Peripheral Devices And Interfacing Of

Microprocessor 8085

Peripheral Devices and Interfacing of Microprocessor 8085: Understanding the Essentials

peripheral devices and interfacing of microprocessor 8085 form a crucial part of

embedded system design and microprocessor-based applications. When working with the

8085 microprocessor, knowing how to connect and communicate with external devices is

key to expanding its capabilities beyond mere processing. This article delves deep into the

essentials of peripheral devices and the interfacing techniques with the 8085, providing a

comprehensive overview that blends technical clarity with practical insights.

What Are Peripheral Devices in the Context of Microprocessor

8085?

Peripheral devices refer to the external hardware units that work alongside the

microprocessor to perform specific functions such as input, output, and storage. The 8085

microprocessor, though powerful for its time, requires these peripherals to interact

effectively with the real world.

Common peripheral devices include:

Input devices like keyboards and sensors

Output devices such as displays and printers

Storage units like RAM, ROM, and external memory

Communication modules including serial and parallel interfaces

By interfacing these devices, the 8085 can control and gather data from the environment,

making it indispensable in embedded control systems.

The Importance of Interfacing in 8085 Microprocessor Systems

Interfacing is the process by which peripheral devices are connected to the

microprocessor so they can communicate and work together seamlessly. The 8085

microprocessor uses various signals and buses (address, data, and control buses) to

interface with peripherals.

Efficient interfacing ensures:

Correct data transfer between the microprocessor and peripherals

Synchronization of signals and timing

Expansion of the microprocessor’s functionality

Simplification of complex hardware designs

Without proper interfacing, the microprocessor cannot effectively control or receive data

from the external devices, limiting its application scope.

Address and Data Bus in 8085 Interfacing

The 8085 microprocessor has a 16-bit address bus and an 8-bit data bus. The address bus

is used to select the peripheral device or memory location to communicate with, while the

data bus carries the actual data.

One key aspect of interfacing is decoding the address to ensure the correct peripheral

responds to the processor’s commands. Address decoding can be done using logic gates

or programmable devices, which enables multiple peripherals to coexist without conflict.

Control Signals and Their Role

Control signals such as RD (Read), WR (Write), IO/M (Input/Output or Memory), and ALE

(Address Latch Enable) play a pivotal role in managing data flow and operations. These

signals indicate whether the processor is reading or writing data and whether the

operation pertains to memory or I/O devices.

Understanding these control signals helps in designing proper interfacing circuits that

respond accurately to the microprocessor’s instructions.

Types of Peripheral Devices Commonly Interfaced with 8085

The versatility of the 8085 microprocessor is largely dependent on the peripheral devices

it can interface with. Let’s explore some of the common categories and how they interact

with the microprocessor.

Input Devices

Input devices provide data to the microprocessor for processing. Examples include:

Keyboards: Sending user inputs via parallel or serial interfaces

Sensors: Temperature, pressure, or light sensors that convert physical quantities

into electrical signals

Switches and Buttons: Simple on/off signals for control purposes

These devices often use input ports or memory-mapped I/O to transmit data to the 8085.

Output Devices

Output devices receive data from the microprocessor to display or act upon. Typical

output peripherals include:

LED displays and seven-segment displays for visual output

Printers and plotters for hardcopy output

Motors and actuators controlled via driver circuits

Interfacing output devices requires careful timing and sometimes additional hardware like

latches or buffer ICs to ensure data integrity.

Memory Devices

Memory interfacing is fundamental because the 8085 needs to read instructions and store

data in memory components such as:

ROM (Read Only Memory): For storing firmware or program code

RAM (Random Access Memory): For temporary data storage during operations

EPROM and EEPROM: Electrically programmable memories allowing reprogramming

of stored data

Memory interfacing often involves address decoding and timing considerations to match

the microprocessor’s clock.

Communication Interfaces

To enable communication with other systems or devices, the 8085 can interface with

serial and parallel communication modules:

Serial Communication Interface (SCI): For data transmission bit by bit, used in UART

or USART chips

Parallel Communication Interface: Faster communication through multiple data lines

simultaneously

Peripheral Interface Adapters like the 8255 PPI, which provide programmable ports

for flexible I/O configurations

These interfaces often require dedicated control and status registers to manage data flow

efficiently.

