PIN DIAGRAM AND PIN DESCRIPTION OF 8085
1. Power supply and Clock frequency signals:

  • Vcc        + 5 volt power supply
  • Vss        Ground
  • X1, X2 :    Crystal or R/C network or LC network connections to set the frequency of internal clock generator.
  • The frequency is internally divided by two. Since the basic operating timing frequency is 3 MHz, a 6 MHz crystal is connected externally.
  • CLK (output)-Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.

  • 8085 is a 40 pin IC, DIP package. The signals from the pins can be grouped as follows

  1. Power supply and clock signals
  2. Address bus
  3. Data bus
  4. Control and status signals
  5. Interrupts and externally initiated signals
  6. Serial I/O ports

Fig (a) - Pin Diagram of 8085 & Fig(b) - logical schematic of Pin diagram.
2. Address Bus:

  • A8 - A15   (output; 3-state)
  • It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address;

3. Multiplexed Address / Data Bus:

  • AD0 - AD7 (input/output; 3-state)
  • These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.
  • During the opcode fetch operation, in the first clock cycle, the lines deliver the lower order address A0 - A7.
  • In the subsequent IO / memory, read / write clock cycle the lines are used as data bus.
  • The CPU may read or write out data through these lines.

4. Control and Status signals:

  • ALE  (output) - Address Latch Enable.
  • This signal helps to capture the lower order address presented on the multiplexed address / data bus.
  • RD (output 3-state, active low) - Read memory or IO device.
  • This indicates that the selected memory location or I/O device is to be read and that the data bus is ready for accepting data from the memory or I/O device.
  • WR (output 3-state, active low) - Write memory or IO device.
  • This indicates that the data on the data bus is to be written into the selected memory location or I/O device.
  • IO/M (output) - Select memory or an IO device.
  • This status signal indicates that the read / write operation relates to whether the memory or I/O device.
  • It goes high to indicate an I/O operation.
  • It goes low for memory operations.

5. Status Signals:

  • It is used to know the type of current operation of the microprocessor.

  • The microprocessor is a clock-driven semiconductor device consisting of electronic logic circuits manufactured by using either a large-scale integration (LSI) or very-large-scale integration (VLSI) technique.

  • The microprocessor is capable of performing various computing functions and making decisions to change the sequence of program execution.

  • In large computers, a CPU implemented on one or more circuit boards performs these computing functions.

  • The microprocessor is in many ways similar to the CPU, but includes the logic circuitry, including the control unit, on one chip.

  • The microprocessor can be divided into three segments for the sake clarity, arithmetic/logic unit (ALU), register array, and control unit.
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