module 12: elementary input/output display example input devices push button proximity switch rotary...

68000 Microprocessor

Module 12: Elementary Input/Output

I/O Concepts

Typical Organization of an I/O Device

Memory-mapped vs Isolated I/O

Elementary output with latch and LED

Elementary input with buffer and switch

7-segment interfacing

© 2006-2008 [email protected] 11-1

7-segment interfacing

Keypad interfacing (extra)


Input and Output (I/O)

One of the basic and essential features designed in a computer system is its ability to exchange data with other external devices, and to allow the user to interact with the system:

Input Devices include: Switches, Keyboards, Mice, Scanners, Cameras, etc.

Output devices include: Lamp/LED/LCD displays, Video monitors, Speakers, Printers, etc.

One or more interface circuits usually are used between I/O devices and the CPU to:


the CPU to:

Handle transfer of data between CPU and I/O interface.

Handle transfer of data between I/O device and interface.

Enable the CPU to request the status of data sent/received by the interface.

Common I/O interfaces:

Elementary I/O: Simple two-state devices such as LED and switches

Peripheral I/O: Non-trivial interaction with complex I/O devices such as printers and keyboards


Elementary I/O

Two-state peripheral devices

May involve more than one bit - e.g., BCD

Requirements

Bit signals can be written to output devices under program control

Bit signals can be read from input devices under program control

Devices required

Latches such as 74LS374 for output

Tri-state buffers such as 74LS244 for input


Tri-state buffers such as 74LS244 for input

Example Output devices

LED, bulbs

Relay coils

7-segment display

Example Input devices

Push button

Proximity switch

Rotary BCD coder


Address

CPU

Memory I/O Device

Data

Read

Write

I/O Schemes

Memory-mapped I/O

The I/O devices and memory

components are resident in the

same memory space

The microprocessor uses the

same instruction set for I/O

operations

Isolated I/O


CPU

MemoryI/O Device

Data

Read

Write

Address

I/O Port

Memory I/O

Isolated I/O

The microprocessor has a

separate address space for I/O

devices

The microprocessor has

dedicated instructions for I/O

operations

Special bus lines

Traditionally, Intel microprocessors use the isolated I/O method, whereas Motorola microprocessors use the memory-mapped I/O method.


Memory Mapped I/O

Part of the CPU's normal memory space is dedicated to I/O operations and all I/O ports look exactly like normal memory locations.

Pros

Rich set of addressing modes for I/O operation (e.g. bit manipulation)

No overhead on special instructions ProgramProgram

000000000000

004000004000


No overhead on special instructions

Cons

Processor’s address space is allocated to I/O devices

Risk of errors due to spurious accesses (elaborated address decoding)

Lack of special purpose I/O signals to control the I/O operation

ProgramProgram

DataData

I/OI/O

220000000000

808000000000808000030003


Peripheral I/O Concepts

I/O devices (peripheral devices) Equipment that exchange data with a computer.

May itself contain a microprocessor!

Examples include printers, modems, keyboards, and disk drives.

Interface (peripheral) chip A chip whose function is to synchronize data transfer between the CPU

and I/O devices.

© 2006-2008 [email protected]

An interface may consist of control registers, status registers, data registers, and control circuitry.

An interface chip has data pins that are connected to the CPU and I/O port pins that are connected to the I/O devices.

Data transfer between an I/O device and the CPU can be proceeded bit-by-bit (serial) or in multiple bits (parallel).


I/O Transfer Synchronization

The role of an interface chip

Synchronizing data transfer between the CPU and the interface chip.

Synchronizing data transfer between the interface chip and the I/O device.



