UNIT IV I/O INTERFACING PART A (2 Marks) 1. …mahalakshmiengineeringcollege.com/pdf/cse/IVsem/CS2252/...There are two types of write operations in 8253 (1) Writing a control word

CS2252 MICROPROCESSORS & MICROCONTROLLERS Department of CSE

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

UNIT IV – I/O INTERFACING

PART A (2 Marks)

1. Name the three modes used by the DMA processor to transfer data? [NOV/DEC

2006]

Signal transfer mode (cycling stealing mode)

Block transfer mode

Demand transfer mode

2. Name the 6 modes of operations of an 8253 programmable interval

timer.[NOV/DEC 2006]

Mode 0:interrupt on terminal count

Mode 1:hardware re -triggerable one-shot

Mode 2 :rate generator

Mode3:square wave rate generator

Mode 4:software triggered strobe

Mode 5:hardware triggered strobe

3. What are the features used mode 1 in 8255?

Two groups – group A and group B are available for strobed data transfer.

Each group contains one 8-bit data I/O port and one 4-bit control/data

port.

The 8-bit data port can be either used as input or output port. The inputs

and outputs both are latched.

Out of 8-bit port C, PC0-PC2 is used to generate control signals for port

B and PC3=PC5 are used to generate control signals for port A. The

lines PC6, PC7 may be used as independent data lines.

4. What is memory mapping?

The assignment of memory addresses to various registers in a memory chip is

called as memory mapping.

5. What is key bouncing?

Mechanical switches are used as keys in most of the keyboards. When a key is

pressed the contact bounce back and forth and settle down only after a small time delay

(about 20ms). Even though a key is actuated once, it will appear to have been actuated

several times. This problem is called Key Bouncing.

What are the different types of methods used for data transmission?

The data transmission between two points involves unidirectional or bi-

directional transmission of meaningful digital data through a medium. There are

basically there modes of data transmission

(a) Simplex

(b) Duplex

(c) Half Duplex


In simplex mode, data is transmitted only in one direction over a single

communication channel. For example, a computer (CPU is received by the computer

(i.e the computer is receiver). However, it is not possible to transmit data from the

computer to terminal and from terminal to the computer simultaneously.

6. What are the various programmed data transfer methods?

i) Synchronous data transfer

ii) Asynchronous data transfer

iii) Interrupt driven data transfer

7. What are the basic modes of operation of 8255?

Mode 0 - Basic I/O mode

Mode 1 - Strobed I/O mode

Mode 2 - Strobed bi-directional I/O

8. What is an USART? USART stands for universal synchronous/Asynchronous Receiver/Transmitter.

It is a programmable communication interface that can communicate by using either

synchronous or asynchronous serial data.

9. What is the use of terminal count register?

Each of the four DMA channels of 8257 has one terminal count register. This

16-bit register is used for ascertaining that the data transfer through a DMA channel

ceases or stops after the required number of DMA cycles.

10. Define HRQ?

The hold request output requests the access of the system bus. In non- cascaded

8257 systems, this is connected with HOLD pin of CPU. In cascade mode, this pin of a

slave is connected with a DRQ input line of the master 8257,while that of the master is

connected with HOLD input of the CPU.

11. What are the basic modes of operation of 8255?[APRIL/MAY2008]

There are two basic modes of operation of 8255, viz.

1. I/O mode.

2. BSR mode.

In I/O mode, the 8255 ports work as programmable I/O ports, while In BSR

mode only port C (PC0-PC7) can be used to set or reset its individual port bits. Under

the IO mode of operation,

further there are three modes of operation of 8 255, So as to support different types of

applications, viz. mode 0, mode 1 and mode 2.

Mode 0 - Basic I/O mode

Mode 1 - Strobed I/O mode

Mode 2 - Strobed bi-directional I/O.

12. Write the features of mode 0 in 8255?

1. Two 8-bit ports (port A and port B) and two 4-bit ports (port C upper and lower) are

available.

