Knowledge activities in daily life

1. ACOE255 1
Microprocessors
Input/Output Interface
(Chapter 10)

2. ACOE255 2
Introduction
• In a typical computer system, the user communicates with the computer via
standard peripheral devices such as the keyboard, mouse, display, printer e.t.c.
• Furthermore computers and microprocessor based systems are used in
instrumentation or automatic control applications. In such cases it is necessary
that the microprocessor reads the state of input devices (switches, sensors) and
activate some output devices (motors, heaters, lights).
• The I/O interface is required to enable the interface between the microprocessor
and the peripheral devices.
• The peripheral devices are connected on a microprocessor system through the
Input/Output ports.
• The I/O interface provides the following:
– Isolation between the buses and the peripheral devices.
– Address decoding.
– Synchronization/Latching.

3. ACOE255 3
Isolated vs Memory-mapped I/O
• Isolated I/O
– I/O locations are separate from memory locations
– Special I/O instructions are used
– The most common technique for Intel microprocessors
– Advantage: More space for memory
– Disadvantage: Additional control signals (IO/M) and instructions increase complexity
• Memory-mapped I/O
– I/O devices are treated as memory locations in the memory map
– Any memory transfer instruction can be used (MOV, LDR, STR etc)
– Advantages: Simpler decoding circuitry, no special instructions required
– Disadvantage: A portion of the memory system is used as the I/O map, reducing the
memory available to applications

4. ACOE255 4
Input/Output Instructions
• The 8088 and 80X86 processors use the 16 lower address lines (A0 to A15) to
address I/O devices. This limits the maximum number of I/O ports to 64K.
• The IN instruction copies the content of an input port to register AL, AX or EAX.
• The OUT instruction copies the content of register AL, AX, or EAX to an output
port.
• Register AL is used for 8-bit ports, AX for 16-bit ports and EAX for 32-bit ports.
• The 8088 and 80X86 processors support two I/O addressing modes.
– Fixed Address. The address is directly specified in the instruction as an 8-bit number.
That is loaded on the address lines A0 to A7. Address lines A8 to A15 are set to 00.
• IN AL,30H ;Read input port 0030H into register AL
• OUT 30H,AL ;Copy register AL to output port 0030H.
– Variable Address. The address is specified through register DX as a 16-bit address.
• MOV DX,3A0H ;Set I/O address
• IN AL,DX ;Read input port [DX] or 3A0H into register AL.
• OUT DX,AL ;Copy register AL to output port [DX] or 3A0H

5. ACOE255 5
High Level Language Input/Output Instructions (Pascal)
• Input/Output using Pascal: I/O is obtained using the ‘Port[ ]’ instruction.
– X := Port[PortAddress]; /* Copy the contents of the specified port into X */
– Port[PortAddress] := X; /* Copy the contents of X into the specified port */
• Input/Output using Delphi: Delphi does not allow direct access to I/O ports.
This can be done by inserting assembly language code into Delphi as follows:
asm ; code equivalent to ‘value:=port[portaddress];
begin
mov dx,portaddress ;specify the port address
in al,dx ;input from the port specified by “dx” into “al”
mov value,al ;copy input data into variable ‘value’
end;
asm ; code equivalent to ‘port[portaddress] := value;
begin
mov dx,portaddress ;specify the port address
mov al,value ;move output data into register “al”
out dx,al ;output data to the port specified by “dx”
end;

6. ACOE255 6
High Level Language Input/Output Instructions (C/C++)
• C/C++ does not allow direct access to I/O ports. This can be done by inserting
assembly language code into the C/C++ code as follows:
__asm ; code equivalent to ‘value:=port[portaddress];
{
mov dx,portaddress ;specify the port address
in al,dx ;input from the port specified by “dx” into “al”
mov value,al ;copy input data into variable ‘value’
}
__asm ; code equivalent to ‘port[portaddress] := value;
{
mov dx,portaddress ;specify the port address
mov al,value ;move output data into register “al”
out dx,al ;output data to the port specified by “dx”
}

