AU : Dec.-09, Marks 4

7. Show and explain the ADC interfacing with 8085 microprocessor.

AU May-11, Marks 8

8. Why do we need A/D converter and D/A converter? Draw the block diagram to interface 8085 microprocessor with A/D convertor and D/A convertor.

 AU: Dec.-11, Marks 8

9. Draw and describe the interfacing of A/D and D/A converter interfacing to 8085 microprocessor.

AU: May-12, Marks 8