The two 4-bit ports can be combined used as a third 8-bit port.

2. Any port can be used as an input or output port.

3. Output ports are latched. Input ports are not latched.

4. A maximum of four ports are available so that overall 16 I/O configurations are

possible.



Two groups – group A and group B are available for strobed data transfer.

1. Each group contains one 8-bit data I/O port and one 4-bit control/data port.

2. The 8-bit data port can be either used as input or output port. The inputs and outputs

both are latched.

3. Out of 8-bit port C, PC0-PC2 is used to generate control signals for port B and

PC3=PC5 are used to generate control signals for port A. The lines PC6, PC7 may be

used as independent data lines.


The single 8-bit port in-group A is available.

1. The 8-bit port is bi-directional and additionally a 5-bit control port is available.

2. Three I/O lines are available at port C, viz PC2-PC0.

3. Inputs and outputs are both latched.

4. The 5-bit control port C (PC3=PC7) is used for generating/accepting handshake

signals for the

8-bit data transfer on port A.

15. What are the modes of operations used in 8253?

Each of the three counters of 8253 can be operated in one of the following six

modes of operation.

1. Mode 0 (Interrupt on terminal count)

2. Mode 1 (Programmable monoshot)

3. Mode 2 (Rate generator)

4. Mode 3 (Square wave generator)

5. Mode 4 (Software triggered strobe)

6. Mode 5 (Hardware triggered strobe)

16. What are the different types of write operations used in 8253?

There are two types of write operations in 8253

(1) Writing a control word register

(2) Writing a count value into a count register

The control word register accepts data from the data buffer and initializes the

counters, as required. The control word register contents are used for

(a) Initializing the operating modes (mode 0-mode4)

(b) Selection of counters (counter 0- counter 2)

(c) Choosing binary /BCD counters

(d) Loading of the counter registers.

The mode control register is a write only register and the CPU cannot read its contents.

17. Give the different types of command words used in 8259?

The command words of 8259A are classified in two groups

1. Initialization command words (ICWs)

2. Operation command words (OCWs).

18. Give the operating modes of 8259a?

(a) Fully Nested Mode

(b) End of Interrupt (EOI)

(c) Automatic Rotation

(d) Automatic EOI Mode

(e) Specific Rotation

(f) Special Mask Mode

(g) Edge and level Triggered Mode

(h) Reading 8259 Status


(i) Poll command

(j) Special Fully Nested Mode

(k) Buffered mode

(l) Cascade mode.

19. Define scan counter?[NOV/DEC2011]

The scan counter has two modes to scan the key matrix and refresh the display.

In the encoded mode, the counter provides binary count that is to be externally decoded

to provide the scan lines for keyboard and display. In the decoded scan mode, the

counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4

scan on SL0-SL3.The keyboard and display both are in the same mode at a time.

20. What is the output modes used in 8279?

8279 provides two output modes for selecting the display options.

1. Display Scan

In this mode, 8279 provides 8 or 16 character-multiplexed displays those can be

organized as dual 4-bit or single 8-bit display units.

2. Display Entry

8279 allows options for data entry on the displays. The display data is entered

for display from the right side or from the left side.

21. What are the modes used in keyboard modes?

1. Scanned Keyboard mode with 2 Key Lockout.

2. Scanned Keyboard with N-key Rollover.

3. Scanned Keyboard special Error Mode.

4. Sensor Matrix Mode.

22. What are the modes used in display modes?

1. Left Entry mode

In the left entry mode, the data is entered from the left side of the display unit.

2. Right Entry Mode

In the right entry mode, the first entry to be displayed is entered on the rightmost

display.

23. What is the use of modem control unit in 8251?

The modem control unit handles the modem handshake signals to coordinate

the communication between the modem and the USART.

24. What is the use of 8251 chip?

Intel’s 8251A is a universal synchronous asynchronous receiver and transmitter

compatible with Intel’s Processors. This may be programmed to operate in any of the

serial communication modes built into it. This chip converts the parallel data into a

serial stream of bits suitable for serial transmission. It is also able to receive a serial

stream of bits and converts it into parallel data bytes to be read by a microprocessor.

