The document discusses the 8255 programmable parallel I/O device that can be used to interface I/O devices to a microprocessor like the 8085. It describes how the 8255 has three I/O ports (Port A, Port B, Port C) that can be configured using a control word. It then provides an example of interfacing an 8-bit analog to digital converter and temperature sensor to the 8255 ports to read temperature values using instructions like OUT, IN.

1. Programmable Peripheral Interface Devices Support chips : 8155 , 8255 , 8279 , 8254 , 8257 , 8259,8251

2. Programmable Interface Devices used to interface a I/O device to the microprocessor 8085 Programmable I/O Interface I/O Devices System Bus Can be programmed/configured to perform various I/O functions by writing software instructions

3. 8255 – A programmable parallel I/O device Designed to be compatible with 8085 It consists of Three I/O ports Port A Port B Port C Port C can be used as two 4-bit ports PC u (Port C upper 4 bits) PC l (Port C lower 4 bits)

4. I/O Interfacing using 8255 8085 8255 8-bit ADC System Bus LED DISPLAY Temperature Sensor Port A Port C u Port B Port C l

5. Interfacing with Analog-to-Digital Converter 8085 8255 8-bit ADC System Bus Temperature Sensor Port A Port C lines SOC EOC OE

6. Application Programming Logic Configure 8255 I/O ports 8085 sends SOC command to ADC 8085 waits for EOC signal from ADC 8085 sends OE command to ADC 8085 reads 8-bit temperature value from port A 8085 displays temperature value on LED display connected to port B

7. Writing the program At this point, it will help us to understand, what we need to know more about 8255

8. 1. Configure 8255 I/O Ports 8085 8255 8-bit ADC System Bus LED DISPLAY Temperature Sensor Port A Port C u Port B Port C l

9. Port A as INPUT port Port B as OUTPUT port Port C (PC u – O/P , PC l – I/P ) SOC (PC 4 ) is a OUTUT line EOC (PC 0 ) is a INPUT line OE (PC 7 ) is a OUTPUT line Know more about Modes of operation of 8255 Control word format of 8255

10. Modes of Operation of 8255 Two primary modes of operation Parallel I/O mode Mode 0 (Simple Input/Output) Mode 1 (Input/Output with Handshake) Mode 2 (Bidirectional data transfer) Bit Set/Reset Mode This application uses Parallel I/O mode - Mode 0 Bit Set/Reset mode

11. Control Word for 8255 8255 has a 8-bit Control word register 8255 ports can be configured for operation by writing a appropriate control word in it

12. for Parallel I/O 1 0 0 0 1 00 1 91H

13. for Bit Set/Reset Mode

14. Application Programming Configure 8255 I/O ports MVI A, 91H OUT CWR Writing value 91H in Control Word Register of 8255 CWR represents the 8-bit port address of Control Word Register

15. Application Programming 8085 sends SOC (PC4=1)command to ADC MVI A, 00001001B OUT CWR Uses Bit Set/Reset mode of 8255

16. Application Programming 8085 waits for EOC signal from ADC WAIT: IN PORTC RAR JC WAIT EOC is taken to be active low

17. Application Programming 8085 sends OE command to ADC MVI A, 00001111H OUT PORTC 8085 reads 8-bit temperature value from port A IN PORTA MOV B, A

18. Application Programming 8085 displays temperature value on LED display connected to port B OUT PORTB

19. Pin diagram of 8255 D7 – D0 Data Bus PA7 – PA0 Port A PB7 – PB0 Port B PC7 – PC0 Port C CS Chip Select A0, A1 Address bits RD Read Input WR Write Input RESET Reset Input Vcc +5V GND 0 Volts

20. Interfacing 8255 with 8085 8255 can be interfaced to 8085 using Memory-mapped I/O Peripheral-mapped I/O As application program uses IN/OUT instructions Peripheral-mapping is used 8-bit addresses are used for ports and control register of 8255

21. For e.g. OUT CWR IN PORTA OUT PORTB In actual program, label CWR will be substituted by 8-bit address of Control Word Register of 8255

22. 8255 Chip selection & Port Addresses 8255 A2 A3 A4 A5 A6 A7 CS A1 A0 A1 A0 A = 80H B = 81H C = 82H A1 A0 Port 0 0 A 0 1 B 1 0 C 1 1 CWR

23. Programming with port addresses OUT 83H IN 80H OUT 81H OUT CWR IN PORTA OUT PORTB

24. Block diagram of 8255