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Last Updated: Sep 6, 2023

Pin Diagram of 8085 Microprocessor


Hey Ninja!!! Welcome to yet another article. Do you know what are microprocessors?? The microprocessor is the core processing unit of a computer system that conducts arithmetic and logic operations such as adding, subtracting, moving numbers from one region to another, and comparing two numbers. It is frequently referred to as a processor, a central processing unit, or a logic chip. Now let’s move on to our main topic, which is the 8085 Microprocessor pin diagram.

8085 Microprocessor Pin Diagram

What is the 8085 Microprocessor?

The 8085 Microprocessor is an 8-bit general-purpose CPU with 64K bytes of memory. This microprocessor has 40 pins and operates on a +5V power source. This CPU has a maximum frequency of 3MHz. This processor is available in three versions: 8085 Microprocessor AH, 8085 Microprocessor AH1, and 8085 Microprocessor AH2, all of which use HMOS technology. The advanced versions consume 20% of the power supply. This processor's CLK frequencies are 8085 Microprocessor A-3 MHz, 8085 Microprocessor AH-3 MHz, 8085 Microprocessor AH2-5 MHz, and 8085 Microprocessor AH1-6 MHz.

Pin Diagram of 8085 Microprocessor

The 8085 Microprocessor Pin Diagram or functional pin diagram of the 8085 Microprocessor is shown below. The signals in the 8085 Microprocessor Pin Diagram may be divided into seven groups based on their functionality.

  • Power Supply and Clock Signals
  • Address Bus and Data Bus
  • Reset Signals
  • Serial I/O Ports
  • Control and Status Signals
  • Interrupts and Externally Initiated Signals
  • Direct Memory Access Signal

Serial I/O Signals

SID (Serial I/P Data): This input signal takes serial data from an external device bit by bit.

SOD (Serial O/P Data): This is an output signal that allows serial data to be sent bit by bit to an external device.

Direct Memory Access (DMA) Signal

HOLD: This signal indicates that another master has requested access to the address, data, and control buses.

HLDA: The HLDA signal is used to acknowledge a HOLD request.

Power Supply and Clock Signals

Vcc: A single +5 V power supply is required.

Vss: Ground reference.

X1 and X2: A tuned circuit such as an LC, RC, or crystal is linked at these two points. Because the internal clock generator divides the oscillator frequency by two, a system operating at 3 MHz requires a crystal with a frequency of 6 MHz.

CLK OUT: This signal is utilized by other devices as a system clock. Its frequency is half that of the oscillator.

Interrupts and Peripheral Initiated Signals

In the 8085 microprocessor, interrupts and externally initiated signals are used to handle critical events, perform I/O operations, and respond to hardware events. Non-maskable interrupts are used for critical events, while maskable interrupts are used for other purposes, such as handling external requests or performing time-critical tasks. 

RST 5.5, RST 6.5, RST 7.5, TRAP, and INTR are the five hardware interrupt signals on the 8085 Microprocessor Pin Diagram. The CPU recognizes interrupt requests on these lines at the end of the current instruction execution.

Interrupts can be enabled or disabled by setting or resetting the interrupt enable flip-flop (IFF). The INTR input is a low-level triggered interrupt, while the RST 7.5 and RST 6.5 interrupts are software interrupts generated by executing specific instructions.

Address Bus and Data Bus

AD0 to AD7: The 8-bit data bus (D0 - D7) is multiplexed with the bottom half of the 16-bit address bus (A0 - A7). Lower 8 bits of memory address or I/O address come on the bus during the initial phase of the machine cycle (T1). These lines are utilized as a bi-directional data bus for the remainder of the machine cycle (T2 and T3).

A8 to A15: The address lines A8 to A15 contain the upper half of the 16-bit address. These lines are dedicated to carrying the most significant 8 bits of the 16-bit address lines.

