1. UNIT-IV 8255 PPI Various Modes Of Operation Interfacing To 8086.
Interfacing Keyboard, Displays
Stepper Motor
D/A And A/D Converter Interfacing.

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. Example: Mode 1 output BIT1 EQU 2 PORTC EQU 62H PORTB EQU 61H
CMD EQU 63H
PRINT PROC NEAR
; check printer ready?
IN AL, PORTC ;get OBF
TEST AL, BIT1 ;test OBF
JZ PRINT ;if OBF=0 buffer is full
;send character to printer
MOV AL, AH ;get data
OUT PORTB, AL ;print data
; send data strobe to printer
MOV AL, 8 ;clear DS
OUT CMD, AL
MOV AL, 9 ;clear DS
;rising the data at the positive edge of DS
RET
PRINT ENDP

13. Fig 4.1 Interfacing of 8255 PPI to 8086
Data bus
8086
D[7:0] A0 A1 RD WR
RESET CS
Control port
PA[7:0]
PB[7:0]
PC[7:0] A7 A6 A5 A4 A3 A2
IO/M
A1 A0
Port PA PB PC
Control
Fig 4.1 Interfacing of 8255 PPI to 8086

14. Fig 4.10 Interfacing of 8255 to 4x4 matrix Keyboard in mode1
Keyboard example
Fig 4.10 Interfacing of 8255 to 4x4 matrix Keyboard in mode1

15. Fig 4.11 Flow chart of a keyboard scanning procedure
Keyboard example
Fig 4.11 Flow chart of a keyboard scanning procedure

16. Fig 4.12 Key bouncing problem

17. Fig 4.13 Key bouncing problem

18. Fig 4.14 Software Key debouncing solution
Software Solution
Fig 4.14 Software Key debouncing solution

19. 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.

20. 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.
The CPU may drive these lines using output port lines in case of multichannel applications.

21. Fig 4.15 Pin diagram of ADC 0808/0809

22. Table 4.1 analog input selection
These are unipolar Analog to Digital (A to D) converters, they are able to convert only positive analog input voltages to their digital equivalents. This chips do not contain any internal sample & hold circuit.

23. 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.

24. 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.

25. 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.

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

27. 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.

28. 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.

29. 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

30. Fig 4.17 Pin diagram of AD7523

31. Fig 4.18 Interfacing of AD7523 to 8086 through 8255

32. Display interface
Interface an 8255 with 8086 at 80h as an I/O address of port-A. interface five 7 segment displays with the write a sequence of instructions to display 1,2,3,4 and 5 over the five displays continuously as per their positions starting with 1 at the least significant position. CWR address is 86h.
Number to be displayed
PA7dp
PA6 a
PA5 b
PA4 c
PA3 d
PA2 e
PA1 f
PA0 g
Code 1 CF 2 92 3 86 4 CC 5 A4
All these codes are stored in a look up table starting at 2000:0001.

33. Fig 4.19 Interfacing multiplexed 7-segment display to 8086 through 8255

34. ALP for display interface
again: mov cl,05h ;count for displays
mov bx,2000h ;initialize the data segment for
mov ds,bx ; look-up table
mov ch,01h ;1st no. to be displayed
mov al,80h
out 86h,al ;load control word in the CWR
mov dl,01h ;enable code for least significant 7-seg display
nxtdgt: mov bx,0000h ;set pointer to look-up table
mov al,ch ;store number to be display
xlat ;find code from table

35. out 80h,al
mov al,dl
out 82h,al ;enable the display
rol dl ;go for next digit display
inc ch
dec cl ;decrement counter
jnz nxtdgt ;go for next digit display
jmp again

36. Stepper Motor Interfacing
A stepper motor is a device used to obtain an accurate position control of rotating shafts.
It employs rotation of its shaft in terms of steps, rather than continuous rotation as in case of AC or DC motors.
In dot-matrix printer one small stepper motor which is used to advance the paper to the next line position & another small stepper motor which is used to move the print head to the next character position.
In floppy disk stepper motor is used to position the read/write head over the desired track.
To rotate the shaft of the stepper motor, a sequence of pulses is needed to be applied to the windings of the stepper motor, in a proper sequence.
The no. of pulses required for one complete rotation of the shaft of the stepper motor are equal to its number of internal teeth on its rotor.

37. The stator teeth the rotor teeth lock with each other to fix a position of the shaft .
With a pulse applied to the winding input, the rotor rotates by one teeth position or an angle x. The angle x may be calculated as:
x = / no. of rotor teeth
The stepper motors have been designed to work with digital circuits. Binary level pulses of 0-5v are required at its winding inputs to obtain the rotation of shafts.
The sequence of pulses can be decided, depending upon the required motion of the shaft.
The count for rotating the shaft of the stepper motor through a specified angle may be calculated from the no. of rotor teeth
C = no. of rotor teeth / 3600 * θ0

38. Motion
Step A B C D
Clockwise 1 2 3 4 5
Anticlockwise
Table 4.2 Excitation sequence for clockwise and anticlockwise rotation of a stepper motor

39. Problem
Design a stepper motor controller and write an ALP to rotate shaft of a 4-phase stepper motor:
In clockwise 5 rotations
In anticlockwise 5 rotations.
The 8255 port A address is 0740h. The stepper motor has 200 rotor teeth.
The port A bit PA0 drives winding Wa, PA1 drives winding Wb and so on.
The stepper motor has an internal delay of 10msec. Assume that the routine for this delay is already available.

40. Solution:
ALP:
Assume cs:Code
Code segment
Start: MOV AL, 80H
OUT CWR, AL
MOV AL, 88H; Bit pattern
MOV CX, 1000
Again1: OUT Port A, AL
CALL DELAY
ROL AL, 01
DEC CX
JNZ Again1
MOV AL, 88H

41. Again2: OUT Port A, AL CALL DELAY ROR AL, 01 DEC CX JNZ Again2 MOV AH, 4CH INT 21H Code ends End start