7. ACOE255 7
Input/Output Using DLLs (C/C++)
• For protection purposes, modern operating systems such as the Windows XP do
not allow the use of the IN and the OUT instructions in the users programs.
• A way to have access to I/O ports is through libraries (DLL files) developed for
this purpose. Such a library is the inout32.dll
• To use this library in Visual Studio (C++) we must:
– Add the inout32.lib file in the Linker properties
• Project ->Properties->Linker->Input->Additional Dependencies
– Define the two functions for input and output and use them accordingly
short _stdcall Inp32(short PortAddress);
void _stdcall Out32(short PortAddress, short data);
main()
{
short Inval = Inp32(889); /* input from port 889 into variable “Inval”*/
Out32(888,0x2f); /* Output the hex value 2f to port 888 */
}

8. ACOE255 8
Simple Input Port
• An input port can be implemented
using a simple octal buffer such as
the 74LS244.
• The address decoding circuit
enables the three-state buffers of
the 74LS244 whenever the port
address is selected by the
processor. In such a case the state
of the input devices appears on the
data bus.
• The address decoding circuit is
enabled only if the IO/M signal is
set High by the 8088 (Low for the
x86 processors) and the RD signal
is Low.
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
LS244
G1 G2
A11
A10
A9
A8
A7
A6
A5
A4
A11A10 A9 A8 A4
A7 A6 A5 A1
A3 A2 A0
0 1 0 0 0
0 1 1 X
X X X
Address
460H to 46FH
Switch Open => Logic 1
Switch Closed => Logic 0

9. ACOE255 9
Simple Output Port (Using the 74LS373)
• An output port can be implemented
using a simple octal latch such as
the 74LS373.
• The address decoding circuit
enables the D-latches of the
74LS373 whenever the port
address is selected by the
processor. In such a case the state
of the data bus is latched to the
output devices.
• The address decoding circuit is
enabled only if the IO/M signal is
set High by the 8088 (Low for the
x86 processors) and the WR signal
is Low.
LS373
D Q
EN
EN OE
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
A11
A10
A9
A8
A7
A6
A5
A4
A11A10 A9 A8 A4
A7 A6 A5 A1
A3 A2 A0
0 1 0 0 0
0 1 1 X
X X X
Address
460H to 46FH
Logic 1 => LED is OFF
Logic 0 => LED is ON

10. ACOE255 10
Simple Output Port (Using the 74HCT573)
• The 74HCT573 is pin compatible
with the 74LS373. There are three
significant differences:
– The ‘573 has all inputs on the left side and
all outputs on the right side of the IC,
while the ‘373 has the even numbered
inputs on the left side and the odd
numbered inputs on the right side. Thus
the ‘573 is easier to by handled on PCB
boards.
– The HCT573 has a 20mA current sink and
a 20mA current source, thus it can drive a
LED connected either to the ground or to
+5V. The LS373 has a 16mA current sink
and a 400uA current source, thus it can
only drive a LED connected to +5V.
– The HCT family is faster than the LS
family.
HCT573
D Q
EN
EN OE
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
A11
A10
A9
A8
A7
A6
A5
A4
A11 A10 A9 A8 A4
A7 A6 A5 A1
A3 A2 A0
1 0 0 0 1
0 1 0 X
X X X
Address
850H to 85FH
+5V
Logic 1 => LED is OFF
Logic 0 => LED is ON
Logic 1 => LED is ON
Logic 0 => LED is OFF

11. ACOE255 11
Simple I/O Example 1 (Hardware)
• Draw a circuit to show how 8 switches can be connected on the input port 3AH
and 8 LEDs on the output port 5CH.

12. ACOE255 12
Simple I/O Example 1 (Software)
Write a program to keep reading the state of the switches and switch ON the LED
at the position form by the switches at D2, D1 and D0 (i.e. 3X8 decoder).
Pseudo Code:
Repeat
Read Input port in InVal
Mask out D2,D1,D0 from InVal
if InVal = 0 then OutVal = 1
if if InVal = 1 then OutVal = 2
if InVal = 2 then OutVal = 4
if InVal = 3 then OutVal = 8
if InVal = 4 then OutVal = 16
if if InVal = 5 then OutVal = 32
if InVal = 6 then OutVal = 64
if InVal = 7 then OutVal = 128
Write OutVal to Output port
Until keypressed
C/C++ Code
main()
{
short InVal, OutVal = 0;
do { InVal = Inp32(0x3a);
InVal = InVal & 0x7;
if (InVal == 0) OutVal = 0x1;
if (InVal == 1) OutVal = 0x2;
if (InVal == 2) OutVal = 0x4;
if (InVal == 3) OutVal = 0x8;
if (InVal == 4) OutVal = 0x10;
if (InVal == 5) OutVal = 0x20;
if (InVal == 6) OutVal = 0x40;
if (InVal == 7) OutVal = 0x80;
Out32(0x5c,OutVal);
} while (!_kbhit());
}

13. ACOE255 13
Mask Operations
(a) Write a program to keep reading the
.
(b) Write a program to keep reading the
state of the switches and switch ON
the LEDs for which the corresponding
switch is closed.

