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Input/Output Ports and Interfacing

ELEC 330Digital Systems Engineering

Dr. Ron ayne

Images Courtesy of Ramesh Gaonkar and Delmar Learning

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!asic I/O Concepts

Perip"erals suc" as LEDs and #eypads are essential

components of microcontroller$%ased systems Input de&ices

Pro&ide digital information to an 'P( E)amples* s+itc", #ey%oard, scanner, and digital camera

Output de&ices Recei&e digital information from an 'P( E)amples* LED, se&en$segment display, LCD, and printer

De&ices are interfaced to an 'P( using I/O ports

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I/O Interfacing

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Interfacing and ddressing

I/O ports !uffers and latc"es on t"e 'C( c"ip

ssigned %inary addresses %y decoding t"e address %us

1enerally %idirectional

Internal data direction registers

2o read %inary data from an input perip"eral

'P( places t"e address of an input port on t"e address %us

Ena%les t"e input port %y asserting t"e RD signal

Reads data using t"e data %us

2o +rite %inary data to an output perip"eral 'P( places t"e address of an output port on t"e address %us

Places data on data %us

sserts t"e R signal to ena%le t"e output port

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PIC56748-/48-0 I/O Ports

'C( includes fi&e I/O ports POR2, POR2!, POR2C, POR2D, POR2E

Ports are multiple)ed Can %e set up to perform &arious functions

Eac" I/O port is associated +it" se&eral S7Rs POR2

7unctions as a latc" or a %uffer

2RIS

Data direction register

Logic 0 sets up t"e pin as an output

Logic 5 sets up t"e pin as an input

L2 Output latc" similar to POR2

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POR2* E)ample of 'ultiple 7ns

Digital I/O* R9$R0

nalog Input* :0$:4

; RE7< * /D Reference Plus ;

; RE7$ * /D Reference 'inus ;

2OC=5* 2imer0 E)t. Cloc#

SS* SPI Sla&e Select Input

L;DI:* Lo+ ; Detect Input

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POR2!

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I/O E)ample

rite instructions to set up pins R!>$R!4 of

POR2! as inputs and pins R!3$R!0 as outputs

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Opcode Operands Comments'O;L 0)70 ?Load !@55550000@ into RE1

'O;7 2RIS! ?Set POR2! 2RIS Reg

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Interfacing Output Perip"erals

Commonly used output perip"erals in em%edded systems LEDs Se&en$Segment Displays LCDs

2+o +ays of connecting LEDs to I/O ports Common Cat"ode

LED cat"odes are grounded Logic 5 from t"e I/O port turns on t"e LEDs Current is supplied %y t"e I/O port called current sourcing

Common node LED anodes are connected to t"e po+er supply Logic 0 from t"e I/O port turns on t"e LEDs Current is recei&ed %y t"e c"ip called current sin#ing

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Interfacing Output Perip"erals

Common Cat"ode Common node

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Se&en$Segment Display

Se&en$segment Displays (sed to display !CD digits

0 t"ru

group of > LEDs p"ysically

mounted in t"e s"ape of t"e num%er

eig"t Plus a decimal point

Eac" LED is called a segment

Aa@ t"roug" Ag@ 2+o types

Common anode

Common cat"ode

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Se&en$Segment Display

Common Cat"ode ll cat"odes are connected toget"er to ground nodes are connected to data lines

Logic 5 turns on a segment

E)ample* 2o display digit 5 ll segments e)cept % and c s"ould %e off 00000550 B 09

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E)ample .4

Interfacing Se&en$Segment Display to POR2! Common node

2a%le Loo#$(p

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Interfacing Input Perip"erals

Commonly used input perip"erals DIP s+itc"es, pus"$%utton #eys, #ey%oards, and /D con&erters

DIP s+itc"

One side of t"e s+itc" is tied "ig" 2o a po+er supply t"roug" a resistor called a pull$up resistor

