Microprocessor - 8085 Architecture and Block Diagram

Microprocessor is a general-purpose chip which has a CPU unit built into it. Adding an additional external circuit can make it work as a Microcomputer. Microprocessor can perform operations like adding, subtracting, comparing two numbers and fetching the data from memory for transferring it from one place to another.

Microprocessor is also called as Basic Input-Output System – BIOS used for processing the input from sensor and produce equivalent output from the system.

Arithmetic and Logic Unit – ALU: ALU is used to perform arithmetical and logical operations on the data stored inside a register.

Accumulator: Accumulator is a register in which intermediate arithmetical and logical operation data is stored.

Registers: These are storage devices used to store the data inside a microprocessor in different address location.

Program Counter: Program counter is used for counting the no. of program executed inside a microprocessor.

Stack Pointer: It acts as a pointer to the address and is used to store the address of the last program request made by processor inside a stack

Clock Circuit: Used to generate clock pulse as a reference signal for the microprocessor.

Interrupt Circuit: Used for generating the interrupt signal when the higher priority process required to be served first on basis of priority by microprocessor.

8085 Microprocessor Architecture and Block Diagram:

• 40 pin IC
• 8-bit Data Bus
• 16-bit Program Counter
• 16-bit Stack Pointer
• Six 8-bit registers arranged in pairs: B-C, D-E, H-L
• Five Status Flags: Carry Flag, Parity Flag, Auxiliary Carry Flag, Zero Flag, Sign Flag

AD0 – AD7: Input/Output Pins. Address/Data bus, serve dual purpose. Used for Least Significant 8 Bits (LSB) of the memory address or I/O address during the 1^st cycle. They are again used for data during 2^nd and 3^rd clock cycles.

A8 – A15: Output Pins. Address bus. Used for the Most Significant 8 Bits (MSB) of the memory address or I/O address.

ALE – Output: Address Latch Enable signal. Goes high during 1^st clock cycle of a machine cycle and enables the lower 8 bits of the address to be latched either into the memory or external latch.

IO/M – Output: Status signal which distinguish whether the address is for memory or I/O device.

S1, S0 – Output: Status signals sent by the microprocessors to distinguish the various types of operation given below

Status Codes:

S1	S0	Operations
0	0	HALT
0	1	WRITE
1	0	READ
1	1	FETCH

 RD – Output: RD is a signal to control READ operation. When it goes low, the selected I/O device or memory is read.

WR – Output: WR 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 – Input: It is used by the microprocessor to sense whether a peripheral is ready to transfer a data or not. If READY is high, the peripheral is ready. If it is low the microprocessor waits till it goes high.

Interrupts Signals:

HOLD – Input: HOLD indicates that another device is requesting for the use of the address and data bus.

HLDA – Output: HLDA is a signal for HOLD Acknowledgement which indicates that the HOLD request has been received. After the removal of this request the HLDA goes low.

INTR – Input: INTR is an Interrupt Request Signal. Among interrupts it has the lowest priority. The INTR is enabled or disabled by software.

INTA – Output: INTA is an interrupt acknowledgement sent by the microprocessor after INTR is received.

RST 5.5, 6.5, 7.5 and TRAP – Inputs: These all are interrupts. When any interrupt is recognized, the next instruction is executed from a fixed location in the memory.

TRAP – 0024, RST 5.5 – 002C, RST 6.5 – 0034, RST 7.5, 003C.

RST è Restart Interrupts

Order of priority of interrupts:

-          TRAP (Highest Priority)

-          RST 7.5

-          RST 6.5

-          RST 5.5

-          INTR (Lowest Priority).

Reset Signals:

RESET IN – Input: Resets program counter to 0. Also resets interrupt enable and HLDA flip-flops. CPU is held in reset condition till RESET is not applied.

RESET OUT = Output: Indicates that the CPU is being reset.

Clock Signals:

X1, X2 – Input: X1 and X2 connected to crystal oscillator which drives internal circuit of microprocessor. Used to produce clock for the operation of microprocessor.

CLK – Output: CLK is clock output for user, which can be used for other digital ICs. Its frequency is same at which processor operates.

Serial I/O Signals:

SIC – Input: SID is data line for serial input. The data on this line is loaded into the seventh bit of the accumulator when RIM instruction is executed.

SOD – Output: SOD is a data line for serial output. The seventh bit of the accumulator is output on SOD line when SIM instruction is executed.