25. What are the different types of methods used for data transmission?

The data transmission between two points involves unidirectional or bi-

directional transmission of meaningful digital data through a medium. There are

basically there modes of data transmission.

(a) Simplex

(b) Duplex

(c) Half Duplex

In simplex mode, data is transmitted only in one direction over a single

communication channel For example, a computer (CPU) may transmit data for a CRT

display unit in this mode.


In duplex mode, data may be transferred between two transceivers in both

directions simultaneously.

In half duplex mode, on the other hand, data transmission may take pace in

either direction, but at a time data may be transmitted only in one direction. For

example, a computer may communicate with a terminal in this mode. When the terminal

sends data (i.e. terminal is sender).

The message is received by the computer (i.e the computer is receiver).

However, it is not possible to transmit data from the computer to terminal and from

terminal to the computer simultaneously.

26. What are the various programmed data transfer methods?

i) Synchronous data transfer

ii) Asynchronous data transfer

iii) Interrupt driven data transfer.

27. What is synchronous data transfer?

It is a data method which is used when the I/O device and the microprocessor

match in speed. To transfer a data to or from the device, the user program issues a

suitable instruction addressing the device. The data transfer is completed at the end of

the execution of this instruction.

28. What is asynchronous data transfer?

It is a data transfer method which is used when the speed of an I/O device does

not match with the speed of the microprocessor. Asynchronous data transfer is also

called as Handshaking.

29. What are the functional types used in control words of 8251?

The control words of 8251A are divided into two functional types.

1. Mode Instruction control word

2. Command Instruction control word

Mode Instruction control word:-

This defines the general operational characteristics of 8251A.

Command Instruction control word:-The command instruction controls the actual

operations of the selected format like enable transmit/receive, error reset and modem

control.

30. Give the register organization of 8257?

The 8257 perform the DMA operation over four independent DMA channels.

Each of the four channels of 8257 has a pair of two 16-bit registers. DMA address

register and terminal count register. Also, there are two common registers for all the

channels; namely, mode set registers and status register. Thus there are a total of ten

registers. The CPU selects one of these ten registers using address lines A0- A3.

31. What is the function of DMA address register?

Each DMA channel has one DMA address register. The function of this register

is to store the address of the starting memory location, which will be accessed by the

DMA channel. Thus the starting address of the memory block that will be accessed by

the device is first loaded in the DMA address register of the channel. Naturally, the

device that wants to transfer data over a DMA channel, will access the block of memory

with the starting address stored in the DMA Address Register.

32. What is the use of terminal count register?

Each of the four DMA channels of 8257 has one terminal count register. This

16-bit register is used for ascertaining that the data transfer through a DMA channel

ceases or stops after the required number of DMA cycles.


33. What is the function of mode set register in 8257?

The mode set register is used for programming the 8257 as per the requirements

of the system. The function of the mode set register is to enable the DMA channels

individually and also to set the various modes of operation.

34. List the operation modes of 8255

a) I.O Mode

i. Mode 0-Simple Input/Output.

ii. Mode 1-Strobed Input/Output (Handshake mode)

iii. Mode 2-Strobed bidirectional mode

b) Bit Set/Reset Mode.

35. What is a control word?

It is a word stored in a register (control register) used to control the operation of

a program digital device.

36. What is the purpose of control word written to control register in

8255?[APRIL/MAY2011]

The control words written to control register specify an I/O function for each I.O port.

The bit D7 of the control word determines either the I/O function of the BSR function.

37. What is the size of ports in 8255?

Port-A: 8-bits

Port-B: 8-bits

Port-CU: 4-bits

Port-CL: 4-bits.

38. What is interfacing?

An interface is a shared boundary between the devices which involves sharing

information. Interfacing is the process of making two different systems communicates

with each other.

39. What is memory mapping?[NOV/DEC 2007]

The assignment of memory addresses to various registers in a memory chip is

called as memory mapping.

