1. 8051 I/O Interfacing Need for more ports PPI 8255
Key board Interfacing
LED Interfacing
7 Segment LED Interfacing

2. 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.

3. 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)

4. 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)

5. The 8255A block diagram.

6. 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
- A1. A0 are lower 2 bits of as address lines.
Issued by µp

7. A1 A0
Port A
Port B
Port C
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.

8. The I/O ports can be programmed to work in any of the 3 modes
Mode 0 - Basic Input –Output
Mode 1 - Strobed Input-Output
Mode 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

9. 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

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

12. Example-
We need to configure with Port A as input, Port B as output, Port C (L) as output, port C (upper) as input using mode=00

13. Note – Mode set flag i. e. D7=1 for all configuration
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

14. I/O and memory are treated separately
8255 – 8051 Interfacing
Let us review I/O mapped I/O and memory mapped I/O [Ref. Page 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.

15. 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

16. 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.

17. 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

18. Assume that 8255 is interfaced at address FE10H
Port A address = FE10H
Port B address = FE11H
Port C address = FE12H
Control register address = FE13H
MOV DPTR, # FEI3H
MOV A, # 98H A
(will configure 8255)
MOV DPTR, # FE10H
MOVX DPTR
(will bring the content of Port A to ACC.)

19. To generate CS for 8255 from given address. Assume address = FE10
F E
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

20. Many Variations of circuits are possible
CS of 8255 A7 A6 A5 A4 A3 A2
Many Variations of circuits are possible

21. If we select 8255 address as 0030H then decoding may be- A5 A4 A7 A6 A5 A4 CS A3 A2

22. 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.
What will be decoding strategy ?

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

24.
A15
A12
A1 A0
CS of
8255 OR
A15
A14
A13
A12
A11
A10
A 9
A 8
Decoding of 8255 Address E000H to generate chip select

25. Reading and writing to 8255 ports
Let us assume that 8255 has been interfaced at address E000H the port addresses will be
Port A - E000H
Port B - E001H
Port C - E002H
Control register - E003H
Consider that the application demands the following ports configuration
Port A - Output
Port B - Input
Port C - Output
Mode = 0
What will be control word of 8255 ?

26. The 8255 control word will be-1000 0010 =82H
We may configure 8255 by writing 82H to control register in two ways:-
(A)
MOV DPTR, # E003H
MOV A, # 82H
Note - E003 is location in external data memory.
Other way of writing (A) will be
(B)
PRA EQH E000H
PRB EQH E001H
PRC EQH E002H
PRCR EQH E003H
ORG 00H
SJMP 030H
ORG 030H

27. MOV DPTR, # PRCR
MOV A, # 82H A
To read data from Port B
MOV DPTR, # PRB
MOVX
To write data to Port A
MOV DPTR,# PRA
MOV A,# Data A

28. 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

29. Basic keyboard operation is shown in figure
Keyboard interfacing
Basic keyboard operation is shown in figure IN
High
OUT (High when the key is pressed)
Basic keyboard operation-single key.

30. Basic keyboard operation- two keys.
OUT 1
OUT 2 IN

31. Basic keyboard operation – four keys.
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 ports
We may represent the above as.

32. C
(1)
OUT1
(2)
IN1
OUT2
(3)
(4)
IN2
2x2 keyboard operation

33. 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.

34. 8x8 keyboard interfacing with microprocessor

35. Code table for 8 x 8 keyboard
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.
XX1
Codes for keys
in the first row
Codes for Keys
in the second row
in the eighth row
XX2
XX8
Code table for 8 x 8 keyboard

36. Keyboard- microprocessor interface software flowchart
Start I
Enable the Ith row
Set the address of the code for the first key on this row
Check for key depression
Key
Depressed ?
Determine the column number and the code of the key
I I+1
I>8
Yes No

37. 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.

38. 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.

39. Fig-33

40. 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.

41. LED interface with microprocessor
Common cathode Anode connected to Port Lines
All cathodes connected together
to grounds
[ Port bit = 1
LED Conducts
i.e glows ]
Port
Bit 7
Bit 0
Microprocessor interface to LED (common cathode)

42. Common Anode Anodes of all LED’s connected together to 5V
Cathodes of LED’s connected to port lines
[ Port bit = 0
LED Conducts
i.e glows ]
Bit 7
Bit 0
+5 V
Microprocessor interface to LED (common anode).

43. 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.

44. Example :- For displaying A, all segments except d and h should glow.
Seven segment LED’s – The LED’s can be arranged in the fashion shown in a b c d e f g h
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.

45. Character formation in seven segment LEDsb c e f g d

46. 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.

47. Representing segment code in byte - two ways
a b c d e f g h
h g f e d c b a
0-Glow the segment for common Anode configuration
1-Do not glow

48. Common Catnode - bit =1, segment glows
bit = 0 – doesn’t glow
Common Anode - bit = 0 – segment glows
bit = 1 – segment doesn't glow
Microprocessor can be interfaced to seven segment LED in parallel or serial way.

49. … Parallel interface +5 V Anode a b h Resisters Out port 7 6
Microprocessor interface to seven-segment LED (parallel interface).

50. in common Anode fashion
Cathode of segments – connected to port bits.
MOV Px, #00H – will glow all the segments
CLR 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.

51. … +5V Anode a b c d e f g h a b h Resisters MSB LSB Shift register
Code bits
Clock
Microprocessor interface to seven-segment LED(serial interface)

52. 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 No
Seventh
bit loaded ?
Load clock
Yes
Stop

53. After 8 clock period the character will appear in 7 segment.
Example:- Display code for A 1
h g f e d c b a a f b g
for common anode configuration . e c

54. 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.

56. 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

57. 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.