Synchronizing µP and the Interface Chip

The polling method

Repeatedly checks the status of an I/O device (through a status register of the associated I/O controller) until the desired condition is met

This process is called polling

Polling involves overhead

Repeated testing of condition


Repeated testing of condition

Can be used to handle only anticipated event

The interrupt-driven method

Processor gets interrupted when a specified event occurs

Very efficient

Can be used to handle unanticipated events


Polled I/O

Usually in the interface chip we have

Control register

a register where the processor is placing commands

Data register

input and output data registers where the data is made available to/from the processor

Status register


a register that shows the status of the peripheral device

For input -- the microprocessor checks a status bit of the interface chip to find out if new data has been received from the input device.

For output -- the microprocessor checks a status bit of the interface chip to find out if it can send new data to the interface chip.


A Generic Interface Chip

Chip select

Register select

Register select

R/W

CS

RS1

RS0

R/W

Timingand

Control

Busbuffers

Bidirectionaldata bus

Port Aregister

Port Bregister

Controlregister

Statusregister

I/O data

I/O data

Control

Status

CPU I/ODevice


register

InputsRegister Selected

E RS1 RS0

1 X X None

0 0 0 Port A Register

0 0 1 Port B Register

0 1 0 Control Register

0 1 1 Status Register


Communicating with External Devices : Parallel Interface

Several ports each 1-byte wide

A full byte of data or more is transferred on data cycle.

Example devices:

MC6821 PIA for 6800 CPU

MC68230 PIT for 68000 CPU

They are general purpose devices

Direction of data flow is programmable, unlike in elementary I/O


Direction of data flow is programmable, unlike in elementary I/O

Strobe and handshaking lines for more efficient data transfers.

May interrupt the CPU if peripheral is ready for data

These devices are often quite complex and can operate in one of several modes


Communicating with External Devices : Serial Interface

Serial data transmission

One bit is transferred for each clock cycle.

cheap, simple, easy to use

Connects the µP to another µP or peripheral such as modem

Example devices:

MC6850 ACIA for 6800 CPU

MC68681 DUART for 68000 CPU


They are programmable devices

To use, the designer must program number of bits per character (not necessarily 8 bit/char), parity etc

Can interrupt µP when peripheral is ready

Possible to connect many device to single serial line (multidrop).


LEDs – Simples Elementary Output

An LED is a semiconductor device that converts electrical energy directly into a discrete color of light

Can be visible or IR

White LEDs are blue LEDs plus phosphor

1.5 - 2 V voltage drop


1.5 - 2 V voltage drop

Typically draws 10-20 mA

Wire BondAnode Lead Cathode LeadEpoxy CaseEpoxy LensSemiconductor Diode Reflective Cup


Turning on an LED

+5VR

output pin1

No current

LED

no light+5V

Note: A zero turns on the LED


+3.2VCurrent

light

R =voltage

current=

2.8

13 x 10-3 ~ 220 ohms

+5VR

output pin0

LED

+0.4V


Interfacing LED with Latch

Unlike diodes, Light-emitting diodes have a forward voltage drop from 1.7 to 2.5 volts and most of them flow a forward current 10-20mA.

Poor circuit

TTL outputs can't source above 1mA so the LED will be very dim.

Fair circuit

The LED will conduct heavily at about 2V and the extra 3V has to be dropped in the TTL circuitry. This


extra 3V has to be dropped in the TTL circuitry. This causes high power dissipation in the TTL or the LED fails.

Good circuit

The resistor limits the current. The resistance can be calculated by assuming its voltage is about 2V and the TTL output is 0.4V. See previous page. The resistor should not less than 100ohm or the LED would fail.


Simple Interfacing CPU to LED

+5V

220ž74LS374

D7

D6

D5

D4

D3

Q7

Q6

Q5

Dari bas data

220ž

220ž

220ž

D7

D6

D5

D4

D3

LED EQU $E00010

START ORG $1000

MOVEQ #1,D0

LOOP MOVE.B D0,LED


D3

D2

D1

D0

LE

OE

Q4

Q3

Q2

Q1

Q0

data 68000

Dari penyahkod alamat

220ž

220ž

220ž

220ž

220ž

D3

D2

D1

D0

MOVE.L #200000,D1

DELAY SUBQ.L #1,D1

BNE DELAY

ROR.B #1,D0

BRA LOOP

END START


7-Segment Display

There are applications where you need to display a decimal digit using a seven segment LED display.

