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8085

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8085 Architecture and its Operations

The microprocessor is a programmable digital

device, design with registers, Flip flops and

timing elements. The microprocessor has set

of instructions, designed internally, to

manipulate data and communicate with

peripherals. This process of data manipulation

and communication is determined by the logicdesign of the microprocessor, called the

architecture.

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All the various functions performed by the

microprocessor can be classified in three

general categories:

Microprocessor-initiated operations

Internal operations

Peripheral (or externally initiated) operations

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Microprocessor Initiated Operations

and 8085 Bus organization The MPU performs primarily four operations:

1. Memory Read: Read data (or instructions) from memory

2. Memory Write : Write data (or instructions) into memory

3. I/O Read: Accepts data from input devices

4. I/O Write: Sends data to output devicesAll the operations are part of the communication process between theMPU and peripheral (including memory). To communicate with aperipheral (or a memory location), the MPU needs to perform thefollowing steps:

Step1: Identify the peripheral or the memory location (with its

address)Step2: Transfer binary information (Data or instruction)

Step3: Provide timing or synchronization signals.

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8085 Bus Structure

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Example: Memory Read Operation

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Internal Data Operations and the 8085

Registers The internal architecture of the 8085 microprocessor

determines how and what operations can be performedwith the data. These operations are:

1. Store 8-bit data.

2. Perform arithmetic and logical operations.3. Test for conditions.

4. Sequence execution of instructions.

5. Store data temporarily during execution in the definedR/W memory location called stack.

To perform these operations, the microprocessor requiresregisters, an arithmetic logic unit and control logic, andinternal buses (paths for information flow)

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Peripheral or Externally Initiated

Operations

External devices can initiate the following

operations, for which individual pins on the

microprocessor chip are assigned : Reset,

Interrupt, Ready, Hold.

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Signal descriptions and pins of

8085 microprocessor Address Bus

The 8085 has 16 signal lines (pins) that are used as theaddress bus.

Multiplexed Address/ Data Bus

The signal lines AD7-AD0 are bidirectional they serve a dualpurpose . They are used as the low order address bus as wellas data bus.

Control and Status Signal

ALE -Address latch enable: This is a positive going pulsegenerated every time the 8085 begin an operation; itindicates that the bits on AD7-AD0 are address bits.

RD-Read: This is a read control signal(active low). This signalindicates that the selected I/O or memory device is to be read

and data are available on the data bus. WR-Write: This is a write control signal (active low). This

signal indicates that the data on the data bus are to be writteninto selected memory or I/O location.

IO/M: This is a status signal used to differentiate between I/Oand memory operations. When it is high, it indicates an I/Ooperation; when it is low, it indicates a memory operation.This signal is combined with RD(Read) an WR(Write) togenerate I/O and memory control signals

S1 and S0: These status signals, similar to IO/M, can identifyvarious operations, but they are rarely used in smallsystems.

Power Supply and Clock Frequency

Vcc: +5V power supply

Vss : Ground Reference

X1,X2 : A crystal is connected at these two pins. Thefrequency is internally divided by two therefore, to operate asystem at 3Mhz, the crystal should have a frequency of 6Mhz.

CLK (OUT)- Clock Ouptut: This signal can be used as thesystem clock for other devices.

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Cont. Externally Initiated Signals, Including Interrupts

INTR(Input) Interrupt Request: This is used as ageneral purpose interrupt .

INTA(Output) Interrupt Acknowledge : This is used

to acknowledge an interrupt. RST 7.5 , RST 6.5, RST 5.5(Inputs) Restart Interrupts

: These are vectored Interrupts that transfer theprogram control to specific memory locations. Theyhave higher priorities than the INTR interrupt.

TRAP (Input )This is a nonmaskable interrupt andhas the highest priority.

HOLD (Input) This signal indicates that a peripheral

such as a DMA (Direct memory access) controlleris requesting the use of the address and databuses.

HLDA (Output) Hold Acknowledge: This signalacknowledge the HOLD request.

Ready (Input) This signal is used to delay themicroprocessor Read or write cycles until a slowresponding peripheral is ready to send or acceptdata. When this signal goes low, the

microprocessor waits for an integral number ofclock cycles until it goes high.

Reset IN : When the signal on this pin goes low, theprogram counter is set to zero, the buses are tri-stated and, the MPU is reset.

Reset OUT : This signal indicates that the MPU isbeing reset. The signal can be used to reset otherdevices.

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Cont..

Serial I/O Ports

The 8085 has two signals to

implement the serial

transmission: SID (Serial Input

Data) and SOD (Serial Output

Data).

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Suggested Reading

Chapter 3 Section 3.1 and Chapter 4 Section

4.1 of Microprocessor Architecture,

Programming, and Application with the 8085

by Ramesh Gaonkar 5th edition.