Control and Status Signals

ALE (Address Latch Enable): We already know that the AD0 to AD7 lines are multiplexed and that the bottom half of the address (A0 - A7) is only available during T1 of the machine cycle. This bottom half of the address is also required to reach a specific location in memory or an I/O port during T2 and T3 of the machine cycle. This means that the lower half of an address must be latched in T1 of the machine cycle for it to be accessible throughout the machine cycle. The lower half of an address bus is latched using an external latch and the ALE signal from the 8085 Microprocessor Pin Diagram.

RD and WR: These signals regulate the data flow between the CPU and the memory or I/O device/port. A low RD value indicates that data must be read through the data bus from the designated memory location or I/O port. A low WR indicates that the data must be written via the data bus into the designated memory location or I/O port.

IO/M, S0, and S1: IO/M shows if an I/O or memory operation is being performed. S1 and S0 show the type of machine cycle currently in use.

READY: The microprocessor uses it to determine whether or not a peripheral is ready for data transmission. Otherwise, the processor waits. As a result, it is utilized to synchronize slower peripherals to the microprocessor.

Reset Signals

RESET IN: This pin is low.

  • The program counter is reset to zero (0000H).
  • The interrupt enables, and HLDA flip-flops are reset.
  • The data bus, address bus and control bus are all tri-stated. (Note: This is only active during RESET.)
  • Randomly affects the contents of the processor's internal registers.

When the PC is reset, it sets to 0000H, causing the 8085 Microprocessor Pin Diagram to execute the first instruction from address 0000H. The reset signal must be kept low for at least three clock cycles for the appropriate reset function. The power-on reset circuit can be utilized to verify that the first instruction from address 0000H is executed.

RESET OUT: This active high signal indicates that the CPU is being reset. This signal is synced with the CPU clock and can be used to reset other system components.

Advantages of the 8085 Microprocessor Pin Diagram

  • The pin diagram is logically and systematically arranged hence it is easy to comprehend.
  • It has a simple structure and follows a standardized layout which makes it easy to integrate into circuits.
  • The pin diagram of the 8085 microprocessor offers flexibility in system design.
  • The pin diagram serves as important documentation for the 8085 microprocessor.
  • Troubleshooting and debugging issues can be solved by referring to the pin diagram.

Disadvantages of the 8085 Microprocessor Pin Diagram

  • It cannot handle applications that require a large amount of memory.
  • It has a limited number of pins, hence it is hard to design complex systems that require more peripherals.
  • It is an older architecture, its pin diagram lacks modern features and enhancements.
  • It may face compatibility issues when used with modern system components.
  • Difficult to design memory-mapped I/O circuits.

Frequently Asked Questions

What are pins in 8085 microprocessor? 

Pins refer to the electrical connections on the integrated circuit(IC) package. Each pin has a specific function such as data transfer, address decoding, power supply, etc.

What are the 40 pins of 8085? 

8 Address Bus, 8 Data Bus, Vcc, GND, ALE, RD, WR, IO/M, READY, RESET, HOLD, HLDA, INTR, INTA, SO, S1, S2 , S3, X1, X2, RESET OUT, HOLD ACK, HLDA ACK, READT, GND  are the 40 pins of 8085 microprocessor.

How many pins does an 8085 microprocessor have? 

The most common 8085 microprocessor has 40 pins. However, there are other variants as well which have different pin counts, such as 44 to 42 pins.

What is the pin 37 of 8085 microprocessor?

Pin 37 on the 8085 microprocessor is the SID (Serial Input Data) pin. It's used for receiving serial data when the microprocessor is in a serial communication mode, such as asynchronous data transfer.

What is the function of ready pin of 8085 microprocessor?

The READY pin on the 8085 microprocessor indicates whether external devices are ready for data transfer. When low (0), the CPU waits for the device to become ready before proceeding with data exchange, ensuring proper synchronization between the microprocessor and external hardware.


In this article, we discussed, in brief, the 8085 Microprocessor Pin Diagram. In conclusion, understanding the 8085 microprocessor's pin diagram is crucial for anyone working with this iconic chip. Each pin serves a specific purpose, facilitating communication with external devices and enabling various operations.

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Happy Learning!

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