2"e ot"er side is grounded

2"e logic le&el c"anges +"en t"e position is s+itc"ed

Pus"$%utton #ey Same as t"e DIP s+itc" e)cept t"at contact is momentary

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Interfacing Dip S+itc"es

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Reading from an I/O Port

Read input s+itc"es on POR2! R!>$R!4 R!0 set I 5

S+itc"es Open B LO 0

S+itc"es Closed B I1 5

Display on POR2C

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Opcode Operands Comments

'O;L 0)70 ?Load !@55550000@ into RE1

'O;7 2RIS! ?Set POR2! 2RIS RegCLR7 2RISC ?Set POR2C as Output

!S7 POR2!,0 ?Set R!0 ig"

'O;7 POR2!, ?Read POR2!

'O;7 POR2C ?Display on POR2C

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Internal Pull$(p Resistor

2urning off t"e internal 7E2

pro&ides a pull$up resistor

!it> R!P( in t"e I:2CO:-

register ena%les or disa%les t"e

pull$up resistor Instruction to Ena%le Pull (p

Resistors*

!C7 I:2CO:-,>

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Interfacing Pus"$!utton =eys

"en a #ey is pressed or released, mec"anical metalcontact %ounces momentarily and can %e read as multipleinputs

=ey de%ounce

Eliminating reading of one contact as multiple inputs ard+are or Soft+are

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=ey De%ounce 2ec"niues

ard+are tec"niue 2+o ::D gates

S$R latc"

2"e output of t"e latc" is

a pulse +it"out a %ounce

Soft+are tec"niue ait for 50 to -0 ms

after detection of a

s+itc" closure

If t"e reading is still t"e

same it is accepted

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Interfacing a 'atri) =eypad

Pro%lem statement Interface a 4 ) 4 e) #eypad to POR2!

rite a program to recogniFe a #ey pressed and

encode t"e #ey in its %inary &alue Display %inary code on POR2C

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ard+are PIC56 Simulator 4 ) 4 matri) #eypad organiFed in t"e ro+ and

column format 7our columns are connected to t"e lo+er "alf of

POR2! R!0$R!3 7our ro+s are connected to upper "alf of POR2!

R!4$R!> "en a #ey is pressed, it ma#es a contact +it"

t"e corresponding ro+ and column

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PIC56 Simulator =eypad 'atri)

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4

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8

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0

3

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C

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2ime 'ultiple) Scanning

ard+are Eig"t data lines of POR2! are connected to t"e anodes of

eac" display

Eac" cat"ode is connected to POR2C RC3$RC0 t"roug" a

transistor

2ransistors and LEDs can %e turned on %y sending logic 5

Eac" display is turned on and off in a seuence to display a

digit

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2ime 'ultiple) Scanning

Soft+are Codes of t"e num%ers to %e displayed are stored in data

registers in seuence 2"e program gets t"e codes from t"e data registers %y

using t"e pointer 7SR0 and sends t"em out to t"e LEDsegments t"roug" POR2!

One display at a time is turned on %y sending logic 5 to t"ecorresponding transistor connected to POR2C

fter an appropriate delay, t"e first display is turned offand t"e ne)t display is turned on 2urning displays on/off is repeated in seuence

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Interfacing LCD

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Interfacing LCD

Dri&er D44>60 6$%it data %us RD>$RD0 2"ree control signals

RS G Register Select R3R/ G Read/rite R-E G Ena%le R5

2"ree po+er connectionsPo+er, ground, and &aria%le resistor to control %rig"tness

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Interfacing LCD

LCD Operation "en t"e 'P( +rites an instruction to IR or data to DR,

t"e controller* Sets D!> "ig" indicating t"at t"e controller is %usy

Sets D!> lo+ after t"e completion of t"e operation 2"e 'P( s"ould al+ays c"ec# +"et"er D!> is lo+

%efore sending an instruction or a data %yte

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Documents

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