14. ACOE255 14
Simple I/O Example 2
Four float switches and four LEDs are used to indicate the level of a liquid in the tank
shown below. The switches and the LEDs are connected on a computer through an
interface board with an input and an output port occupying the address 460H to 46FH.
• Draw the circuit diagram of the interface board.
• Write a program to keep reading the state of the float switches and display the liquid
level on the LEDs. If an error is detected then switch ON all LEDs.
Interface
Board
Empty
Low
Half
Full
DI3
DI2
DI1
DI0
DO3
DO2
DO1
DO4
Closed
Closed
Open
Open
High
DO0
DI3 DI2 DO3 DO2 DO1
0 0 0 0 0 0 0
0 0 0 1 0 0 0
0 0 1 0 1 1 1
0 0 1 1 0 0 1
0 1 0 0 1 1 1
0 1 0 1 1 1 1
0 1 1 0 1 1 1
0 1 1 1 0 1 0
1 0 0 0 1 1 1
1 0 0 1 1 1 1
1 0 1 0 1 1 1
1 0 1 1 1 1 1
1 1 0 0 1 1 1
1 1 0 1 1 1 1
1 1 1 0 1 1 1
1 1 1 1 1 0 0
DI1 DI0
Empty
Low
Error
Half
Error
Error
Error
High
Error
Error
Error
Error
Error
Error
Error
Full
0 1
1 0
1 1
0 0
1 1
1 1
1 1
0 0
1 1
1 1
1 1
1 1
1 1
1 1
1 1
0 0
DO0
DO4 State

15. ACOE255 15
Simple I/O Example 2 (Circuit Diagram)
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
HCT573
D Q
EN
EN OE
LS244
G1 G2
DI0
DI1
DI2
DI3
DO4
DO3
DO2
DO1
DO0
A11
A10
A9
A8
A7
A6
A5
A4
A11A10 A9 A8 A4
A7 A6 A5 A1
A3 A2 A0
0 1 0 0 0
0 1 1 X
X X X
Address
460H to 46FH

16. ACOE255 16
Simple I/O Example 2 (Software using Case/Switch Statement)
Pseudo Code:
Set OutVal to 0, i.e switch off all leds
Repeat
Read Input port in InVal
Mask out D3,D2,D1,D0 from InVal
Case of InVal
0: OutMask = 1
1: OutMask = 2
3: OutMask = 4
7: OutMask = 8
FH: OutMask = 10H
else OutMask = 1FH
Mask in D4 to D0 of OutMask in OutVal
Write OutVal to Output port
Until keypressed
C/C++ Code
main()
{
short InVal, OutVal = 0;
do {
InVal = Inp32(0x460);
InVal = InVal & 0xf;
switch (InVal) {
case 0: OutMask = 1; break;
case 1: OutMask = 2; break;
case 3: OutMask = 4; break;
case 7: OutMask = 8; break;
case 0xf: OutMask = 0x10; break;
default OutMask = 0x1f; }
OutVal = (OutVal & 0xe0) | OutMask;
Out32(0x460,OutVal);
} while (!_kbhit());
}

17. ACOE255 17
Simple I/O Example 2 (Software using array as a lookup table)
Pseudo Code:
Table[0] = 00001 /*
Table[1] = 00010 /*
Table[3] = 00100 /*
Table[7] = 01000 /*
Table[15] = 10000 /*
Table[rest] = 111111 /*
Repeat
Read Input port in InVal
Mask out D2,D1,D0 from InVal
Mask in D2,D1,D0 of Table[InVal] to OutVal
Write OutVal to Output port
Until keypressed
C/C++ Code
main()
{
short InVal, OutVal = 0;
short Table[16] = 0x1, 0x2, 0x1f, 0x4,
0x1f, 0x1f, 0x1f, 0x8,
0x1f, 0x1f, 0x1f, 0x1f,
0x1f, 0x1f, 0x1f, 0x10;
do {
InVal = Inp32(0x460);
InVal = InVal & 0xf;
OutMask = Table[InVal];
OutVal = (OutVal & 0xe0) | OutMask;
Out32(0x460,OutVal);
} while (!_kbhit());
}

