8051 I/O Interfacing

Need for more ports PPI 8255 8255 – 8051 Interfacing Key board Interfacing LED Interfacing 7 Segment LED Interfacing

I/O Interfacing• I/O Devices connected through ports• 8051 has 4 I/O ports P0 to P3.• In case 8051 needs external program and/or data memory

P0 and P2 are used for address bus & P0 is used for data bus.

Only 2 ports (P1,P3) remain i.e. only 2 I/O devices can be connected.

In case system application needs interrupt, serial I/O, i.e. alternate functions of P3

Only 1 port (P1) remain i.e only 1 I/O device can be interfaced.

• In case of 8031(Rom less version )• Program memory is external i.e. (P0 and P2) are not available for I/O

interfacing.• If P3 in used for alternate functions then

Only one port (P1) in available. • We need more ports.• Intel has designed programmable peripheral Interface (PPI) Intel

8255 for this purpose.• Intel 8255 PPI has 24 I/O lines distributed to four ports.

Port A – (8 Lines)Port B – (8 Lines)Port C – (Upper) – (4 Lines)Port C- (Lower) – (4 Lines)

• Each port can be programmed to be input or output.• Individual port lines can’t be read or written to - different from 8051 ports. • Ports have been put in two groups.• Group A - Port A and Port C (Upper) (PA7 – PA0) (PC7 – PC4)• Group B - Port B and Port C (Lower) (PB7 – PB0) (PC3 – PC0)

The 8255A block diagram.

• D7- D0 – Used for data transfer between µp and 8255(Bidirectional)• RD (Input) - Read control Signal • WR (Input) - Write control Signal • RESET (Input)- Resets the ports. Ports are configured an input ports on Reset.• CS - Used to select the 8255 chip. Chip is

selected based on the address decoding. • A1, A0 - Are used to select the specified ports in

8255. - A1. A0 are lower 2 bits of as address lines.

Issued by µp

A1 A00 0 - Port A0 1 - Port B1 0 - Port C1 1 - Control Register

Note – There is no separate selection for Port C (Upper) or Port C (Lower) Port C is selected as a whole i.e. both upper and Lower ports are selected when A1, A0 = 10.

• Control Register is used to program the - Ports as input or output- Program the I/O mode.

• The I/O ports can be programmed to work in any of the 3 modes

Mode 0 - Basic Input –OutputMode 1 - Strobed Input-OutputMode 2 - Strobed Bidirectional Bus

• Mode 0 – Basic Input-Output majority of application fall under this mode.

- Any port A , B, C(U) or C(L) can be input or output

• Mode 1- Strobed input-output - Provides means for transferring I/O data to or from

a specified port in conjunction with strobes or hand shaking signals.

- Port C lines are used for hand shaking signals.- Port A and Port B can be used as input or output. - Port A uses port C (Upper) lines for hand shaking –

Group A- Port B uses port C (Lower) lines for hand shaking – Group B

• Mode 2- Strobed Bidirectional Bus - Port A can be used for input as well or output for transmitting

as well as receiving the data in conjunction with hand shaking signals on Port C.

- Hand shaking signals are provided to maintain proper bus flow direction

- 5 bits (PC7-PC3) of port C are used for hand shaking. Mode 2 is available for only Group A.

- Parallely Group B i.e port B can be used in mode 0 or 1.• Configuring 8255- We have already mentioned that 8255 ports can be configured

using control register (A1 A0 = 11).- The control word format for configuration of 8255 is shown

• Example- We need to configure 8255 with Port A as input,

Port B as output, Port C (L) as output, port C (upper) as input using mode=00

Note – Mode set flag i.e. D7=1 for all configuration. If this bit is one then only mode setting with be done by 8255.

• Thus D7=1 , D5, D6=00-Mode 0, D4=1 (Port A=Input), D3=1[Port C(Upper)=Input], D2=0, (Mode=0) D1=0 (port B=output), D0=0 [ Port C(lower)=output]

• So the Configuration byte = D7 D6 D5 D4 D3 D2 D1 D0

=98 H• By transferring 98H to control Register of 8255 will configure

8255 in the above manner.• How -? We shall see after interfacing of 8255 with 8051. Note – The concept of 8255 configuration using control

register has been used in 8051 in case of Timer, Serial I/O and interrupts.

