8085 Microprocessor Features, Pin Diagram, Architecture

Feature of Intel 8085 Microprocessor:

• It is an 8 bit processor with 64 KB addressing capacity.
• It provides 16 address lines so it can access 64 KB(KiloBytes) of memory i.e 2^16 bytes.
• Address ranges from 0000 H to FFFF H i.e 64 KB.
• The operands or data values are also of 8 bit.
• General Purpose Registers are of size 8 bit.
• Data ranges from 00 H to FF H i.e 256 bytes.
• It is called 8 bit microprocessor because it reads or write 8 bit data from or to memory in a single read write machine cycle.
• 8085 microprocessor is a Accumulator based microprocessor.
• The maximum clock frequency is 3 MHz while minimum frequency is 500 KHz.
• It provides 74 instructions with 5 different addressing modes. 

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Pin Diagram of 8085 Microprocessor

Figure: 8085-pin-configuration

From the above figure of Pin Configuration of 8085 Microprocessor, the signal pin are categorized into different signal groups. There are altogether 40 pins in 8085 microprocessor. They are:

1. Multiplexed Address and Data Bus:
• AD0-AD7:  Bidirectional.
2. Address Bus : 
• A8-A15: They carry high order address. Unidirectional.
3. Control Signal groups:
• ALE(Address Latch Enable): It is an output signal which goes high when operation is started by Microprocessor.
• S0 and S1: They are Status signal used to indicate the type of operation.
• IO/M̅: stands for Input Output and Memory. When there is high signal Input Output is activated and when there is low signal Memory is activated.
• RD (Read Bar): This is an active low signal which performs Read Operation when signal goes low. 
• WR(Write Bar): This is an active low signal that performs Write operation when signal goes low.
4.  Clock and Frequency Signal: 
• X1 , X2: These are clock input signals and are connected to external LC or RC circuit.
• Vcc and Vss: They are power supply and Ground respectively.
5. Serial Input/Output port:
• SID: Serial Input Data is an input pin that help to accept 1 bit data.
• SOD:  Serial output data is output pin which is used to send serial 1 bit data.
6. Interrupts:
• There are 5 Interrupt signals in this group.
• TRAP:  It is highest priority non maskable interrupt.
• INTR: stands for Interrupt request signal that has the lowest priority.
• RST 5.5, 6.5, 7.5: These are maskable interrupts.
7. Externally Initiated Signal:
• READY: This is an output signal to check the status of output device.
• HOLD: HOLD signal is used by DMA(Direct Memory Access) controller to make request to microprocessor.
• RESET IN: It is an active low signal to clear the Microprocessor.
8. External signal Acknowledge:
• INTA : is an active low signal for Interrupt Request Acknowledgement.
• HLDA :  is an active low signal to acknowledge HOLD signal request by DMA controller.

Architecture of 8085 Microprocessor

Architecture of 8085 Microprocessor

Architecture of 8085 Microprocessor can be explained as a whole structure where there are different blocks which make it to operate.  The internal structure of 8085 consists of  different sections like:

1. Register array
2. Arithmetic and logic unit
3. Timing and control unit.
4. Interrupt Control
5. Serial I/O

A. Register Array:

The 8085 has both 8 bit and 16 bit registers. It has 8 addressable 8 bit registers and three 16 bit registers. These registers can be classified as 

• General purpose register
• Special purpose register. 

1. General Purpose Registers:
•  The 8085 has 6 general purpose registers to store 8 bit data during program execution. B,C, D ,E, H, L are 8 bit registers and can be used singly or in 16 bit register pairs as BC, DE, HL. 
• They are used during programming with Accumulator.
2. Special Purpose Register:
• Accumulator: It is a 8 bit register used in arithmetic logic load and store operations as well in input output instructions. Most of the Operations are done in Accumulator.
• Temporary Register:  It is 8 bit register not accessible to the programmer while executing the instruction. It is used for internal processing and storage.
• Stack Pointer(SP): is a 16 bit register to store the Stack Address. Stack is the portion of Memory arranged in LIFO(Last In First Out).
• Program Counter(PC): is a 16 bit register that stores the memory address of 8085 microprocessor. It holds the address of the next instruction to be fetched.
• Instruction Register and Decoder:  When an instruction is fetched from memory, it is loaded in the instruction register. The decoder decodes the instruction and establishes the sequence of events to flow. The instruction register is not programmable and cannot be accessed through any instructions. 
• Flag Register:  It is an 8-bit register which consists of 5 flip flops used to know status of various operations done. 5 Flags are available which are:
• Sign Flag(S): Sign flag is set when result of an operation is negative. 
• Zero Flag(Z):  Zero flag is set when result of an operation is 0.
• Auxiliary Flag(AC): Auxiliary carry flag is set when there is a carry out of lower nibble or lower four bits of the operation.
• Carry Flag(CY): Carry flag is set when there is carry generated by an operation. 
• Parity Flag (P): Parity flag is set when result contains even number of 1’s. 
• Rest are don’t care flip flops and reserved for future use.

• S	Z	-	AC	-	P	-	CY

B. Arithmetic and Logic Unit:

The ALU performs the computing function. It includes the accumulator, the temporary register, arithmetic and logic circuits and the flag register. The temporary register is used to store hold data during an arithmetic/ logic operation. The result is stored in the accumulator and the flags are set or cleared according to the result of operation. 

C. Timing and Control Unit:

This is a unit to provide necessary timing, control and synchronization signal to ALU. There are different pins attached to this unit such as RD, WR, IO/M, ALE etc.

For example: To perform the read operation by ALU, the Timing and Control Unit should issue RD signal to ALU.

D. Interrupt Control:
This is a unit that handles different interrupt signals such as INTR, RST 5.5, RST 6.5, RST 7.5 and TRAP. They are basically generated by I/O devices to microprocessor through these pins.
INTA is an Interrupt Acknowledge signal by microprocessor to I/O devices.

E. Serial I/O: 
This unit handles the transmission of 1 bit serial data. There are two pins connected to this unit which are SID and SOD.

Related Posts: 

• 8085 Microprocessor Features, Pin Diagram, Architecture
• 8086 Microprocessor Features, Architecture
• All 8085 Instructions with Operand, Size, Addressing Mode, Machine Cycle, T-States
• Timing Diagram [8085 Microprocessor]