Introduction to microprocessor 8085 and pin diagram part1

Introduction:-

Today the microprocessor based products have revolutionized every area of human  activity and have made a deep impact on quality of life. Right from small chip to supercomputer Microprocessor have become an integral part of the system. Microprocessor evolved from the developments in computer. One such microprocessor is Intel’s 8085, which is most popularly used and regarded as a basic microprocessor. The microprocessors operation can be described through the organization of computer model.

Generally a computer organization is as follows

INPUT> ALU+CONTROL UNIT+MEMORY> OUTPUT
      (1)                           (2)                                                  (3)

Unit 2 is generally replaced with microprocessor. So a microprocessor can be represented as :

Microprocessor=ALU+Registers+Program Counter+Control and Timing circuit+Stack Pointer +Interrupt Circuit.

PIN DIAGRAM:-

8085 is a general purpose microprocessor having 40 pins and works on single power supply. To study the pin diagram we group
 the signals into 6 categories:

1.Power Supply
2.Interrupt Signals
3.Address and Data bus
4.Control and Status signals
5.Serial I/O Port
6.DMA Request Signals

 Power supply and Clock frequency signals:

• VCC:-Vcc is to be connected to +5V power supply.
• Vss:-Ground reference
• X1 and X2:- This pin is used for providing the clock frequency to the microprocessor. Generally Crystal oscillator or LC oscillator is used to generate the frequency.
•  The frequency generated here is internally divided into two.As we know that the basic operating timing frequency of the microprocessor is 3 MHz so 6 MHz frquency is applied.
• CLK (output)-Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.

 Multiplexed Address / Data Bus:

• AD0-AD7:-These are multiplexed address and data bus. So it can be used to carry the lower order 8 bit address as well as the data. Generally these lines are demultiplexed using the Latch.
• During the opcode fetch operation, in the first clock cycle the lines deliever the lower order address bus  A0-A7.
• In the subsequent IO/M read or write it is used as data bus D0-D7. CPU can read or write data through these lines.
• A8-A15:- These are address bus used to address the memory location.

 Interrupt signal:

• Pin 6 to 11:- These pins are used for interrupt signals. Generally and external devices are connected here which requests the microprocessor to perform a particular task.
• There are 5 pins for hardware interrupts-
  TRAP, RST7.5, RST 6.5, RST5.5 and INTR
• INTA is used for acknowledgement. Microprocessor sends the acknowledgement to external devices through the INTA pin. 

Control And Status Signal:

• S0 and S1:-It is used for the status signal in microprocessor
• .ALE(Airthmetic Latch Enable):-This signal is used to capture the lower address presented on multiplexed address and data bus.
• RD:-This is active low input generally
   used for reading operation.
• WR:-This is active low input used for writing operation.
• IO/M:-This pin is used to select the memory or input-output through which we want to communicate the data.
• READY:-As we know that memory and input -output have slower response than microprocessor. So a microprocessor may now be able to handle further data till it completes the present job. So it is in waiting state. As it completes the present job it sets the READY pin. Microprocessor enters into wait state while READY pin is disabled.
• RESET IN:-This is active low input. This pin is used to reset the microprocessor. An active low signal applied to this pin reset the program counter inside the microprocessor. The busses are tristated.
• RESETOUT:-If we want to reset the external devices connected to the microprocessor then a signal applied to this pin resets the external devices.

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