18. ACOE255 18
DC Motor Speed and Direction Control
• The speed of a DC motor is proportional to the DC voltage applied at its
terminal.
– Thus the speed can be controlled by controlling the DC voltage.
• The DC voltage can be controlled by:
– Inserting a resistor in series with the DC motor.
• Unnecessary dissipation of energy on the resistor.
– Using Pulse Width Modulation (PWM)
• The direction of rotation a DC motor is reversed by reversing the polarity of the
DC voltage applied at its terminal.
– This can be obtained using a relay (change-over connection).
– Connecting a relay on an output port requires the use of high current drivers such as
the ULN2803 and diodes to protect the output transistors due to Lenz’s law.
• Special drivers (H-drivers) can also be used to provide both speed control and
direction control.
Average Voltage

19. ACOE255 19
DC motor interface example
A DC motor is connected to a computer as shown below. The switches and the
motor are connected on an input and an output port occupying the address
460H to 46FH.
• Draw a circuit of the interface and the connections to the motor
• Write a program to keep reading the state of the switches and control the
operation of the motor accordingly.
0
1
0
1
0
1
Start/Stop Fast/Slow Fwrd/Rev.
Interface Board
DC Motor

20. ACOE255 20
DC motor interface example(Circuit Diagram)
A11A10 A9 A8 A4
A7 A6 A5 A1
A3 A2 A0
0 1 0 0 0
0 1 1 X
X X X
Address
460H to 46FH
RL
1
ULN2803
Com
RL
2
RL
2
M
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
HCT573
D Q
EN
EN OE
LS244
G1 G2
DI7
DI6
DI5
DI4
DO2
DO1
DO0
A11
A10
A9
A8
A7
A6
A5
A4
On/Off
Fast/Slow
Frwd/Rev.
+5V
+5V
+5V
DI3
DI2
DI1
DI0

21. ACOE255 21
DC motor interface example (Software using conditional execution)
Pseudo Code:
Set OutVal to 0, i.e motor is OFF
Repeat
Read Input port in InVal
If On/Off bit = 1 then OutVal(D0) = 1
else OutVal(D0) = 0
If Fast/Slow bit = 1 then OutVal(D1) = 1
else OutVal(D1) = 0
If Frwd/Rev bit = 1 then OutVal(D2) = 1
else OutVal(D2) = 0
Write OutVal to Output port
Until keypressed

22. ACOE255 22
DC motor interface example (Software using a lookup table)
Pseudo Code:
Table[0] = 000 /* 000 ->motor is Off
Table[1] = 000 /* 001 ->motor is Off
Table[2] = 010 /* 010 -> On-Slow-Rev
Table[3] = 011 /* 011 -> On-Slow-Frwd
Table[4] = 000 /* 100 -> motor is Off
Table[5] = 000 /* 101 -> motor is Off
Table[6] = 110 /* 110 ->On-Fast-Rev
Table[7] = 111 /* 111 ->On-Fast-Frwd
Repeat
Read Input port in InVal
Mask out D2,D1,D0 from InVal
Mask in D2,D1,D0 of Table[InVal] to OutVal
Write OutVal to Output port
Until keypressed

23. ACOE255 23
7-Segment Displays
• Used to display the digits from 0 to 9, as well as many letters (A,b,C,d,E,F,L,H)
• Consists of seven leds connected to a common point
• Can be common-anode or common-cathode
a
b
c
d
e
f
g
a
b
c
d
e
f
g
a b c d e f g
a b c d e f g
+Vcc
0: OFF
1: ON
0: ON
1: OFF
Common Cathode Common Anode
Can be connected directly on an O/P port Through a BCD/7 segment decoder
OR
HCT573
D Q
EN
EN OE
DO4
DO3
DO2
DO1
DO0
DO5
DO6
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
HCT573
D Q
EN
EN OE
DO4
DO3
DO2
DO1
DO0
DO5
DO6
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
CD4511
D
OE
A
/LE
C
B
D
/BL
/LT
EN
Q
Latch
BCD-7seg
decoder
Buffer