1 0 0 1 1 0 0 0

8255 – 8051 Interfacing • Let us review I/O mapped I/O and memory mapped I/O [Ref.

Page 38-39 of Krishna Kant book.]• In I/O mapped I/O- The µp has separate instructions for

memory and I/O read-write.• In addition µp will have separate control signals for I/O read-

write and memory read-write. • A variation of this may be that µp has same read-write control

signal for I/O and memory but may have another signal to identify whether the read-write is for memory or for I/O.

I/O and memory are treated separately Example – 8086 has IN and OUT as I/O read and write

instructions. It has same RD (for read),WR (for write), signal for I/O and memory read- write. It has M/IO to identify memory or I/O read-write.

In Memory mapped I/O • I/O ports are treated as memory locations.• µp having memory mapped I/O.• Will not have separate instructions for I/O and memory read-

write.• Will have only single read and write control signals for read and

write.• Will not have any signal to identify memory or I/O read-write. Example - • 8051 has no separate instructions for memory and I/O read -

write • 8051 has only single read and write control signal

• PSEN – external program memory read • RD – External data memory read• WR - External data memory write• It has no signal to indentify whether read - write in for I/O or

memory.• Thus – I/O ports are treated as memory locations.• Thus if we have to interface 8255 to 8051 , 8255 must be

interfaced in external memory• External program memory can’t be written to .• Thus 8255 must be interfaced in the external data memory.• We need to select the address ‘XXXXH’ of 8255 • The address on address lines to be decoded such that when

‘XXXXH’ is there, CS for 8255 is generated i.e 8255 needs to be selected.

• RD (8051) connect to RD (8255)• WR (8051) connect to WR (8255)• Movx instructions to be used for data transfer from / to 8255• 8255 has four ports• When address lines

A1 – A0 = 00 - Port A is selected= 01 - Port B is selected= 10 - Port C is selected= 11 - Control register is

selected• Thus last two bits starting address of 8255 address must be 00

• Assume that 8255 is interfaced at address FE10HPort A address = FE10HPort B address = FE11HPort C address = FE12H

Control register address = FE13HMOV DPTR, # FEI3HMOV A, # 98HMOVX, @DPTR, A

(will configure 8255)MOV DPTR, # FE10HMOVX A, @ DPTR

(will bring the content of Port A to ACC.)

• To generate CS for 8255 from given address.Assume address = FE1015 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

F E 1 0• A1 , A0 are directly connected to 8255 and used for selection

of I/O ports A,B,C and Control Regester. • For selection of 8255 through CS, the address bits A2 to A15

may be decoded using simple gates.

1 1 1 1 1 1 1 0 0 0 0 1 0 0 0 0

A15A14

A13

A10A11

A12

A8A9

A6A7

A5

A2

CS of 8255

A4

A3

Many Variations of circuits are possible

• If we select 8255 address as 0030H then decoding may be- A5 A4

0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 A7

A6

A5A4

A3

A2

CS

• But it will generate Cs for 8255 for all address where A7 to A2 = 00 11 00

• i.e. 0030H, 0130H …………….FF30H• All these address will be valid for 8255 selection and are

called address aliases – (aliases is used in general for other names identifying a person)

Problem - 1• If selected address for 8255 = 8000H i.e. • 1000 0000 0000 0000• What will be decoding strategy ?

Problem -2 • If 8255 address = 1110H • Then what will be decoding• Decoder 74LS138 may also be used.

1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0A15

A12

A1 A0

CS of 8255

OR

Decoding of 8255 Address E000H to generate chip select

A15

A13

A11

A12

A14

A10

A 9

A 8

Reading and writing to 8255 ports• Let us assume that 8255 has been interfaced at address

E000H the port addresses will be Port A - E000HPort B - E001HPort C - E002HControl register - E003H

• Consider that the application demands the following ports configuration Port A - OutputPort B - InputPort C - Output

Mode = 0 What will be control word of 8255 ?

