Pin diagram of 8085 microprocessor

The 8085 microprocessor is a popular 8-bit microprocessor developed by Intel. It has 40 pins, each with a specific function for interfacing with memory, input/output devices, and other components.

Pin diagram of 8085 microprocessor is shown below: 

Key Pin Descriptions

1. Address Bus and Data Bus

Address Bus (A8 to A15): The address bus is unidirectional, i.e., bits flow in one direction from the microprocessor unit to the peripheral devices and uses the higher order address bus. 

Address Data Bus (AD0 to AD7): These are bi-directional data pins used to transfer data between the microprocessor and memory or I/O devices. The microprocessor is an 8-bit processor, so it uses 8 data lines. These pins serve the dual purpose of transmitting lower order address and data byte. During 1st clock cycle, these pins act as lower half of address. In remaining clock cycles, these pins act as data bus.

2. Control and Status Signals

ALE - It is an Address Latch Enable signal. It goes high during first T state of a machine cycle and enables the lower 8-bits of the address, if its value is 1 otherwise data bus is activated. 

IO/M' - It is a status signal which determines whether the address is for input-output or memory. When it is high(1) the address on the address bus is for input-output devices. When it is low(0) the address on the address bus is for the memory. 

SO, S1 - These are status signals. They distinguish the various types of operations such as halt, reading, instruction fetching or writing.  

RD' - It is a signal to control READ operation. When it is low the selected memory or input-output device is read. 

WR' - It is a signal to control WRITE operation. When it goes low the data on the data bus is written into the selected memory or I/O location. 

READY - It senses whether a peripheral is ready to transfer data or not. If READY is high(1) the peripheral is ready. If it is low(0) the microprocessor waits till it goes high. It is useful for interfacing low speed devices.  

3. Power Supply and Clock Frequency

Vcc - +5v power supply 

Vss - Ground Reference 

XI, X2 - A crystal is connected at these two pins. The frequency is internally divided by two, therefore, to operate a system at 3MHZ the crystal should have frequency of 6MHZ. 

CLK (OUT) - This signal can be used as the system clock for other devices.  

4. Interrupts and Peripheral Initiated Signals: 

The 8085 has five interrupt signals that can be used to interrupt a program execution. 

(i) INTR 
(ii) RST 7.5 
(iii) RST 6.5 
(iv) RST 5.5 
(v) TRAP 

The microprocessor acknowledges Interrupt Request by INTA' signal. In addition to Interrupts, there are three externally initiated signals namely RESET, HOLD and READY. To respond to HOLD request, it has one signal called HLDA. 

1. INTR (Interrupt Request): This pin is used to request an interrupt from an external device.
2. RST7.5, RST6.5, RST5.5: These are vectored interrupts with different priority levels.
3. TRAP: This is a non-maskable interrupt and has the highest priority.
4. INTA (Interrupt Acknowledge): This pin is used to acknowledge interrupts from external devices.

5. Reset Signals

RESET IN - When the signal on this pin is low(0), the program-counter is set to zero, the microprocessor unit is reset. 

RESET OUT - This signal indicates that the MPU is being reset. The signal can be used to reset other devices. 

6. DMA Signals

HOLD - It indicates that another device is requesting the use of the address and data bus. Having received HOLD request the microprocessor relinquishes the use of the buses as soon as the current machine cycle is completed. Internal processing may continue. After the removal of the HOLD signal the processor regains the bus.  

HLDA - It is a signal which indicates that the hold request has been received after the removal of a HOLD request, the HLDA goes low.  

7. Serial I/O Ports

Serial transmission in 8085 is implemented by the two signals.

SID and SOD - SID is a data line for serial input whereas SOD is a data line for serial output.  

Features of 8085 Microprocessor

1. 8-bit Processor: Processes 8 bits of data at a time.
2. 16-bit Address Bus: Can address up to 64 KB of memory.
3. 5 MHz Clock Speed: Provides a decent processing speed for basic tasks.
4. Low Power Consumption: Ideal for embedded systems with minimal power needs.
5. Interrupt System: Features 5 interrupt pins for external signal response.

Limitations of 8085 Microprocessor

1. Limited Memory Addressing: Can address only up to 64KB of memory due to its 16-bit address bus.
2. No Multiprocessing Support: Cannot connect multiple processors for parallel processing.
3. Limited Instruction Set: Supports only 50 instructions, restricting its functionality for complex tasks.
4. No Direct Memory Access: Lacks DMA (Direct Memory Access) support, which slows data transfer between memory and peripherals.
5. Slower Processing: With a clock speed of up to 5 MHz, it is slower compared to modern processors.