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
