Interfacing 8255
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1. The 8254 contains three independent 16-bit counters/timers that can be programmed to operate in different modes. 2. Each counter can be programmed to count from 1 to 65535 and has a programmable control word to select the operating mode. 3. The 8254 supports various timer modes like one-shot, continuous square wave, event counter, and software/hardware triggered one-shot for applications like timing, delay generation, and pulse width modulation.
![8255 Programmable Peripheral Interface
Data bus
D[7:0]
PA[7:0]
A0
8086 A1 PB[7:0]
RD Control port
WR PC[7:0]
RESET
A7 CS
A6
A5
A4
A3
A2 A1 A0 Port
IO/M
0 0 PA
0 1 PB
1 0 PC
1 1 Control](https://image.slidesharecdn.com/interface-8255-120813131326-phpapp02/85/Interfacing-8255-5-320.jpg)
![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] PA[7:0]
PC4 STBA PC7 OBFA
PC5 IBFA PC6 ACKA
PC3 INTRA PC3 INTRA
8255 PB[7:0] 8255 PB[7:0]
PC2 STBB PC2 OBFB
PC1 IBFB PC1 ACKB
PC0 INTRB PC0 INTRB
PC6, 7 PC4, 5](https://image.slidesharecdn.com/interface-8255-120813131326-phpapp02/85/Interfacing-8255-8-320.jpg)
![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]
PC7 OBFA
PC6 ACKA
PC4 STBA
8255 PC5 IBFA
PC3 INTRA
PC0 In Out STBB OBFB
PC0 In Out IBFB ACKB
PC0 In Out INTRB INTRB
PB[7:0]
Mode 0 Mode 1
1. Can you design a decoder for an 8255 chip such that its base address is 40H?
2. Write the instructions that set 8255 into mode 0, port A as input, port B as output,
PC0-PC3 as input, PC4-PC7 as output ?](https://image.slidesharecdn.com/interface-8255-120813131326-phpapp02/85/Interfacing-8255-11-320.jpg)
Interfacing 8255
- 1. PPI : 82C55 The 82C55 is a popular interfacing component, that can interface any TTL-compatible I/O device to the microprocessor. It is used to interface to the keyboard and a parallel printer port in PCs (usually as part of an integrated chipset). Requires insertion of wait states if used with a microprocessor using higher that an 8 MHz clock. PPI has 24 pins for I/O that are programmable in groups of 12 pins and has three distinct modes of operation. In the PC, an 82C55 or its equivalent is decoded at I/O ports 60H-63H.
- 3. Pin layout of 8255
- 5. 8255 Programmable Peripheral Interface Data bus D[7:0] PA[7:0] A0 8086 A1 PB[7:0] RD Control port WR PC[7:0] RESET A7 CS A6 A5 A4 A3 A2 A1 A0 Port IO/M 0 0 PA 0 1 PB 1 0 PC 1 1 Control
- 6. 8255 Programmable Peripheral Interface
- 7. Programming 8255 8255 has three operation modes: mode 0, mode 1, and mode 2
- 8. 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] PA[7:0] PC4 STBA PC7 OBFA PC5 IBFA PC6 ACKA PC3 INTRA PC3 INTRA 8255 PB[7:0] 8255 PB[7:0] PC2 STBB PC2 OBFB PC1 IBFB PC1 ACKB PC0 INTRB PC0 INTRB PC6, 7 PC4, 5
- 11. 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] PC7 OBFA PC6 ACKA PC4 STBA 8255 PC5 IBFA PC3 INTRA PC0 In Out STBB OBFB PC0 In Out IBFB ACKB PC0 In Out INTRB INTRB PB[7:0] Mode 0 Mode 1 1. Can you design a decoder for an 8255 chip such that its base address is 40H? 2. Write the instructions that set 8255 into mode 0, port A as input, port B as output, PC0-PC3 as input, PC4-PC7 as output ?
- 12. Timing diagram is a combination of the Mode 1 Strobed Input and Mode 1 Strobed Output Timing diagrams.
- 13. Example: Mode 1 Input 8255 keyboard BIT5 EQU 20H PA0 PORTC EQU 22H PORTA EQU 20H PA7 READ PROC NEAR Read: STB PC4 DAV • IN AL, PORTC ; read portc • TEST AL, BIT5 ;test IBF • JZ Read ;if IBF=0 • IN AL, PORTA ;Read Data READ ENDP
- 14. Example: Mode 1 output 8255 Printer PB0 PB7 ACK Data Strobe : to tell PC2 ACK the printer to latch the incoming data. PC4 DS Generated Externally
- 15. Example: Mode 1 output BIT1 EQU 2 ;send character to printer PORTC EQU 62H MOV AL, AH ;get data OUT PORTB, AL ;print PORTB EQU 61H data CMD EQU 63H ; send data strobe to PRINT PROC NEAR printer ; check printer ready? MOV AL, 8 ;clear DS IN AL, PORTC ;get OBF OUT CMD, AL TEST AL, BIT1 ;test OBF MOV AL, 9 ;clear DS JZ PRINT ;if OBF=0 buffer OUT CMD, AL is full ;rising the data at the positive edge of DS RET PRINT ENDP
- 19. 8254 Programming
- 20. 8254 Programming Each counter may be programmed with a count of 1 to FFFFH. • Minimum count is 1 all modes except 2 and 3 with minimum count of 2. Each counter has a program control word used to select the way the counter operates. • If two bytes are programmed, then the first byte (LSB) stops the count, and the second byte (MSB) starts the counter with the new count.
- 21. 8254 Read Back Command 8254 Read Back Command 1 1 COUNT STATUS CNT2 CNT1 CNT0 0 8254 status word format NULL OUTPUT COUNT RW1 RW0 M2 M1 M0 BCD NULL COUNT: goes low when the new count written to a counter is actually loaded into the counter
- 22. 8254 Modes Mode 0: An events counter enabled with G. • The output becomes a logic 0 when the control word is written and remains there until N plus the number of programmed counts. Mode 1: One-shot mode. • The G input triggers the counter to output a 0 pulse for `count' clocks. • Counter reloaded if G is pulsed again.
- 23. 8254 Modes Mode 2: Counter generates a series of pulses 1 clock pulse wide. • The seperation between pulses is determined by the count. • The cycle is repeated until reprogrammed or G pin set to 0. • Mode 3: Generates a continuous square-wave with G set to 1. If count is even, 50% duty cycle otherwise OUT is high 1 cycle longer.
- 24. 8254 Modes Mode 4: Software triggered one-shot • (G must be 1). Mode 5: Hardware triggered one- shot. G controls similar to Mode 1.