40. What is I/O mapping? [NOV/DEC 2007]

The assignment of addresses to various I/O devices in the memory chip is called

as I/O mapping.

41. What is an USART?[NOV/DEC 2007]

USART stands for universal synchronous/Asynchronous Receiver/Transmitter.

It is a programmable communication interface that can communicate by using either

synchronous or asynchronous serial data.

42. What is the use of 8251 chip?

8251 chip is mainly used as the asynchronous serial interface between the

processor and the external equipment.

43. The 8279 is a programmable __________ interface.

Keyboard/Display.

44.. List the major components of the keyboard/Display interface.

a. Keyboard section

b. Scan section

c. Display section


d. CPU interface section.

45. What is Key bouncing?

Mechanical switches are used as keys in most of the keyboards. When a key is

pressed the contact bounce back and forth and settle down only after a small time delay

(about 20ms). Even though a key is actuated once, it will appear to have been actuated

several times. This problem is called Key Bouncing.

46. Basic concepts in memory interfacing

The primary function of memory interfacing is that the microprocessor should

be able to read from and write into a given register of a memory chip. To perform these

operations the microprocessor should

• Be able to select the chip

• Identify the register

• Enable the appropriate buffer.

47. Define HRQ?

The hold request output requests the access of the system bus. In non- cascaded

8257 systems, this is connected with HOLD pin of CPU. In cascade mode, this pin of a

slave is connected with a DRQ input line of the master 8257, while that of the master is

connected with HOLD input of the CPU.

48. What is the use of stepper motor?

A stepper motor is a device used to obtain an accurate position control of

rotating shafts. A stepper motor employs rotation of its shaft in terms of steps, rather

than continuous rotation as in case of AC or DC motor.

49. What is TXD?

TXD- Transmitter Data Output

50. What is RXD?

RXD- Receive Data Input

This input pin of 8251A receives a composite stream of the data to be received by

8251A.

42. How many address lines in a 4096 x 8 EPROM CHIP?

12 address lines.

43. Control signals used for DMA operation are ____________

HOLD & HLDA.

44. Differentiate between absolute and linear select decoding?


Part – B (16 Marks)

1. Memory interfacing: The memory is made up of semiconductor material used to store the programs and

data. Three types of memory is,

Process memory

Primary or main memory

Secondary memory Typical EPROM and Static RAM

A typical semiconductor memory IC will have n address pins, m data pins (or

output pins).Having two power supply pins (one for connecting required supply voltage (V and

the other for connecting ground). The control signals needed for static RAM are chip select (chip enable), read

control (output enable) and write control (write enable). The control signals needed for read operation in EPROM are chip select (chip

enable) and read control (output enable).

Decoder It is used to select the memory chip of processor during the execution of a program.

No of IC's used for decoder is,

2-4 decoder (74LS139)

3-8 decoder (74LS138)

Number of Address Pins and Data Pins in Memory ICs


Example for Memory Interfacing Consider a system in which the full memory space 64kb is utilized for EPROM memory.

Interface the EPROM with 8085 processor. The memory capacity is 64 Kbytes. i.e

2^n = 64 x 1000 bytes where n = address lines. So,

n = 16. In this system the entire 16 address lines of the processor are connected to address input

pins of memory IC in order to address the internal locations of memory. The chip select (CS) pin of EPROM is permanently tied to logic low (i.e., tied to ground).

Since the processor is connected to EPROM, the active low RD pin is connected to active low

output enable pin of EPROM. The range of address for EPROM is 0000H to FFFFH.