• The 8255 control word will be-1000 0010 =82HWe may configure 8255 by writing 82H to control register in two ways:-

(A)MOV DPTR, # E003HMOV A, # 82HMOVX @DPTR,ANote - E003 is location in external data memory.Other way of writing (A) will be(B)PRA EQH E000HPRB EQH E001HPRC EQH E002HPRCR EQH E003H

ORG 00HSJMP 030HORG 030H

MOV DPTR, # PRCRMOV A, # 82HMOVX @DPTR, A

• To read data from Port BMOV DPTR, # PRBMOVX A,@DPTR

• To write data to Port AMOV DPTR,# PRAMOV A,# DataMOVX @DPTR, A

• Note :- 8255 ports ie . A,B,C have no addressable bits.• That to find status of individual bits –

– Read port context– Use logical operations ANL ORL etc

Or - Use shift/rotate RRL , RRL etc• In keil bit (24 MHz, 8051) 8255 has been interfaced at addres

E000H• In program (B) way of writing is normally used.• Now let us take some I/O interface• We shall first take up • Keyboard interfacing followed by • LED interfacing and • 7 segment LED interfacing

• Keyboard interfacingBasic keyboard operation is shown in figure

INHigh

OUT (High when the key is pressed)

Basic keyboard operation-single key.

Basic keyboard operation- two keys.

OUT 1

OUT 2

IN

OUT 3

OUT 2

OUT 1

OUT 4

IN

Basic keyboard operation – four keys.

• Above Configuration can’t be adopted for more no. of keys A 64 key keyboard will require 8 bit portsWe may represent the above as.

C

OUT1

OUT2

IN1

IN2

(1)

(2)

(3)

(4)

2x2 keyboard operation

• Contains two rows – IN1 and IN2 and two columns OUT1 and OUT2

• Activate Row1 i.e IN1 check OUT1 (If 1 then key No. 1)• Else cheek OUT2 (If 1 then key No. 2)• Activate Row2 i.e IN2 check OUT2 (If 1 then key No. 3)• Else check OUT2 (If 1 then key No. 4)• 8 x 8 key board can be arranged in 8 rows and 8 columns.• Only two ports are required to interface with µp.

8x8 keyboard interfacing with microprocessor

• The codes of keys of key board may be staored row wise is memory.

• When a Key is pressed.

row no. and column no.

i.e. position of key is identified

key code is taken from table and used in the program.

Codes for keysin the first row

Codes for Keysin the second row

Codes for keysin the eighth row

XX1

XX2

XX8

Code table for 8 x 8 keyboard

Start

I 1

Enable the Ith row

Set the address of the code for the first key on this row

Check for key depression

KeyDepressed

?

Determine the column number and the code of the key

I I+1

I>8?

Yes

No

YesNo

Keyboard- microprocessor interface software flowchart

Key Debouncing

• When a key is depressed and released contact is not broken permanently.

• Key makes and breaks the contact several time for few milliseconds before the contact is broken permanently.

• Thus a key depression detected by µp may be false i.e it may be due to bouncing of key.

• Thus key debouncing i.e. as certaining that key depression in true is important.

• Debouncing by HW and SW both in possible. • Software debouncing after key depression detected. • µp executes a delay routine for few milliseconds. • µp then again checks for key depression.• If key is depression is detected then real depression else false

depression.

• Let us interface 4x4 keyboard with 8051• Both rows and columns can be connected to 8 lines of one

port.• Rows can be activated by using SET B bit instruction.• Key Depression can be checked by using JB or JNB

instructions. • Hex key pad interface in Lab. - 16 keys (0 to F) arranged in - 4 x 4 key board• Rows connected to – PB0 to PB3• Columns connected to – PA0 to PA3• You have to determine the key depression• Store the code of the key in memory.

Fig-33

LED Interface • LED i.e Light Emitting Diode

emits light energy when conducts

Anode is held at higher voltage than cathod

+5 V

LED operation.

LED interface with microprocessor • Common cathode Anode connected to Port Lines

All cathodes connected together

to groundsPort

Bit 7

Bit 0

Microprocessor interface to LED (common cathode)

[ Port bit = 1LED Conducts i.e glows ]

• Common Anode Anodes of all LED’s connected

together to 5V

Cathodes of LED’s connected to port lines

Bit 7

Bit 0+5 V

Microprocessor interface to LED (common anode).

