PPI : 82C55PPI : 82C55 The 82C55 is a popular interfacing component, The 82C55 is a popular interfacing component,

that can interface any TTL-compatible I/O device that can interface any TTL-compatible I/O device to the microprocessor. to the microprocessor.

It is used to interface to the keyboard and a It is used to interface to the keyboard and a parallel printer port in PCs (usually as part of an parallel printer port in PCs (usually as part of an integrated chipset). integrated chipset).

Requires insertion of wait states if used with a Requires insertion of wait states if used with a microprocessor using higher that an 8 MHz clock. microprocessor using higher that an 8 MHz clock.

PPI has 24 pins for I/O that are programmable in PPI has 24 pins for I/O that are programmable in groups of 12 pins and has three distinct modes of groups of 12 pins and has three distinct modes of operation. operation.

In the PC, an 82C55 or its equivalent is decoded In the PC, an 82C55 or its equivalent is decoded at I/O ports 60H-63H. at I/O ports 60H-63H.

8255 Block Diagram8255 Block Diagram

Pin layout of 8255Pin layout of 8255

Interfacing 8255 PPIInterfacing 8255 PPI

8255 Programmable Peripheral Interface

Data bus

8086

D[7:0]

A0A1

RDWR

RESET

CS

Control port

PA[7:0]

PB[7:0]

PC[7:0]

A7A6A5A4A3A2

IO/MA1 A0 Port

0 00 11 01 1

PAPBPCControl

8255 Programmable Peripheral 8255 Programmable Peripheral InterfaceInterface

Programming 8255

8255 has three operation modes: mode 0, mode 1, and mode 2

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

INTRAPC3PC5PC4

PB[7:0]

STBB

IBFB

INTRBPC0PC1PC2

PC6, 7

8255

PA[7:0]

OBFA

ACKA

INTRAPC3PC6PC7

PB[7:0]

OBFB

ACKB

INTRBPC0PC1PC2

PC4, 5

8255

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

PC6PC7

STBA

IBFA

PC0

PC3PC58255

PC0

PC0

PB[7:0]

In Out In OutIn Out

Mode 0

STBB OBFB IBFB ACKB

INTRB INTRB

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 ?

Timing diagram is a combination of the Mode 1 Strobed Input and Mode 1 Strobed Output Timing diagrams.

Example: Mode 1 InputExample: Mode 1 Input

BIT5BIT5 EQU EQU 20H20H PORTCPORTC EQUEQU 22H22H PORTAPORTA EQUEQU 20H20H

READ READ PROCPROC NEARNEAR Read:Read:

• IN AL, PORTCIN AL, PORTC ; read portc; read portc• TEST AL, BIT5TEST AL, BIT5 ;test IBF;test IBF• JZ ReadJZ Read ;if IBF=0;if IBF=0• IN AL, PORTAIN AL, PORTA ;Read Data;Read Data

READ READ ENDPENDP

keyboard

PA0

PA7

STBPC4 DAV

8255

Example: Mode 1 outputExample: Mode 1 output

Printer

PB0

PB7

ACKPC2 ACK

8255

PC4 DS

Data Strobe : to tell the printer to latch the incoming data. Generated Externally

BIT1BIT1 EQUEQU 22PORTCPORTC EQUEQU 62H62HPORTBPORTB EQUEQU 61H61HCMDCMD EQUEQU 63H63HPRINTPRINT PROCPROC NEARNEAR

; check printer ready?; check printer ready?IN AL, PORTC ;get OBFIN AL, PORTC ;get OBFTEST AL, BIT1 ;test OBFTEST AL, BIT1 ;test OBFJZ PRINT ;if OBF=0 buffer JZ PRINT ;if OBF=0 buffer

is fullis full

;send character to printer;send character to printerMOV AL, AH ;get dataMOV AL, AH ;get dataOUT PORTB, AL ;print dataOUT PORTB, AL ;print data; send data strobe to ; send data strobe to

printerprinterMOV AL, 8 ;clear DSMOV AL, 8 ;clear DSOUT CMD, ALOUT CMD, ALMOV AL, 9 ;clear DSMOV AL, 9 ;clear DSOUT CMD, ALOUT CMD, AL;rising the data at the ;rising the data at the

positive edge of DSpositive edge of DSRETRET

PRINT ENDPPRINT ENDP

Example: Mode 1 outputExample: Mode 1 output

Keyboard example 1/2Keyboard example 1/2

Keyboard example 2/2Keyboard example 2/2

Programmable Timer 8254 Programmable Timer 8254

8254 Programming 8254 Programming

8254 Programming 8254 Programming

Each counter may be programmed Each counter may be programmed with a count of 1 to FFFFH. with a count of 1 to FFFFH. • Minimum count is 1 all modes except 2 Minimum count is 1 all modes except 2

and 3 with minimum count of 2. and 3 with minimum count of 2. Each counter has a program control Each counter has a program control

word used to select the way the word used to select the way the counter operates. counter operates. • If two bytes are programmed, then the If two bytes are programmed, then the

first byte (LSB) stops the count, and the first byte (LSB) stops the count, and the second byte (MSB) starts the counter with second byte (MSB) starts the counter with the new count.the new count.

8254 Read Back Command

8254 Read Back Command

1 1 COUNT STATUS CNT2 CNT1 CNT0 0

NULL COUNT: goes low when the new count written to a counter is actually loaded into the counter

8254 status word format

OUTPUTNULL

COUNT RW1 RW0 M2 M1 M0 BCD

8254 Modes8254 Modes

Mode 0: An events counter enabled with G. Mode 0: An events counter enabled with G. • The output becomes a logic 0 when the control word is The output becomes a logic 0 when the control word is

written and remains there until N plus the number of written and remains there until N plus the number of programmed counts. programmed counts.

Mode 1: One-shot mode. Mode 1: One-shot mode. • The G input triggers the counter to output a 0 pulse for The G input triggers the counter to output a 0 pulse for

`count' clocks. `count' clocks. • Counter reloaded if G is pulsed again. Counter reloaded if G is pulsed again.

8254 Modes8254 Modes

Mode 2: Counter generates a series of pulses 1 clock pulse Mode 2: Counter generates a series of pulses 1 clock pulse wide. wide. • The seperation between pulses is determined by the count. The seperation between pulses is determined by the count. • The cycle is repeated until reprogrammed or G pin set to 0. The cycle is repeated until reprogrammed or G pin set to 0.

• Mode 3: Generates a continuous square-wave with G set to 1. 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 If count is even, 50% duty cycle otherwise OUT is high 1 cycle

longer. longer.

8254 Modes8254 Modes

Mode 4: Software triggered one-shot Mode 4: Software triggered one-shot • (G must be 1). (G must be 1).

Mode 5: Hardware triggered one-Mode 5: Hardware triggered one-shot. G controls similar to Mode 1. shot. G controls similar to Mode 1.