2. Parallel communication interfacing:

Programmable peripheral interface (8255): Architecture of 8255:

The parallel input-output port chip 8255 is also called as programmable peripheral input

output port. The Intel‟s 8255 is designed for use with Intel ‟s 8-bit, 16-bit and higher capability

microprocessors. It has 24 input/output lines which may be individually programmed in two

groups of twelve lines each, or three groups of eight lines. The two groups of I/O pins are

named as Group A and Group B. Each of these two groups contains a subgroup of eight I/O

lines called as 8-bit port and another subgroup of four lines or a 4-bit port. Thus Group A

contains an 8-bit port A along with a 4-bit port. C upper.The port A lines are identified by

symbols PA0-PA7 while the port C lines are identified as PC4-PC7. Similarly, GroupB contains

an 8-bit port B, containing lines PB0-PB7 and a 4-bit port C with lower bits PC0 - PC3. The

port C upper and port C lower can be used in combination as an 8-bit port C. Both the port C are

assigned the same address. Thus one may have either three 8-bit I/O ports or two 8-bit and two

4-bit ports from 8255. All of these ports can function independently either as input or as output

ports. This can be achieved by programming the bits of an internal register of 8255 called as

control word register ( CWR ). This buffer receives or transmits data upon the execution of

input or output instructions by the microprocessor. The control words or status information is

also transferred through the buffer.


Block diagram of 8255:

Pin diagram of 8255:


The signal description of 8255 are briefly presented as follows :

PA7-PA0: These are eight port A lines that acts as either latched output or buffered input

lines depending upon the control word loaded into the control word register.

PC7-PC4 : Upper nibble of port C lines. They may act as either output latches or input

buffers lines.This port also can be used for generation of handshake lines in mode 1 or

mode 2. PC3-PC0 : These are the lower port C lines, other details are the same as PC7-PC4 lines.

PB0-PB7 : These are the eight port B lines which are used as latched output lines or

buffered input lines in the same way as port A.

RD : This is the input line driven by the microprocessor and should be low to indicate

read operation to 8255.

WR : This is an input line driven by the microprocessor. A low on this line indicates

write operation. CS : This is a chip select line. If this line goes low, it enables the 8255 to respond to RD and

WR signals, otherwise RD and WR signal are neglected.

A1-A0 : These are the address input lines and are driven by the microprocessor. These

lines A1-A0 with RD, WR and CS from the following operations for 8255. These

address lines are used for addressing any one of the four registers,i.e. three ports and a control

word register as given in table below. In case of 8086 systems, if the 8255 is to be interfaced with lower order data bus, the

A0 and A1 pins of 8255 are connected with A1 and A2 respectively. D0-D7 : These are the data bus lines those carry data or control word

to/from the microprocessor.

RESET : A logic high on this line clears the control word register of 8255. All ports are

set as input ports by default after reset.

3. Modes of operation in 8255:

Operational Modes of 8255 4. There are two operational modes of 8255.

1. Input / Output mode 2. Bit set / reset mode.

Input / Output Mode

There are three types of the input/output mode. They are as follows: Mode 0 5.

In this mode, the ports can be used for simple input/output operations without

handshaking. If both port A and B are initialized in mode 0, the two halves of port C can

be either used together as an additional 8-bit port, or they can be used as individual 4-bit

ports. Since the two halves of port C are independent, they may be used such that one-half

is initialized as an input port while the other half is initialized as an output port. The input

output features in mode 0 are as follows: 1. O/p are latched. 2. I/p are buffered not latched.

3. Port do not have handshake or interrupt capability.


Mode 1 6.

When we wish to use port A or port B for handshake (strobed) input or output

operation, we initialize that port in mode 1 (port A and port B can be initialized to operate

in different modes, ie, for eg, port A can operate in mode 0 and port B in mode 1). Some of

the pins of port C function as handshake lines.

