1. Review 8255: 8255 PPI Various Modes Of Operation Interfacing To 8086.
Interfacing Techniques:
D/A And A/D Converter Interfacing.
Stepper Motor

2. 8255Programmable Peripheral Interface(PPI)
For most of the applications 8086 needs parallel input ,output data to/from the peripherals.
8255 is a device used to interface different real time peripherals to the microprocessors which can be initialized for different modes operations like simple I/O, Strobed I/O and handshaking I/O.
It has three 8-bit parallel ports :port A, port B, port C

3. Fig pin diagram

4. 8255 has three operation modes: mode 0, mode 1, and mode 2
Fig 4.3 Control Word Register Bit Format of 8255 for I/O mode and BSR mode

5. Fig 4.4 Input and Output control signals of 8255 in mode1
Programming 8255
Mode 0:
Ports A, B, and C can be individually programmed as input or output ports
Port C is divided into two 4-bit ports which are independent from each other
Mode 1:
Ports A and B are programmed as input or output ports
Port C is used for handshaking
PA[7:0]
STBA
IBFA
INTRA
PC3
PC5
PC4
PB[7:0]
STBB
IBFB
INTRB
PC0
PC1
PC2
PC6, 7
8255
OBFA
ACKA
PC6
PC7
OBFB
ACKB
PC4, 5
Fig 4.4 Input and Output control signals of 8255 in mode1

7. Fig 4.5 Input and Output control signals of 8255 in mode1

8. Fig 4.6 Input and Output control signals of 8255 in mode2
Programming 8255
Mode 2:
Port A is programmed to be bi-directional
Port C is for handshaking
Port B can be either input or output in mode 0 or mode 1
PA[7:0]
OBFA
ACKA
INTRA
PC4
PC6
PC7
STBA
IBFA
PC0
PC3
PC5
8255
PB[7:0]
In Out
Mode 0
STBB OBFB
IBFB ACKB
INTRB INTRB
Mode 1
Fig 4.6 Input and Output control signals of 8255 in mode2

9. Fig 4.7 Input and Output control signals of 8255 in mode2

10. Fig 4.8 Interfacing of 8255 to Keyboard for input in mode1
Example: Mode 1 Input
BIT5 EQU 20H
PORTC EQU 22H
PORTA EQU 20H
READ PROC NEAR
Read:
IN AL, PORTC ; read portc
TEST AL, BIT5 ;test IBF
JZ Read ;if IBF=0
IN AL, PORTA ;Read Data
READ ENDP
keyboard
8255
PA0
PA7
STB
PC4
DAV
Fig 4.8 Interfacing of 8255 to Keyboard for input in mode1

11. Fig 4.9 Interfacing of 8255 to Printer for output in mode1
Example: Mode 1 output
Printer
8255
PB0
PB7
ACK
PC2
ACK
PC4 DS
Fig 4.9 Interfacing of 8255 to Printer for output in mode1

12. ADC/DAC Interfacing
8086 MP
DAC – IC 0800
ADC – IC 0808

14. ADC Interfacing
The time taken by the ADC from the active edge of SOC pulse till the active edge of EOC signal is called as the conversion delay of the ADC.
General algorithm for ADC interfacing contains the following steps
Ensure the stability of analog input, applied to the ADC
Issue start of conversion SOC pulse to ADC
Read end of conversion EOC signal to mark the end of conversion process
Read digital data output of the ADC as equivalent digital output.

15. ADC 0808/0809
The analog to digital converter chips 0808 and are 8-bit CMOS, successive approximation converters. It is fastest technique.
The conversion delay is 100 µs at a clock frequency of 640 kHz, which is quite low as compared to other converters.
Block Diagram of ADC 0808/0809
This converter internally has a 3:8 analog multiplexer, so that at a time 8 different analog inputs can be connected to the chips.
Out of these 8 inputs only one can be selected for conversion by using 3 address lines A,B,C.