24. ACOE255 24
7-segment Displays – Example1
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
HCT573
D Q
EN
EN OE
LS244
G1 G2
DI0
DI1
DI2
DI3
DO4
DO3
DO2
DO1
DO0
A11
A10
A9
A8
A7
A6
A5
A4
DO5
DO6
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
x 0 1 1 1 1 1 1 3F
x 0 0 0 1 1 0 0 0C
x 1 1 1 0 1 1 0 76
x 1 0 1 1 1 1 0 5E
x 1 0 0 1 1 0 1 4D
x 1 0 1 1 0 1 1 5B
x 1 1 1 1 0 1 1 7B
x 0 0 0 1 1 1 0 0E
x 1 1 1 1 1 1 1 7F
x 1 0 0 1 1 1 1 4F
x 1 1 0 1 1 1 1 6F
x 1 1 1 1 0 0 1 79
x 0 1 1 0 0 1 1 33
x 1 1 1 1 1 0 0 7C
x 1 1 1 0 0 1 1 73
x 1 1 0 0 0 1 1 63
- g f e d c b a
D0
D1
D2
D3
D4
D5
D6
D7
• Draw a circuit to show how a common cathode 7-segment display can be
connected on an o/p port and four switches on an i/p port both occupying the
address range 860h to 86Fh.
• Write a program to read the state of the switches and display the binary number
formed by the switches as a hexadecimal digit.

25. ACOE255 25
7-segment Displays – Example1: Program

26. ACOE255 26
7-Segment Displays: Example 2
Four float switches and a 7-segment display are used to indicate the level of a liquid in the
tank shown below. The switches and the display are connected on a computer through an
interface board with an input and an output port occupying the address 460H to 46FH.
Draw the circuit diagram of the interface board.
Write a program to keep reading the state of the float switches and display the liquid level
on the display using the digits F(Full), H(High), A (Half), L(Low) and E(Empty). If an error
is detected then switch ON the decimal point.
Interface
Board
DI3
DI2
DI1
DI0
DO3
DO2
DO1
DO4
DO0
DO6
DO5
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
dp
Open
Open
Closed
Closed
DI3 DI2
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
DI1 DI0
Empty
Low
Error
Half
Error
Error
Error
High
Error
Error
Error
Error
Error
Error
Error
Full
State
E
L
.
A
.
.
.
H
.
.
.
.
.
.
.
F
Digit Code

27. ACOE255 27
7-segment Displays – Example1: Program

28. ACOE255 28
7-segment Displays – Example3
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
HCT573
D Q
EN
EN OE
LS244
G1 G2
DI0
DI1
DI2
DI3
DO4
DO3
DO2
DO1
DO0
A11
A10
A9
A8
A7
A6
A5
A4
DO5
DO6
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
x 0 1 1 1 1 1 1 3F
x 0 0 0 1 1 0 0 0C
x 1 1 1 0 1 1 0 76
x 1 0 1 1 1 1 0 5E
x 1 0 0 1 1 0 1 4D
x 1 0 1 1 0 1 1 5B
x 1 1 1 1 0 1 1 7B
x 0 0 0 1 1 1 0 0E
x 1 1 1 1 1 1 1 7F
x 1 0 0 1 1 1 1 4F
x 1 1 0 1 1 1 1 6F
x 1 1 1 1 0 0 1 79
x 0 1 1 0 0 1 1 33
x 1 1 1 1 1 0 0 7C
x 1 1 1 0 0 1 1 73
x 1 1 0 0 0 1 1 63
- g f e d c b a
D0
D1
D2
D3
D4
D5
D6
D7
• Draw a circuit to show how a common cathode 7-segment display can be
connected on an o/p port and four switches on an i/p port both occupying the
address range 860h to 86Fh.
• Write a program to read the state of the switches and display the binary number
formed by the switches as a hexadecimal digit.