Techniques for Interfacing Peripheral Devices with 8085

Interfacing peripheral devices with the 8085 involves both hardware and software

considerations. Here are some key techniques and tips for effective interfacing.

Memory-Mapped I/O vs. Isolated I/O

There are two primary methods to interface I/O devices:

Memory-Mapped I/O: The I/O devices are assigned specific memory addresses, and

the microprocessor accesses them as if they were memory locations. This method

simplifies programming but uses up part of the address space.

Isolated I/O (Port-Mapped I/O): Separate address spaces are maintained for memory

and I/O devices. The 8085 uses specific control signals (IO/M) to differentiate

between them. This allows more efficient use of memory space but requires special

instructions like IN and OUT.

Choosing between these depends on the system’s complexity and resource availability.

Using Programmable Peripheral Interface (8255)

The Intel 8255 chip is widely used in 8085 systems to expand the number of I/O ports. It

offers three 8-bit ports, which can be programmed as input or output.

Key features include:

Mode selection for simple I/O or handshake operations

Easy interfacing with control signals from the 8085

Flexibility in handling parallel data transfer

Using the 8255 greatly simplifies the process of connecting multiple peripheral devices

and managing their data flow.

Interrupt-Driven I/O

Instead of constant polling, the 8085 can use interrupts to communicate with peripherals

more efficiently. When a peripheral needs the processor’s attention, it sends an interrupt

request, temporarily halting the current process.

Benefits of interrupt-driven I/O include:

Better CPU utilization by avoiding busy-wait loops

Faster response to peripheral events

Support for multiple peripheral devices via interrupt priorities

The 8085 supports five hardware interrupts, which can be used to interface devices like

keyboards, timers, or serial communication units.

Practical Tips for Effective Peripheral Interfacing with 8085

When working on microprocessor projects involving the 8085, keep these practical tips in

mind:

**Plan your address decoding carefully:** Avoid overlapping address ranges to

prevent conflicts.

**Use buffering where necessary:** Data bus buffers help protect the

microprocessor and peripherals from voltage mismatches.

**Consider timing constraints:** Ensure peripherals meet the timing requirements of

the 8085 clock and control signals.

**Implement proper handshaking:** For asynchronous devices, handshaking signals

ensure data integrity.

**Test each interface module individually:** Debugging smaller sections before

integrating helps isolate issues.

These practices can save time and improve the reliability of your microprocessor system.

Expanding Beyond Basic Peripherals

While traditional peripherals like keyboards, displays, and memory form the foundation of

8085 interfacing, modern applications often require more advanced modules. Examples

include:

Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs) for

sensor interfacing

Serial communication controllers for networking

Real-time clocks and timers for time-sensitive applications

Integrating such devices requires understanding additional protocols and sometimes using

interface ICs or microcontroller co-processors alongside the 8085.

Peripheral devices and interfacing of microprocessor 8085 thus remain a fascinating area

that blends hardware design with software control. Mastering this domain opens up

numerous possibilities for creating custom embedded solutions and understanding the

fundamentals of computer architecture.

Question

Answer

What are peripheral devices in

the context of the 8085

microprocessor?

Peripheral devices are external hardware components

like keyboards, displays, printers, and storage devices

that are connected to the 8085 microprocessor to

expand its functionality and allow interaction with the

external environment.

How does the 8085

microprocessor interface with

peripheral devices?

The 8085 microprocessor interfaces with peripheral

devices through its address, data, and control buses

using techniques such as memory-mapped I/O or

isolated I/O, where specific input/output instructions

and dedicated ports are utilized for communication.

What is the role of the 8255

Programmable Peripheral

Interface (PPI) with the 8085

microprocessor?

The 8255 PPI is a commonly used peripheral device

that provides 24 programmable I/O lines divided into

three 8-bit ports, allowing the 8085 microprocessor to

interface with multiple input and output devices

efficiently.

