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Pin Diagram of Intel 8085 Microprocessor

  • There are 40 pins in Intel 8085 microprocessor. The photo image is shown in figure and pin diagram is shown in figure.
    Intel 8085 microprocessor

    Pin diagram of Intel 8085 microprocessor
    The signals are classified into the following categories.

    S.No.
    Group
    1
    Address bus
    2
    Multiplexed Address / Data bus
    3
    Control and Status signals
    4
    Peripheral initiated signals
    5
    Clock signals
    6
    Reset signals
    7
    Interrupt signals
    8
    Serial I/O signals
    9
    Power supply and Ground signals

    The signal diagram is shown in figure below.
    Signal diagram of Intel 8085 microprocessor

    Pin number
    Signal name
    Description
    Address Bus
    21 – 28
    A8 – A15
    Higher order byte (8 bits) of address bus. The address bus is unidirectional (one-way, ie. microprocessor to memory).
    Multiplexed Address / Data bus
    12 -19
    AD0 – AD7
    Lower order byte (8 bits) address bus (A0 - A7). And Data bus (8 bits) (D0 – D7).The data bus is bidirectional (two-wayie. microprocessor to memory and memory to microprocessor).
    Control and Status signals
    30
    ALE
    ALE - Address Latch Enable. Used for de-multiplexing the Address / Data bus. i.e. separating the lower order address lines (A0 – A7) and data lines (D0 – D7) from (AD0 – AD7).
    29
    S0
    The microprocessor indicates its status (memory write, memory read, I/O write, I/O read, Opcode fetch, Halt, Hold, Reset) through these three output pins. IO/M’ will be ‘1’ for I/O operation and ‘0’ for memory operation.
    33
    S1
    34
    IO / M' (M - bar)
    32
    RD’
    This is an active low signal. It will be 0 during memory read and I/O read operation.
    33
    WR’
    This is an active low signal. It will be 0 during memory write and I/O write operation.
    Peripheral initiated signals
    35
    READY
    This pin is used for interfacing slow devices with the microprocessor.
    39
    HOLD
    DMA (Direct Memory Access) signals:
    HOLD (hold request)
    HLDA (hold acknowledge)
    38
    HLDA
    Clock signals
    1, 2
    X1, X2
    Used for connecting the crystal for generating the clock signal.
    37
    CLK OUT
    Clock signal for the peripheral devices.
    Reset signals
    36
    RESETIN’
    Used to apply reset signal to the microprocessor. When this pin goes low, the microprocessor is reset and begins executing the instruction from the memory location 0000. All the registers inside the microprocessor is reset to zero.
    3
    RESET OUT
    Used to reset the peripheral devices.
    Interrupt signals
    6
    TRAP
    Interrupt request input signals to the microprocessor.
    7
    RST 7.5
    Interrupt request input signals to the microprocessor.
    8
    RST 6.5
    9
    RST 5.5
    10
    INTR
    11
    INTA’
    Interrupt Acknowledge signal from the microprocessor to the peripheral devices.
    Serial I/O signals
    5
    SID
    Serial Input Data. Used to receive serial data bits.
    4
    SOD
    Serial Output Data. Used to transmit serial data bits.
    Power supply and Ground signals
    40
    VCC
    Power supply pins.
    +5V to VCC
    GND to VSS
    20
    VSS



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