[ Port bit = 0LED Conducts i.e glows ]

• In common cathode- by making individual port bits as “1” we may glow the LED.

• SETB P2.7 i.e LED connected to P2.7 will glow• MOV P2,#OFFH – All LED’s connected to port Port P2 will glow• In common Anode making a particular port bit ‘0’ will glow

the LED connected to the port line.• CLR P2.7- LED connected to P2.7 will glow• MOV P2, # 00H- Will glow all the LED’s connected to port P2.

• The structure has eight segments a,b,c,d,e,f,g and h. Very use full in displaying numeric and alphanumeric data.

Example :- For displaying A, all segments except d and h should glow.

a

b

c

d

e

fg

h

Seven segment LED’s – The LED’s can be arranged in the fashion shown in

Character formation in seven segment LEDs

a

b

ce

fg

a

c

d

e

fg

• For displaying 6, all segments except b and h should glow.• h is used for displaying decimal point. Note – Previously there were only seven segments. Eighth

segment h has been added later. But name seven segment has remained stock.

• The On/Off in formation of segments can be arranged in one byte.

• Called control byte for 7 segment LED• The bits (port lines) can be connected to segments as

common catnode or common anode fashion. Common Catnode - Anodes connected to bits catnodes

connected to ground.Common Anode – Anodes connected to +5V catnodes

connected to bit.

Representing segment code in byte- two ways

7 6 5 4 3 2 1 0

a b c d e f g h

7 6 5 4 3 2 1 0

h g f e d c b a

0-Glow the segment for common Anode configuration 1-Do not glow

Common Catnode - bit =1, segment glows bit = 0 – doesn’t glow

Common Anode - bit = 0 – segment glowsbit = 1 – segment doesn't glow

• Microprocessor can be interfaced to seven segment LED in parallel or serial way.

Parallel interface +5 V

Anode

a b h

…Resisters

Out port

7 6 0 Microprocessor interface to seven-segment LED (parallel interface).

in common Anode fashion • Cathode of segments – connected to port bits.

MOV Px, #00H – will glow all the segmentsCLR Px .3 – will glow segment No. 3 i.e segment ‘e’.

• Simple Hard ware and software interface• For one 7 segment LED i.e display of 1 character one port is

dedicated.• Thus for displaying 20 characters, 20 ports will be required.

Generation of ports will require extra hardware.• Parallel interface is very fast but we don’t require very fast

changing display due to limitation of our eyes.

Serial Interface- Over comes the limitation of parallel interface in case of large no. of 7 segment displays.

Anode

a b h

…Resisters

+5V

MSBShift register

LSB

Clock

Microprocessor interface to seven-segment LED(serial interface)

a

b

c

d

e

fg

h

Code bits

• Contains – shift register connected to 7 segment LED.• Two input lines- one for code bits and other for clock.• The operation sequence is shown in

Start

Load LSB from display code

Load clock

Load next bit

Load clockSeventh bit loaded ?

Stop

No

Yes

1 0 0 0 1

0

0 0

• After 8 clock period the character will appear in 7 segment.Example:- Display code for A

7 6 5 4 3 2 1 0

h g f e d c b a

for common anode configuration .

a

b

ce

fg

• As bits move in shift register different segments glow and at the end of 8 clock periods ‘A’ appears.

• Connecting more segments doesn’t require any extra port or line. Using just clock and code bit line we may connect many segments in serial fashion. MSB of a segment is connected to input code bit line of shift register of next segment.

• The characters will appear almost instantenously. • Large No. of 7 segment displays can be connected• 74164 shift register is normally used. Any two port lines-one

for clock and other for code bits may be used for displaying.• Control byte may also be arranged as a b c d e f g h

• In this case we shall - Input MSB- Input Clock

• And continue doing it After 32 clocks, the 4 characters will appear in 4 -7 segment LED’s

• Coder bits of characters to be displayed may be stored in consecutive memory locations.

D0 D1 D2 D3

• Code bits are sent in the order of D0 to D3.• A segment counter is initialized to 32.• After every clock – decremented • When counter =0, - stop i.e. operation is completed • 7 segment display experiment is port of microcontroller lab.