For port B in this mode (irrespective of whether is acting as an input port or output port),

PC0, PC1 and PC2 pins function as handshake lines. If port A is initialised as mode 1 input port, then, PC3, PC4 and PC5 function as

handshake signals. Pins PC6 and PC7 are available for use as input/output lines. The mode 1 which supports handshaking has following features: 1. Two ports i.e. port A

and B can be use as 8 -bit i/o port. 2. Each port uses three lines of port c as handshake

signal and remaining two signals can be function as i/o port. 3. interrupt logic is

supported. 4. Input and Output data are latched. Mode 2

Only group A can be initialised in this mode. Port A can be used for bidirectional

handshake data transfer. This means that data can be input or output on the same eight lines

(PA0 - PA7). Pins PC3 - PC7 are used as handshake lines for port A. The remaining pins of

port C (PC0 - PC2) can be used as input/output lines if group B is initialised in mode 0. In

this mode, the 8255 may be used to extend the system bus to a slave microprocessor or to

transfer data bytes to and from a floppy disk controller. Bit Set/Reset (BSR) mode

In this mode only port b can be used (as an output port). Each line of port C (PC0

- PC7) can be set/reset by suitably loading the command word register.no effect occurs in

input-output mode. The individual bits of port c can be set or reset by sending the signal

OUT instruction to the control register.

Control Word Format Input/output mode format

Control Word format in input/output mode The figure shows the control word format in the input/output mode. This mode is

selected by making D7 = '1' . D0, D1, D3, D4 are for lower port C, port B, upper port C and port A respectively.

When D0 or D1 or D3 or D4 are "SET", the corresponding ports act as input ports. For

eg, if D0 = D4 = '1', then lower port C and port A act as input ports. If these bits are

"RESET", then the corresponding ports act as output ports. For eg, if D1 = D3 = '0', then port

B and upper port C act as output ports. D2 is used for mode selection for group B (Port B and Lower Port C). When D2

= '0', mode 0 is selected and when D2 = '1', mode 1 is selected. D5, D6 are used for mode selection for group A (Upper Port C and Port A). The

format is as follows: D6 D5 mode

0 0 0

0 1 1

1 x 2


BSR mode format

Control Word format in BSR mode

The figure shows the control word format in BSR mode. This mode is selected by making D7='0'.

D0 is used for bit set/reset. When D0= '1', the port C bit selected (selection of a port C bit

is shown in the next point) is SET, when D0 = '0', the port C bit is RESET. D1, D2, D3 are used to select a particular port C bit whose value may be altered using

D0 bit as mentioned above. The selection of the port C bits are done as follows:


D4, D5, D6 are not used.

4. Programmable Interrupt Controller(8259)

Features

8 levels of interrupts.

Can be cascaded in master-slave configuration to handle 64 levels of interrupts.

Internal priority resolver.

Fixed priority mode and rotating priority mode.

Individually maskable interrupts.

Modes and masks can be changed dynamically.

Accepts IRQ, determines priority, checks whether incoming priority > current level

being

serviced, issues interrupt signal.

In 8085 mode, provides 3 byte CALL instruction. In 8086 mode, provides 8 bit vector

number.

Polled and vectored mode.

Starting address of ISR or vector number is programmable.

No clock required.

Pin diagram:


Pin description:


Block diagram:




5. Serial communication interface:

UNIVERSAL SYNCHRONOUS ASYNCHRONOUS RECEIVER TRANSMITTER (USART)

The 8251 is a USART (Universal Synchronous Asynchronous Receiver Transmitter) for serial

data communication. As a peripheral device of a microcomputer system, the 8251receives parallel data

from the CPU and transmits serial data after conversion. This device also receives serial data from the

outside and transmits parallel data to the CPU after conversion.

Block Diagram of 8251

Control Words

There are two types of control word. 1. Mode instruction (setting of function) 2. Command (setting of operation)


1) Mode Instruction Mode instruction is used for setting the function of the 8251. Mode instruction will be in "wait

for write" at either internal reset or external reset. That is, the writing of a control word after

resetting will be recognized as a "mode instruction." Items set by mode instruction are as follows: Synchronous/asynchronous mode Stop bit length (asynchronous mode) Character length

Parity bit

Baud rate factor (asynchronous mode)

Internal/external synchronization (synchronous mode)

Number of synchronous characters (Synchronous mode) The bit configuration of mode instruction is shown in Figures 2 and 3. In the case of synchronous

mode, it is necessary to write one-or two byte sync characters. If sync characters were written, a

function will be set because the writing of sync characters constitutes part of mode instruction


2) Command Command is used for setting the operation of the 8251. It is possible to write a

command whenever necessary after writing a mode instruction and sync characters. Items to be set by command are as follows:

Transmit Enable/Disable

Receive Enable/Disable DTR, RTS Output of data.

