Microprocessor Tutorial

8085 Microprocessor

8086 Microprocessor

Difference between

Pin Diagram of 8085

Pin Diagram

Functions of Pins

The signals from the pins can be grouped as follows

  • Power supply and Clock signals
  • Address Bus
  • Data Bus
  • Control and Status signals
  • Interrupt signals
  • Serial I/O ports


  • Power supply and Clock Signals

    Power Supply
    Vcc: +5v
    Vss: Ground Signal

    Clock Signal
    X1,X2: Crystal or R/C network or LC network connections to set the frequency of internal clock generator.
    CLK OUT: Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.

    Address Bus

    PINs from A8 - A15.
    It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address

    Data Bus

    PINs from AD0 - AD7.
    They are multiplexed pins.
    These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.


    Control and Status Signal


    ALE (Address Latch Enable) : This signal helps to capture the lower order address presented on the multiplexed address / data bus. When the Signal is 1 it indicates AD0-AD7 as address else data.

    RD : Indicates that the selected data from memory or I/O is to be read and data bus is ready for accepting data.

    WR : indicates that the data on the data bus is to be written into the selected memory location or I/O device.


    Inerrupt and externally initiated signals


    INTA : Interrupt Acknowledgment signal.

    RESET IN : Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA Flip-flops and makes address, data and control lines tri-stated.

    RESET OUT : Active hig signal used to indicate that CPU is being reset.
    Also it can be used as system reset.

    READY : This indicates that CPU is ready to send or receive data.

    HOLD : indicates that another Master is requesting the use of the Address and Data Buses.

    HLDA : indicates that the CPU has received the Hold request and that it will withdraw the buses in the next clock cycle

    Serial I/O signals


    SID(Serial Input Data) : Active high, the data is loaded into accumulator whenever a RIM instruction is executed.

    SOD(Serial Output Data) : Active high, The output SOD is set/reset as specified by the SIM instruction.

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