16. Fig 4.15 Pin diagram of ADC 0808/0809

17. Block Diagram of ADC 0808 / 0809 Successive Approximation
Analog Comparator
DAC

18. Pin Diagram of ADC 0808 / 0809
Add A, B, C
Digital Data

19. Pin Diagram of ADC 0808 / 0809

20. Pin No
Signal Name
Description 11
Vcc
Supply pins +5V 13
GND 12
Vref+
Reference voltage positive (+5 Volts maximum) 16
Vref -
Reference voltage negative (0 Volts minimum)
26 – 28
1 - 5
I/P0 –I/P7
Analog inputs
25, 24, 23
ADD A,B,C
Address lines for selecting analog inputs.

21. Pin Description : ADC 0808 / 0809 Pin No Signal Name Description
8, 14, 15,
O7 – O0
Digital 8-bit output with O7 MSB and O0 LSB 6
SOC
Start of conversion signal pin 7
EOC
End of conversion signal pin 9 OE
Output latch enable pin, if high enables output 10
CLK
Clock input for ADC 22
ALE
Address Load Enable. Once loaded the multiplexer sends the appropriate channel to the converter on the chip.

22. Analog input selection
These are unipolar Analog to Digital (A to D) converters, they are able to convert only positive analog input This chips do not contain any internal sample & hold circuit.

23. Interfacing 0808 with 8086

24. Interfacing between ADC to Microprocessor
Problem:-
Interface ADC 0808 with 8086 using 8255 ports. Use Port A of 8255 for transferring digital data output of ADC to the CPU & Port C for control signals. Assume that an analog input is present at I/P2 of the ADC and a clock input of suitable frequency is available for ADC.
Draw the schematic & timing diagram of different signals of ADC0808.

25. Solution:-
The analog input I/P2 is used & therefore address pins A,B,C should be 0,1,0 respectively to select I/P2.
The OE (Out put latch Enable) & ALE pins are already kept at +5v to select the ADC and enable the outputs.
Port C upper acts as the input port to receive the EOC signal while Port C lower acts as the output port to send SOC to ADC.
Port A acts as a 8-bit input data port to receive the digital data output from the ADC.

26. 8255 Control Word: D7 D6 D5 D4 D3 D2 D1 D0 1 0 0 1 1 0 0 0 = 98H
Program:
MOV AL,98H ; Initialize 8255, send AL to control word (CWR)
OUT CWR, AL
MOV AL, 02H ;Select I/P2 as analog I/P
OUT Port B, AL ;Port B as output
MOV AL, 00H ; Give start of conversion pulse to the ADC
OUT Port C, AL
MOV AL, 01H
MOV AL, 00H
OUT Port C, AL
WAIT: IN AL, Port C ; check for EOC by reading Port C upper & rotating
RCL ; through carry.
JNC WAIT
IN AL, Port A ; if EOC, read digital equivalent in AC
HLT ; stop.

27. Fig 4.16 Interfacing of ADC 0808 to 8086 through 8255

28. Interfacing D/A Converters
The Digital to Analog Converters (DAC) convert binary numbers into their analog equivalent voltages.
The DAC find applications in areas like
Digitally controlled gains
Motor speed controls
Programmable gain amplifiers etc.
AD Bit Multiplying DAC:--
Intersil’s AD 7523 is a 16 pin DIP, multiplying digital to analog converter, containing R-2R ladder (R=10K) for digital to analog conversion.

29. Power supply +5v to +15v
Vref -> -10v to +10v
The maximum analog output voltage will be +10v
A Zener is connected between OUT1 & OUT2 to save the DAC from negative transients.
An operational amplifier is used as a current – to – voltage converter at the output of AD 7523.
An external feedback resister acts to control the gain.

30. Interfacing of AD 7523 with 8086 Problem:--
Interface DAC AD7523 with the 8086 running at 8MHz & write ALP to generate a saw tooth waveform of period 1ms with Vmax 5v.
Solution:--
Code segment
Assume cs:code
Start: MOV AL, 80H
OUT CWR, AL
AGAIN: MOV AL, 00H
BACK: OUT Port A, AL
INC AL
CMP AL, 0F2H
JB BACK
JMP AGAIN
Code ends
End Start

31. Fig 4.17 Pin diagram of AD7523

32. Fig 4.18 Interfacing of AD7523 to 8086 through 8255

33. END