Can you explain the difference

between memory-mapped I/O

and isolated I/O in 8085

interfacing?

Memory-mapped I/O treats peripheral devices as if

they are memory locations, allowing the use of regular

memory instructions, while isolated I/O uses a

separate address space with dedicated IN and OUT

instructions for communication with peripheral

devices.

What is the significance of the

control signals in interfacing

peripherals with the 8085

microprocessor?

Control signals like RD (Read), WR (Write), ALE

(Address Latch Enable), and IO/M distinguish between

memory and I/O operations, enabling proper timing

and control for data transfer between the 8085 and

peripheral devices.

How do interrupt-driven I/O

operations improve peripheral

interfacing in the 8085

microprocessor?

Interrupt-driven I/O allows peripheral devices to alert

the 8085 microprocessor when they need attention,

enabling efficient handling of data transfer without

continuous polling, thus improving system

performance and responsiveness.

Peripheral Devices and Interfacing of Microprocessor 8085: A Comprehensive Review

peripheral devices and interfacing of microprocessor 8085 represent a

foundational topic in the study of microprocessor-based systems, especially for those

exploring embedded systems design and hardware interfacing. The Intel 8085

microprocessor, introduced in the mid-1970s, remains a key subject for understanding

classic microprocessor architecture and its interaction with external components. This

article delves into the critical aspects of peripheral devices and the interfacing

mechanisms that enable the 8085 microprocessor to communicate effectively with the

external world.

The Role of Peripheral Devices in Microprocessor Systems

Peripheral devices are essential hardware components that extend the functionality of a

microprocessor beyond its intrinsic processing capabilities. In the context of the 8085

microprocessor, peripherals include input/output (I/O) devices such as keyboards,

displays, printers, memory modules, and communication interfaces. These devices are

crucial for enabling user interaction, data storage, and real-world signal processing.

The interfacing of peripheral devices with the 8085 microprocessor involves creating a

communication bridge that allows data exchange and control signal transmission. Since

the 8085 itself contains no built-in peripherals, the design and implementation of efficient

interfacing circuits become paramount to system performance and reliability.

Understanding Interfacing in the 8085 Microprocessor

Interfacing refers to the method and hardware used to connect peripheral devices to the

microprocessor. It includes managing address decoding, control signals, data transfer

protocols, and synchronization, ensuring coherent communication between the CPU and

external devices.

The 8085 microprocessor features a 16-bit address bus and an 8-bit data bus, supporting

direct interfacing with memory and I/O devices. It uses a separate I/O address space

accessed via IN and OUT instructions, distinguishing memory-mapped and I/O-mapped

interfacing techniques.

I/O Interfacing Techniques

Two primary methods exist for interfacing input/output devices with the 8085

microprocessor:

Memory-Mapped I/O: In this approach, peripheral devices are assigned specific

1.

memory addresses within the system's address space. The microprocessor accesses

these devices using standard memory instructions, making device registers appear

as memory locations.

Port-Mapped I/O (Isolated I/O): This technique uses a separate address space for

2.

I/O devices, accessed via dedicated IN and OUT instructions. The 8085 supports 256

I/O ports, providing flexibility and minimizing memory address space usage.

Both methods have their advantages. Memory-mapped I/O simplifies programming by

treating peripherals as memory but consumes valuable address space. Port-mapped I/O

preserves memory but requires special instructions.

Key Peripheral Devices for the 8085 Microprocessor

Several peripheral devices are commonly interfaced with the 8085 to enhance its

capabilities:

Programmable Peripheral Interface (8255): A versatile 8-bit parallel I/O device

1.

that provides three 8-bit ports, configurable as input or output. The 8255 is widely

used for interfacing keyboards, displays, and other parallel devices.

Programmable Interval Timer (8253/8254): Used for generating accurate

2.

timing and counting operations, critical for event counting, generating time delays,

and waveform generation.

Serial Communication Interface (8251): Enables serial communication by

3.

converting parallel data from the microprocessor to serial form and vice versa,

facilitating communication with serial devices like modems.

