Externally Initiated Operations in 8085 Microprocessor

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The 8085 Microprocessor is one of the most widely used 8-bit microprocessors in various applications, ranging from household appliances to industrial systems. This microprocessor supports several internal and external operations to perform tasks.

In this chapter, we will learn about the main Externally Initiated Operations in the 8085 Microprocessor, along with their advantages and disadvantages.

Basically, the externally initiated operations are the operations that allow external peripheral devices to interact with the microprocessor. Hence, these operations are also referred to as peripheral operations.

What is an Externally Initiated Operation?

An externally initiated operation is a process or task which is triggered or started by an external I/O device. These operations play an important role in microprocessors, as allowing them to handle operations like communication with and control of external peripherals. The 8085-microprocessor has dedicated pins to enable these externally initiated operations.

Externally Initiated Operations in 8085 Microprocessor

Some of the key externally initiated operations in the 8085-microprocessor are as follows −

• RESET
• Interrupt
• READY
• HOLD

These operations are described in detail below.

RESET

In the 8085-microprocessor, RESET is an externally initiated operation which is used to clear the program counter (PC) and initialize it. This operation updates the program counter with the 0000H memory address.

When an external signal is applied to the RESET pin to activate it, the microprocessor suspends all the current operations for that time and restarts the execution of program from the 0000H memory address.

The RESET operation in the 8085-microprocessor is mainly used to restart the system or get out from an error state.

Interrupt

In a microprocessor, interrupts are the signals that stops the normal instructions and shifts the execution to perform high-priority or emergency operations, which are known as service routines.

The 8085-microprocessor have several interrupts such as TRAP, RST 5.5, RST 6.5, and RST 7.5. Where, the TRAP is a non-maskable interrupt, having highest priority, while the RST 5.5, RST 6.5, and RST 7.5 are maskable and vectored interrupts.

In the 8085-microprocessor, when an interrupt signal is applied, the processor stops the current execution and performs the service routine. After completing the service routine, the microprocessor resumes the original task.

READY

READY is another important externally initiated operation in the 8085-microprocessor. The READY operation in the 8085-microprocessor provides synchronization between the microprocessor and the external peripherals which are operating at different speeds.

If a low state signal is applied to the READY pin of the 8085-microprocessor, then the microprocessor enters into the wait state until the requested external device is ready to communicate.

The READY operation plays an important role in providing coordination between the microprocessor and slow-speed peripherals like I/O devices or memory.

HOLD

When the HOLD pin of the 8085-microprocessor is activated by applying an external signal, the microprocessor releases the system buses (data and address buses) and allows the external peripheral devices to directly use them. This operation basically enables the Direct Memory Access (DMA) feature of the microprocessor and allows external devices to establish a direct communication with the memory without involving the processor.

The HOLD operation is mainly used in applications that require high-speed data transfer between memory and external devices.

Advantages of Externally Initiated Operations

The externally initiated operations in 8085-microprocessor offer the following key benefits −

• The externally initiated operations provide flexibility to customize tasks and handle different types of external signals and operations efficiently.
• These operations offload tasks like I/O operations from the microprocessor, so the microprocessor can be available for executing complex tasks.
• Externally initiated operations allow external devices to perform operations, this reduces the latency and enhance the overall performance of the system.
• Externally initiated operations allow for sharing resources like memory, processing power, etc. between the microprocessor and the external devices. This results in improved efficiency.

Disadvantages of Externally Initiated Operations

Despite several advantages, the following are some key disadvantages of externally initiated operations in the 8085-micropcorssor −

• These operations make the microprocessor dependent on external devices. Thus, any kind of failure of the external device can make the microprocessor unable to perform tasks.
• Externally initiated operations require frequent communication between the microprocessor and the external peripherals. This introduces communication overhead in the system and reduces the overall performance. It becomes a more serious issue in the case of time-critical applications.
• These operations also increase the security risks of the system. This is because the external devices involved in these operations can act as the entry points for malicious factors.
• Externally initiated operations also increase the complexity and cost of the system, as it requires to manage and control additional hardware and software components.

Conclusion

Externally initiated operations play an important role in the 8085-microprocessor, as they allow the processor to communicate with and control the external peripherals. Understanding these operations is important for system designers to develop more efficient systems using the 8085 microprocessor.