29. ACOE255 29
7-segment Displays– Example1: Program

30. ACOE255 30
7-segment Displays– Example1
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
HCT573
D Q
EN
EN OE
LS244
G1 G2
DI0
DI1
DI2
DI3
DO4
DO3
DO2
DO1
DO0
A11
A10
A9
A8
A7
A6
A5
A4
DO5
DO6
DO7
a
b
c
d
e
f
g
a
b
c
d
e
f
g
x 0 1 1 1 1 1 1 3F
x 0 0 0 1 1 0 0 0C
x 1 1 1 0 1 1 0 76
x 1 0 1 1 1 1 0 5E
x 1 0 0 1 1 0 1 4D
x 1 0 1 1 0 1 1 5B
x 1 1 1 1 0 1 1 7B
x 0 0 0 1 1 1 0 0E
x 1 1 1 1 1 1 1 7F
x 1 0 0 1 1 1 1 4F
x 1 1 0 1 1 1 1 6F
x 1 1 1 1 0 0 1 79
x 0 1 1 0 0 1 1 33
x 1 1 1 1 1 0 0 7C
x 1 1 1 0 0 1 1 73
x 1 1 0 0 0 1 1 63
- g f e d c b a
D0
D1
D2
D3
D4
D5
D6
D7
• Draw a circuit to show how common cathode 7-segment display can be
connected on an o/p port and four switches on an i/p port both occupying the
address range 860h to 86Fh.
• Write a program to read the state of the switches and display the binary number
formed by the switches as a hexadecimal digit.

31. ACOE255 31
Homework- Question 1
a) Draw a circuit diagram to show how four switches and two 7-segment displays
can be interfaced to an 8088 based system occupying the address 10H to
1FH, 20H to 2FH and 30H to 3FH respectively.
b) Write a program to read the binary number formed by the four switches and
display the equivalent decimal number on the 7-segment displays. For
example if the switches form the number 0110, the displays should display the
number 06, and if the switches form the number 1110, then the displays should
display the number 14.

32. ACOE255 32
a) Draw a circuit diagram to show how four switches and a 7-segment display can
be interfaced to an 8088 based system occupying the address A0H to AFH,
F0H to FFH respectively.
b) Write a program to read the state of the switches and display on the 7-segment
display the letter:
• H, if there are more switches closed than open,
• L, if there are less switches closed than open,
• E, if the number of switches closed is equal to the number of switches open.
Homework- Question 2

33. ACOE255 33
a) Draw a circuit diagram to show how six switches and eight Leds can be
interfaced to an 8088 based system occupying the address 30H to 3FH, 20H to
2FH respectively.
b) Write a program to read the binary number formed by the six switches and
display the equivalent Binary Coded Decimal number on the Leds. For
example if the switches form the number 100101, the displays should display
the number 00110111, since (100101)2 = (00110111)BCD.
Homework- Question 3

34. ACOE255 34
a) Draw a circuit diagram to show how the following can be interfaced to an 8088
based system:
i. Four switches connected on an I/P port occupying the address range from 30H to
3FH.
ii. Eight Leds connected on an O/P port occupying the address range from 20H to
2FH.
b) Write a program to read the BCD number formed by the four switches, and
display it on the eight Leds the square of the input number in BCD form. If the
number formed by the switches is an invalid BCD number, then all of the Leds
should be switched ON.
– For example if the switches form the number 00000101 then the Leds should
display 00100101, since (00000101)BCD = (05)10 and 52 = 25 = (00100101)BCD
– If the number formed by the switches is 00001100 then the Leds should display
11111111 since 1100 is an invalid BCD number.
Homework- Question 4

35. ACOE255 35
a) Draw a circuit diagram to show how a common cathode 7-segment display can
be connected on an output port, occupying the address CAH.
b) Write a program to display the digits from 0 to 9 on the 7-segment display with
a 1-second delay between digits.
c) Write a procedure to display on the 7-segment display, the letter E if the
contents of the variable INVAL is equal to 80H, the letter H if INVAL is greater
than 80H, or the letter L if INVAL is less than 80H.
d) Write a program that keeps reading the binary number from the input port
0AAH and displays on the 7-segment display, a letter according to the table
given below.
Homework- Question 5
xxxxx000 xxxxx001 xxxxx010 xxxxx011 xxxxx100 xxxxx101 xxxxx110 xxxxx111
E I L U H Γ Ξ F