Direct Memory Access (DMA) Controller (8257): Allows peripherals to access

4.

memory directly without CPU intervention, enhancing data transfer efficiency.

These peripherals interface with the 8085 through dedicated control and data lines, often

requiring specific programming sequences to configure and operate.

Interfacing Challenges and Solutions

Interfacing peripheral devices with the 8085 microprocessor involves overcoming several

challenges such as timing synchronization, signal integrity, and address decoding.

Address Decoding and Memory Mapping

Efficient address decoding ensures that the microprocessor’s signals are routed to the

correct peripheral device. For example, in a memory-mapped system, a decoder circuit

converts the microprocessor’s address bus signals into device-select lines.

Using devices like the 74LS138 3-to-8 line decoder helps simplify the selection of up to

eight devices by decoding three address lines. Proper address mapping avoids conflicts

and enables seamless device integration.

Synchronization and Timing Constraints

Since the 8085 operates at relatively low clock speeds (up to 3 MHz), timing issues can

arise when interfacing with faster or slower peripherals. Synchronization circuits, including

latches and buffers, are often employed to match timing requirements.

Control signals such as RD (Read), WR (Write), and ALE (Address Latch Enable) help

coordinate data transfer timing and ensure stable data exchange between the CPU and

peripherals.

Data Transfer Modes

The 8085 supports various data transfer modes to interface with peripherals:

Programmed I/O: The CPU actively manages all data transfers, suitable for simple

1.

or low-speed peripherals.

Interrupt-Driven I/O: Peripherals signal the CPU via interrupts when ready for

2.

data transfer, increasing efficiency by reducing polling overhead.

DMA Transfer: Allows peripherals to transfer data directly to memory without CPU

3.

intervention, improving throughput.

Choosing the appropriate data transfer mode depends on the application requirements

and the nature of the peripheral device.

Practical Interfacing Examples with 8085

To further illustrate the interfacing concept, consider the following examples involving

common peripheral devices:

Interfacing a 7-Segment Display Using 8255 PPI

The 7-segment display is a prevalent output device for displaying numerical data. By

connecting the display segments to the 8255’s output ports, the microprocessor can

control which segments illuminate.

Programming the 8255 to configure one port as output and sending appropriate bit

patterns enables the display of digits 0-9. This setup demonstrates the ease of parallel

interfacing and the importance of port configuration.

Keyboard Interfacing via 8255

A matrix keyboard can be interfaced using the 8255 by configuring rows as outputs and

columns as inputs. The microprocessor scans the rows by activating them sequentially

and reads the inputs to detect key presses.

Such interfacing requires careful timing and debouncing logic to ensure accurate key

detection, highlighting the interplay between hardware and software in peripheral

interfacing.

Serial Communication through 8251 USART

Serial communication is vital for data exchange over long distances or with external

devices. The 8251 USART (Universal Synchronous/Asynchronous Receiver Transmitter)

converts parallel data from the 8085 into serial form.

Interfacing the 8251 involves setting up baud rate, data bits, parity, and stop bits through

control registers, followed by transmitting and receiving data. This peripheral underlines

the complexity introduced by asynchronous data transfer and the necessity of protocol

adherence.

Advancements and Legacy Considerations

Although the 8085 and its peripheral interfacing techniques are considered legacy today,

understanding these fundamentals is invaluable for modern embedded system design.

Contemporary microcontrollers integrate many peripheral functions internally, reducing

the need for external interfacing.

However, the principles of address decoding, control signaling, and data transfer modes

remain relevant. Moreover, the modular approach of the 8085 system architecture

informs current practices in system design and debugging.

In educational settings, studying the peripheral devices and interfacing of microprocessor

8085 provides a clear illustration of hardware-software co-design, bridging theoretical

knowledge with practical application.

Exploring these classic interfacing methods also offers insights into optimizing data flow,

managing hardware constraints, and designing robust communication protocols, all

critical skills in today’s increasingly complex embedded environments.

microprocessor 8085, peripheral devices, interfacing, input/output ports, memory-mapped

I/O, programmed I/O, interrupt-driven I/O, direct memory access (DMA), serial

communication, parallel communication