Resetting of error flag. Sending to break characters

Internal resetting Hunt mode (synchronous mode)



7. Keyboard / display controller: Programmable Keyboard/Display Interface – 8279 8.

A programmable keyboard and display interfacing chip. Scans and encodes up to a 64-

key keyboard. Controls up to a 16-digit numerical display. Keyboard section has a built-in

FIFO 8 character buffer. The display is controlled from an internal 16x8 RAM that stores the

coded display information.

Pin diagram:

A0: Selects data (0) or control/status (1) for reads and writes between micro and 8279. BD: Output that blanks the displays.

CLK: Used internally for timing. Max is 3 MHz.

CN/ST: Control/strobe, connected to the control key on the keyboard

CS: Chip select that enables programming, reading the keyboard, etc.

DB7-DB0: Consists of bidirectional pins that connect to data bus on micro.

IRQ: Interrupt request, becomes 1 when a key is pressed, data is available.

OUT A3-A0/B3-B0: Outputs that sends data to the most significant/least significant

nibble of display.

RD(WR): Connects to micro's IORC or RD signal, reads data/status registers.

RESET: Connects to system RESET.

RL7-RL0: Return lines are inputs used to sense key depression in the keyboard matrix. Shift: Shift connects to Shift key on keyboard. SL3-SL0: Scan line outputs scan both the keyboard and displays.


Block diagram of 8279:

Display section: The display section has eight output lines divided into two groups A0-A3 and B0-B3.

The output lines can be used either as a single group of eight lines or as two groups of four lines, in conjunction with the scan lines for a multiplexed display.

The output lines are connected to the anodes through driver transistor in case of

common cathode 7-segment LEDs. The cathodes are connected to scan lines through driver transistors. The

display can be blanked by BD (low) line. The display section consists of 16 x 8 display RAM. The CPU can read from or write

into any location of the display RAM. Scan section:

The scan section has a scan counter and four scan lines, SL0 to SL3. In decoded scan mode, the output of scan lines will be similar to a 2-to-4 decoder. In encoded scan

mode, the output of scan lines will be binary count, and so an external decoder should be used to

convert the binary count to decoded output. The scan lines are common for keyboard and display. The scan lines are used to form the rows of a matrix keyboard and also connected to digit

drivers of a multiplexed display, to turn ON/OFF.

CPU interface section: The CPU interface section takes care of data transfer between 8279 and the

processor. This section has eight bidirectional data lines DB0 to DB7 for data transfer between


8279 and CPU. It requires two internal address A =0 for selecting data buffer and A = 1 for

selecting control register of8279. The control signals WR (low), RD (low), CS (low) and A0 are used for read/write to 8279.

It has an interrupt request line IRQ, for interrupt driven data transfer with

processor. The 8279 require an internal clock frequency of 100 kHz. This can be obtained by dividing the input clock by an internal prescaler.

The RESET signal sets the 8279 in 16-character display with two -key lockout

keyboard modes.

8. DMA controller (8237):

Block diagram:


Pin diagram:






9. I / O interfacing with 8085:

Interfacing I/O devices with 8085 Techniques for I/O Interfacing: 10. 1.Peripheral-mapped I/O 2.Memory-mapped I/O

Memory-mapped I/O 8085 uses its 16-bit address bus to identify a memory location

Memory address space: 0000H to FFFFH

8085 needs to identify I/O devices also

I/O devices can be interfaced using addresses from memory space

8085 treats such an I/O device as a memory location

This is called Memory-mapped I/O.