The display could represent e.g. the number of times a switch was pressed.

Digits 0-9 and hex A-F can be displayed by giving the proper 7-segment codes

q g f e d c b a q g f e d c b a

0 0 1 1 1 1 1 1 8 1 1 1 1 1 1 1


0 0 1 1 1 1 1 1 8 1 1 1 1 1 1 1

1 0 0 0 0 1 1 0 9 1 1 0 0 1 1 1

2 1 0 1 1 0 1 1 A 1 1 1 0 1 1 1

3 1 0 0 1 1 1 1 b 0 0 1 1 1 1 1

4 1 1 0 0 1 1 0 C 0 1 1 1 0 0 1

5 1 1 0 1 1 0 1 d 1 0 1 1 1 1 0

6 1 1 1 1 1 0 1 E 1 1 1 1 0 0 1

7 0 0 0 0 1 1 1 F 1 1 1 0 0 0 1


Types of 7 Segment Displays

Common-anode (CA):

LED ON when Output is LOW.

8 cathode legs, 1 anode

Common-cathode (CC):

LED ON when Output is HIGH.

8 anode legs, 1 cathode

TTL and CMOS devices are normally not used to drive the


normally not used to drive the common-cathode display directly because of current (mA) requirement. A buffer circuit is used between the decoder chips and common-cathode display


Using Lookup Tables

Using lookup table to drive the 7-segment displays will result in efficient programs.; Driver subroutine to display digits

; Digit to be displayed in D0

; Address of interface is "DISP"

; Table of segments is "DIGTAB"

DSPLAY: MOVEM.L A0/DO,-(A7)

MOVE.L #DIGTAB,A0

EXT.W D0


EXT.W D0

EXT.L D0

ADD.L D0,A0

MOVE.B (A0),DISP ;OUTPUT

MOVEM.L (A7)+,A0/D0

RTS

; Format of 7-segment: 0gfedcba Common-anode

DIGTAB: DC.B %00111111, %00000110

DC.B %01011011, %01001111

DC.B %01100110, %01101101

DC.B %01111101, %00000111

DC.B %01111111, %01100111


Elementary Input with 68000

Simplest input device is the three-state buffer .

Buffer is very fast (around 20 ns), so DTACK* does not need to be delayed.

Needs only 1 memory address.

D0-D7 atau

68000 Penimbal

D0-D7


atau D8-D15

A1-A23

AS

DTACK

Penyahkod alamat

OE

D0-D7

Punca DTACK daripada peranti lain

Vcc


Interfacing Switch with 68000

Poor circuit

Logic 1 is stable when the switch is closed. But when the switch is open, the input floats to a noise-sensitive high rather than a low. An open TTL pin is usually read as logic 1 but the pin may picks up noise like an antenna. Impossible to get a stable low input.

Fair circuit


Fair circuit

Requires a very small the pull-down resistor. Otherwise, the pin will rise above 0.9V when the resistor passes the 1.6mA sinking current. When the switch is closed, the circuit waste a large current since virtually no current flows into the pin. The only advantage is that a switch closure gives logic 1.



Good Circuit

When the switch is open, the 10k resistor

supplies very small current needed for logic

1. When it is closed, the port pin is short to

ground. The voltage is 0V and all the sinking

current requirement is met, so it is logic 0.

The 10k resistor will pass 0.5 mA (5 Volt/10k

ohm). Thus the circuits waste very little

current in either state. The drawback is that

the closure of switch gives logic 0 and


the closure of switch gives logic 0 and

people like to think of a switch closure gives

logic 1. But this is not a matter because it is

easy to handle in software.

To conclude, driving a TTL input should always consider current sinking (pulling input to 0V).



