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Programmable Peripheral Interface Devices
Support chips: 8155, 8255, 8279,8254,8257, 8259,8251
Programmable Interface Devices
used to interface a I/O device to the microprocessor
8085Programmable
I/O Interface
I/O Devices
System Bus
Can be programmed/configured to perform various I/O functions by writing software instructions
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 PCu (Port C upper 4 bits)
PCl (Port C lower 4 bits)
I/O Interfacing using 8255
8085 8255
8-bit ADC
System Bus
LED DISPLAY
Temperature Sensor
Port A
Port Cu
Port B
Port Cl
Interfacing with Analog-to-Digital Converter
8085 82558-bit ADC
System Bus
Temperature Sensor
Port A
Port C lines
SOC
EOC
OE
Application Programming Logic
1. Configure 8255 I/O ports2. 8085 sends SOC command to ADC3. 8085 waits for EOC signal from ADC4. 8085 sends OE command to ADC5. 8085 reads 8-bit temperature value
from port A6. 8085 displays temperature value on
LED display connected to port B
Writing the program
At this point, it will help us to understand, what we need to know more about 8255
1. Configure 8255 I/O Ports
8085 8255
8-bit ADC
System Bus
LED DISPLAY
Temperature Sensor
Port A
Port Cu
Port B
Port Cl
Port A as INPUT port Port B as OUTPUT port Port C (PCu – O/P , PCl – I/P )
SOC (PC4) is a OUTUT line EOC (PC0) is a INPUT line OE (PC7) is a OUTPUT line
Know more about Modes of operation of 8255 Control word format of 8255
Modes of Operation of 8255
Two primary modes of operation1. Parallel I/O mode
Mode 0 (Simple Input/Output) Mode 1 (Input/Output with Handshake) Mode 2 (Bidirectional data transfer)
2. Bit Set/Reset Mode This application uses
Parallel I/O mode - Mode 0 Bit Set/Reset mode
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
for Parallel I/O
1
0
0
0
1001
91H
for Bit Set/Reset Mode
Application Programming
1. Configure 8255 I/O portsMVI A, 91HOUT CWR
Writing value 91H in Control Word Register of 8255
CWR represents the 8-bit port address of Control Word Register
Application Programming
2. 8085 sends SOC (PC4=1)command to ADC
MVI A, 00001001BOUT CWR
Uses Bit Set/Reset mode of 8255
Application Programming
3. 8085 waits for EOC signal from ADCWAIT: IN PORTC
RARJC WAIT
EOC is taken to be active low
Application Programming
4. 8085 sends OE command to ADCMVI A, 00001111HOUT PORTC
5. 8085 reads 8-bit temperature value from port A
IN PORTAMOV B, A
Application Programming
6. 8085 displays temperature value on LED display connected to port B
OUT PORTB
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
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
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
8255 Chip selection & Port Addresses
8255A2
A3
A4
A5
A6
A7
CS
A1
A0
A1
A0
A = 80H
B = 81H
C = 82HA1 A0 Port
0 0 A
0 1 B
1 0 C
1 1 CWR
Programming with port addresses
OUT 83H IN 80H OUT 81H
OUT CWR IN PORTA OUT PORTB
Block diagram of 8255
