Basic Input and Output Interface - Microprocessors and Computer Systems - Lecture Slides, Slides of Microprocessors

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Basic Input & Output Interface

Outline

a

Input-output Interface

a

Serial/Parallel Interface

a

8255 Programmable Peripheral Interface

a

Keyboard

a

8254 Software-ProgrammableTimer/Counter

I/O interface

a

Input-output involves the transfer to (or from) peripheral devicesfrom (or to) the data bus of the cpu; eg. data transfer to the CRTdisplay, from the keyboard, to/from a hard-disk drive, to/from amodem to another computer system.

a

Input-output operations fall into one of the following types: •^ Programmed I/O

• cpu polls peripherals to check if I/O is needed -^

Interrupt I/O

• peripheral sends an interrupt request to cpu for I/O -^

Direct memory access (DMA)

• peripheral writes directly to memory

a

Considering the address of I/O, 8088 uses Isolated I/O (I/Oaddresses are not part of memory address) as distinct fromMemory mapped I/O (peripherals are mapped to locations in thememory address space).

Q: pros and cons of isolated I/O and memory mapped I/O?

1M

×

Memory

FFFFF^00000

FFFF

64K

×

I/O

(a) Memory + I/O

I/O

FFFFF^00000

(b)

The Memory and I/Omaps for the 8086/8088microprocessor.(a) Isolated I/O.(b) Memory mapped I/O.

Memory and I/O Map

Q: In Isolated I/O, an I/O port has thesame address as one of the memorybyte. How does it know it is beingaddressed?

I/O interface

a

I/O interface functions include •^

data storage buffer for sending and receiving data

-^

low-level communications protocol (handshaking)

-^

data format conversion (eg. parallel/serial)

-^

error detection

-^

addressing of different peripherals

a

I/O interface are typically implemented by LSI (large scaleintegration) - many different types are available from differentmanufacturers.

a

Data can be transferred through I/O interface by eitherprogrammed I/O or interrupt I/O. DMA typically needs a separatecontroller.

8088 maximum mode I/O interface

AIOWC

Bus controller

I/O

interfacecircuitry

I/O device

I/O device

N-

I/O device

A^8

-A

19

AD

-AD 0

7

S^0

-S

2 CLK

ALE DEN

Generalized I/O interface connections to maximum mode 8088

IORCIOWC

R

DT/

M

IO/

Parallel and Serial Data Transfer

(cont.)

a

Serial data transfer involves sending the data on a single line, bitby bit. The I/O interface converts the data from parallel to serial orvice-versa using shift registers.

a

Serial connections are commonly used for data transfer overlonger distances (eg over telephone line). A popular standard forserial data transmission is the RS232C standard.Q: Why serial data transfer is more commonly used in long distance transmission?

Serial I/O

a

Serial I/O can be either(1) Synchronous - data are sent in blocks, with

start

and

end-of-block

markers.

Individual characters within a block do not need start and stop

bits since the receiver identifies every 8 bits as one character, eg.

syn

syn

stx

data field

etx

bcc

pad

one frame

syn = sync character (ascii code 16)stx = start of text (ascii code 02)etx = end of text (ascii code 03)bcc = block check characters (error detection)pad = end of frame pad ( ascii code ff)

Handshaking for I/O

a

Three types of handshaking: •^ Simple parallel I/O - no handshaking implemented •^ Single handshake I/O - uses STB-ACK handshake •^ Double handshake I/O - uses STB-ACK and STB-ACK

data

simple (no handshake)

Handshaking for I/O (cont.)

data

DoubleHandshake

strobeACK

Data transferred after thefirst STB and ACK.

Tx Rx

data

SingleHandshake

strobeACK

: means the circle will trigger theevent pointed by the arrow aftera given time

8255A Programmable Parallel

Port Device

Internal structureblock diagram

GROUP APRRT CUPPERGROUP BPRRT CLOWER GROU BPORT B

(8)

8 BITSINTERNALDATA BUS

Bi-directionalData BusD0-D

DATABUSBUFFER

RDWRA1A RESET

CS

READ/WRITECONTROLLOGIC

PowerSupply

+5VGND

GROUPACONTROL GROUP B CONTROL

GROUP APORTA

(8)

I/O PA7-PA

I/O PC7-PC I/O PC3-PC I/O PB7-PB

8255A Mode 0 Operation

a

In Mode 0 operation, no handshaking will be used.

a

If both port A and port B are initialized as mode 0 operation, port Ccan be used together as an additional 8-bit port, or two 4-bit ports.

a

When used as an outputs, port C line can be set/reset by sendingspecial control word to the

control register address.

a

The two halves of port C are independent and can be set as inputor output port independently.

8255A Mode 1 Operation

a

In Mode 1 operation, single handshaking (strobed) is used.

a

In this mode, some of the port C pins are used for handshakingpurpose.

`

If Port A is set to mode 1 & input port:

PC3, PC4, and PC5 pins are used.

`

If Port A is set to mode 1 & output port: PC3, PC6, and PC7 pins are used.

`

If Port B is set to mode 1 & input port:

PC0, PC1, and PC2 pins are used.

`

If Port B is set to mode 1 & output port: PC0, PC1, and PC2 pins are used.

8255A Mode 1 Operation

MODE 1

PB7-PB

PA7-PA

I/O

I/O

B^8

A

PC

PC

PC

PC

PC

PC

PC

PC

I/O

I/O

A

ACK

OR

OR

I/O

OR I/O

OR

OR

OR

INTR

B^

INTR

A

B

OBF

A

OBF

B

ACK

IBF

B^

IBF

A

PORT A, PORT B CONTROL

INPUT HANDSHAKING

SIGNAL

OUTPUT HANDSHAKING

SIGNAL

INTR

B^

INTR

A

STB

B^

A STB