36. ACOE255 36
a) Draw a circuit diagram to show how the following can be interfaced to an 8088 system
i. Three switches connected on an I/P port occupying the address range from 60H to 6FH.
ii. Eight Leds connected on an O/P port occupying the address range from 50H to 5FH.
b) Write a program to
i. read the code formed by the three switches in a 1-second time intervals,
ii. convert the input code into a binary value according to the table below and store it in an array
called BINV, and
iii. display it as a bar graph on the eight Leds as shown in figure below
For example if the switches form the number 101 then the number 110 must be stored in the
array BINV and the Leds should display 11111110.
0 0 0
0 0 0
0 0 1
0 0 1
0 1 0
0 1 1
0 1 1
0 1 0
1 0 0
1 1 0
1 0 1
1 0 0
1 1 0
1 0 1
1 1 1
1 1 1
Input Binary Bar Graph
Homework- Question 6

37. ACOE255 37
THE 82C55 PROGRAMMABLE PERIPHERAL INTERFACE (PPI)
VCC: The +5V power supply pin.
GND: GROUND
D0-D7: I/O DATA BUS
RESET: A high on this input clears the control register and all ports
(A, B, C) are set to the input mode with the “Bus Hold” circuitry
turned on.
CS: Chip select is an active low input used to enable the 82C55A onto
the Data Bus for CPU communications.
RD: Read is an active low input control signal used by the CPU to
read status information or data via the data bus.
WR: Write is an active low input control signal used by the CPU to
load control words and data into the 82C55A.
A0-A1: Address input signals, in conjunction with the RD and WR
inputs, control the selection of one of the three ports or the control
word register (00 – Port A, 01 – Port B, 10 – Port C, 11 – CR).
PA0-PA7 (I/O PORT A): 8-bit input and output port.
PB0-PB7 (I/O PORT B): 8-bit input and output port.
PC0-PC7 (I/O PORT C): 8-bit input and output port.
•A popular interfacing component, for interfacing TTL-compatible I/O devices to the microprocessor.
•Requires insertion of wait states if used with a microprocessor using higher that an 8 MHz clock.
•PPI has 24 pins for I/O that are programmable in groups of 12 pins and three modes of operation
(Basic I/O, Strobed I/O, Strobed Bi-directional Bus I/O).
•In the PC, an 82C55 or its equivalent is decoded at I/O ports 60H-63H, interfacing keyboard and
Parallel printer port).

38. ACOE255 38
INTERFACING 82C55 TO AN 8088 SYSTEM (ports 60H-63H)
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
DI0
DI1
DI2
DI3
A7
A6
A5
A4
A3
A2
82C55
D7
D0
RD
WR
A0
A1
PA7
PA0
PB7
PB0
PC7
PC0
CS
A0
A1

39. ACOE255 39
EXAMPLE
• Use a 82C55 PPI to interface four switches at address 60H
and a SSD at address 62H
A19
D7
D0
RD
WR
IO/M'
8088
System
A0
+5V
DI0
DI1
DI2
DI3
A7
A6
A5
A4
A3
A2
a
b
c
d
e
f
g
a
b
c
d
e
f
g
82C55
D7
D0
RD
WR
A0
A1
PA7
PA0
PB7
PB0
PC7
PC0
CS
A0
A1
DI0
DI1
DI2
DI3
DI0
DI1
DI2
DI3

40. ACOE255 40
PROGRAMMING THE 82C55
• Mode 0 operation causes the PPI to function as either buffered input or latched
output
• The 82C55 is programmed through two internal command registers, selected
through bit 7.
• Command byte A
– 0 Port C, PC3-PC0 (1-input, 0-output)
– 1 Port B (1-input, 0-output)
– 2 Mode (00-mode 0, 01-mode1)
– 3 Port C, PC7-PC4 (1-input, 0-output)
– 4 Port A (1-input, 0-output)
– 5-6 Mode (00-mode 0, 01-mode 2, 1X-mode 2)
– 7 Command byte select (always 1 for Command byte A)
• Command byte B
– Used in mode 1 and mode 2

41. ACOE255 41
Example
• Rewrite the program of the example of slide 30, this time using the PPI and
setting up the ports as shown in the previous slide

42. ACOE255 42
Homework
• All examples of slides 31-36 can be modified to include PPI interfacing