+5V

1kž

1kž

74LS244

D7

D6

Q7

Q6

D7

D6

;

; ATURCARA UNTUK MENGUJI SUIS

;

SUIS EQU $800001 ;ALAMAT SEBENAR BERGANTUNG SISTEM

LED EQU $A00001

ORG $1000

START MOVE.B SUIS,LED


1kž

1kž

1kž

1kž

1kž

1kž

1kžOE1

OE2

D6

D5

D4

D3

D2

D1

D0

Q6

Q5

Q4

Q3

Q2

Q1

Q0

Ke bas data 68000

Dari penyahkod alamat

D6

D5

D4

D3

D2

D1

D0

BRA START

END


VDD

Set

Reset

R

R

Q

Reset Set

Q

(a) Set-reset latch

RV

DD

Switch Debouncing

When mechanical switches are opened or closed there are brief oscillations due to mechanical bouncing (switch bounce).

key bounce, tBOUNCE


4050 Vout

(b) CMOS gate debouncer

VDD

Vout

H

L

Threshold level

Switch closed

(c) Integrating RC circuit debouncer

Figure 7.42 Hardware debouncing techniques

R

C

3 basic hardware debouncing methods SR latches

Non-inverting CMOS gates

Integrating debouncer

The most popular and simple one has been the wait and see method. The program simply waits for about

10 ms and reexamines the same key again to see if it is still pressed.


One Hardware Debounce Method



BCD-to-7-Segment Decoder/Driver

(a) BCD-to-7-segment decoder/driver driving a common-anode 7-segment


segment LED display;

(b) segment patterns for all possible input codes.


CPU checks

Keypad Decoding: 4x4 or 4x3 keypad


checks these lines

CPU sends low signal to these

lines


Row 0

Row 1

Row 2

Row 3

0 1 2 3

4 5 6 7

8 9 A B

C D E F

Row 1

01 01

Decoding the Keyboard Matrix

Pressing 5Example: press 5 and E

Col 1

5

E


Col 0

Col 1

Col 2

Col 3

11 10

Pressing E (=14)

Row 3 Col 2


123

171615

Y1Y2

DOADOBConnect to column pins Row & col position

74C922 Keypad Encoder

Performs the decoding and debouncing in hardware


74C922

34

111087

56

13

1514

12

Y2Y3Y4X1X2X3X4

OSCKBM

OE

DOBDOCDOD

DA

Connect to column pins

Connect to row pins

Connect to oscillator

Low to enable output

Row & col position

DA means Data

Available

High when a key

is pressed


74LS244

To 68000Data Bus

D1D2D3D4D5D6

74C9221234

1110

8

17161514

Y1Y2Y3Y4X1X2X3

DOADOBDOCDOD

74C922 Keypad Encoder

D0


HL

U9C

74LS14

56

D6D7

C1

0.068µF

C1

0.68µF

87

56

13 12

X3X4

OSCKBM

OE DA

Wait until DA pin high before accepting the 4-bit data


Interfacing Other Devices with Latches

Buzzers: Most latches do not have the current

capacity to drive a buzzer directly so a

transistor is added.

Diode protects coil transient voltages.

Relays:

A relay is an excellent way of isolating

the latch & CPU from high voltage

circuits.

Use a relay to control motors,

solenoids, lamps etc.

+

+5V

2N3904

+5VRelay

N.S.To Indicating


From latch2.2KΩ

Q1

C

B

E

D 1

2N3904NPN

PiezoBuzzer

-

2.2KΩ

Q1

C

B

E

D 1

2N3904NPN

N.O.

N.S.To Indicating Light

DC Motor

12 VDC

From latch


Example: System Having Only Elementary I/O



Liquid Crystal Displays: HD44780

The HD44780 is an industry standard LCD controller.

An I/O port can easily be used to interface with the HD44780.

By controlling the value of RS and R/W the MCU can easily either sends instructions or data to the controller.

Most operations require 40 micro seconds to complete.



Multiplexing


module 12: elementary input/output display example input devices push button proximity switch rotary...

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