Peripheral-mapped I/O 8085 has a separate 8-bit addressing scheme for I/O devices

I/O address space: 00H to FFH

This is called Peripheral-mapped I/O or I/O-mapped I/O 8085 Communication with I/O devices Involves the following three steps

1. Identify the I/O device (with address)

2. Generate Timing & Control signals 3. Data transfer takes place

8085 communicates with a I/O device only if there is a Program Instruction to do so

Identify the I/O device (with address)

1. Memory-mapped I/O (16-bit address) 2. Peripheral-mapped I/O (8-bit address)

Timing & Control Signals: Memory-mapped I/O

Reading Input: IO/M = 0, RD = 0

Write to Output: IO/M = 0, WR = 0

Peripheral-mapped I/O

Reading Input: IO/M = 1, RD = 0

Write to Output: IO/M = 1, WR = 0


8085 Communication with I/O devices Involves the following three steps

Identify the I/O device (with address)

Generate Timing & Control signals

Data transfer takes place

8085 communicates with a I/O device only if there is a Program Instruction to do so

Peripheral I/O Instructions IN Instruction

Inputs data from input device into the accumulator

It is a 2-byte instruction

Format: IN 8-bit port address

Example: IN 01H

OUT Instruction Outputs the contents of accumulator to an output device

It is a 2-byte instruction

Format: OUT 8-bit port address

Example: OUT 02H

Memory-mapped I/O Instructions I/O devices are identified by 16-bit addresses

8085 communicates with an I/O device as if it were one of the memory locations

Memory related instructions are used For e.g. LDA,

STA LDA 8000H Loads A with data read from input device with 16-bit address

8000H STA 8001H Stores (Outputs) contents of A to output device with 16-bit address

8001H

11. Stepper motor:

Stepper Motor:

The hardware setup consists of a microprocessor motherboard and stepper motor

interface board. The motherboard consists of 8085 MPU, 8KB EPROM, 8KB RAM, Keyboard

and display controller 8279, 21-key Hex-keypad and six numbers of seven segment LEDs and

Bus Expansion connector. The stepper motor interface consists of driver transistors for stepper

motor windings and address decoding circuit. The microprocessor output the binary sequence

through data bus, which are converted to current pulses by the driver transistors and used to drive

stepper motor. The software for the system is developed in 8085 assembly language.


12. Temperature control:

The temperature controller can be used to control the temperature of any plant. Typically

it contains a Processor unit, Temperature input unit and Control output unit. The 8085 based

motherboard forms the processing unit. The Analog-to-Digital unit together with temperature

sensor forms the temperature input unit. The relay driver forms the control output unit. Electric

power to the heating element (coil) is supplied through relay contacts. The switching ON/OFF of

the relay controls the heat supplied to the plant.

CS2252 – MICROPROCESSORS & MICROCONTROLLERS Department of CSE 34

Operationally, the system requires two set points-upper and lower, to be entered by the

user. Whenever the temperature of the plant exceeds the upper limit or recede the lower limit relay

is turned-off, so that a temperature is maintained within limits. The software for the temperature

controller is developed in 8085 assembly language programs. The hardware consists of 8085 microprocessor motherboard, ADC interface board, andrelay and

driver unit.

The motherboard consists of 8085 MPU, 8KB EPROM, 8KB RAM keyboard and display

controller 8279, programmable peripheral interface 8255, 21 key hex-keypad and six numbers of

seven segment LED’s. Ports Expansion connector parallel port connectors are provided for external

interfacing. The temperature input board or ADC interface board consists of ADC 0809, which is an 8-

bit converter with eight channels of input. It is interfaced with the motherboard through 50-pin bus

expansion connector. The temperature sensor ADC590 is used to sense the temperature of the plant

and its analog output is applied to the channel-0 of ADC. Relay is switched ON/OFF by driving the transistor to saturation/cut-off which is

connected to port A of 8255.

UNIT IV I/O INTERFACING PART A (2 Marks) 1. …mahalakshmiengineeringcollege.com/pdf/cse/IVsem/CS2252/...There are two types of write operations in 8253 (1) Writing a control word

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