Pin Diagram and Pin description of 8085 Microprocessor




8085 pin diagram
8085 pin diagram

The 8085 and Its Buses

  • The 8085 is an 8-bit general purpose microprocessor that can address 64K Byte of memory.
  • It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz.
    • The pins on the chip can be grouped into 6 groups:
      • Address Bus.
      • Data Bus.
      • Control and Status Signals.
      • Power supply and frequency.
      • Externally Initiated Signals.
      • Serial I/O ports.

8085 Pin description 

  • Higher Order Address pins- A15 – A8
    • The address bus has 8 signal lines A8 – A15 which are unidirectional.
  • Lower Order Address/ Data Pins- AD7-AD0
    • These are time multiplexed pins and are de-multiplexed using the pin ALE
    • So, the bits AD0 – AD7 are bi-directional and serve as A0 – A7 and D0 – D7 at the same time.
      • During the execution of the instruction, these lines carry the address bits during the early part, then during the late parts of the execution, they carry the 8 data bits.
    • In order to separate the address from the data, we can use a latch to save the value before the function of the bits changes.
  • Control Pins – RD, WR
    • These are active low Read & Write pins
  • Status Pins – ALE, IO/M (active low), S1, S0
    • ALE (Address Latch Enable)-Used to de-multiplex AD7-AD0
    • IO/M – Used to select I/O or Memory operation
    • S1,S0 – Denote the status of data on data bus
  • Interrupt Pins – TRAP, RST7.5, RST 6.5, RST 5.5, INTR, INTA
    • These are hardware interrupts used to initiate an interrupt service routine stored at predefined locations of the system memory.
  • Serial I/O pins – SID (Serial Input Data), SOD (Serial Output Data)
    • These pins are used to interface 8085 with a serial device.
  • Clock Pins- X1, X2, CLK(OUT)
    • X1, X2– These are clock input pins. A crystal is connected between these pins such that fcrystal= 2f8085 where fcrystal= crystal frequency & f8085 = operating frequency of 8085
    • CLK(OUT) – This is an auxiliary clock output source
  • Reset Pins – Reset In (active low), Reset Out
    • Reset In is used to reset 8085 whereas Reset Out can be used to reset other devices in the system
  • DMA (Direct Memory Access) pins – HOLD, HLDA
    • These pins are used when data transfer is to be performed directly between an external device and the main memory of the system.
  • Power Supply Pins – +VCC, VSS

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