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Pin Diagram of 8085

The below Diagram shows the Pin details of 8085:
Pin Diagram of 8085

Functions of various Pins of 8085

A8-A15 Higher Order Address bus:
  • These are o/p tri-state (a state of high impedance) signals used as higher order 8 bits of 16 bit address.

  • These signals are unidirectional and are given from 8085 to select memory or I/O devices.

  • AD0-AD7 Multiplexed Address/Data bus:
  • These are I/O tri-state signals, having 2 sets of signals. They are address and data.

  • The lower 8 bit of 16 bit address is multiplexed/time shared with data bus.

  • Address latch Enable(ALE):
  • It is an output signal used to give information of AD0-AD7 contents.

  • It is a positive going pulse generated when a new operation is started by microprocessor.

  • When pulse goes high it indicates that AD0-AD7 lines are address.

  • When it is low it indicates that the contents are data.

  • IO/M(bar):
  • This is an output status signal used to give info of operation to be performed with memory or I/O devices.

  • When = 0, the microprocessor is performing memory related operation.

  • When = 1, the microprocessor is performing I/O device related operation.

  • This signal separates memory and I/O devices.

  • Status signals(S0 and S1):
  • These are output status signals used to give information of operation performed by microprocessor.

  • The S0 and S1 lines specify 4 different conditions of 8085 machine cycles.

  • Pin Diagram of 8085
  • This is an active low output control signal used to read data from memory or an I/O device.

  • Write:
  • This is an active low output signal used to write data to memory or an I/O device.

  • Ready:
  • This is an active high input control signal.

  • It is used by microprocessor to detect whether a peripheral has completed (or is Ready for) the data transfer or not.
  • Pin Diagram of 8085
  • The main function of this pin is to synchronize slower peripheral to faster microprocessor.

  • If ready pin is high the microprocessor will complete the operation and proceeds for the next operation.

  • If ready pin is low the microprocessor will wait until it goes high.

  • Trap:
  • This is an active high, level and edge triggered, non-maskable higher priority interrupt.

  • When TRAP is active, the program counter of µp jumps automatically at address 0024.

  • RST 7.5,RST 6.5 and RST 5.5:
  • These are active high, edge (RST 7.5) or level (RST 6.5 and RST 5.5) triggered maskable interrupts.

  • The priorities of these are TRAP, RST 7.5, RST 6.5, and RST 5.5.

  • When RST 7.5, RST 6.5 and RST 5.5 are active, the program counter jumps automatically at address 003C, 0034, 002C respectively.

  • INTR and INTA(Bar):
  • INTR is an active high, level triggered general purpose interrupt.

  • When INTR is active µp generates an interrupt acknowledge signal.

  • If INTR is active, the Program Counter (PC) will be restricted from incrementing and an will be issued.

  • During This cycle a RESTART or CALL instruction can be inserted to jump to the interrupt Service routine.

  • The INTR is enabled and disabled by software. It is disabled by Reset and immediately after an interrupt is accepted.

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

  • The CPU, upon receiving the Hold request, will withdraw the use of buses as soon as the completion of the current machine cycle. Internal processing can continue.

  • The processor can regain the buses only after the Hold is removed.

  • When the Hold is acknowledged, the Address, Data, RD, WR, and IO/M lines are tri-stated.

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

  • HLDA goes low after the Hold Request is removed.

  • The CPU takes the buses one half clock cycles after HLDA goes Low.

  • ResetIN(Bar):
  • 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.

  • The CPU is held in the reset condition as long as Reset is applied.

  • After reset status internal register and flag are unpredictable.

  • After reset microprocessor starts executing from instruction from 0000H onwards.
  • Pin Diagram of 8085

  • This is an active high output signal used to indicate CPU is being reset and can be used as a system RESET.

  • The signal is synchronized to the processor clock.

  • This signal is also used to reset the peripherals once the µP is reset.

  • It is an acknowledgement signal to RESET IN (bar).

  • Serial input data(SID):
  • This is an active high Serial input data line the data on this line is loaded into accumulator bit 7 whenever a RIM instruction is executed.

  • Serial output data(SOD):
  • This is an active high Serial output data line.

  • The output SOD is set or reset as specified by the SIM instruction.

  • X1,X2:
  • Crystal or R/C network connections to set the internal clock generator X1 can also be an external clock input instead of a crystal.

  • The input frequency is divided by 2 to give the internal operating frequency as shown in fig.

  • Pin Diagram of 8085

    CLK OUT:
  • Clock Output for use as a system clock when a crystal or R/ C network is used as an Input to the CPU.

  • Clock input to all other peripherals is provided through CLK OUT pin.

  • The period of CLK is twice the X1, X2 input period.

  • Pin Diagram of 8085

    VCC and VSS:
  • +5 volt supply and Ground Reference.

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