Pin Diagram of 8085 Microprocessor

8085 MPU is an 8-bit microprocessor using NMOS technology in 40 pin dual in- line (DIP) package.

Pin Diagram of 8085 Microprocessor

Power supply and frequency signals:

• Vcc : Single +5V power supply.

• Vss : Ground reference.

• X1 and X2: A crystal oscillator is connected to these pins.

Data bus and address bus:

• AD0 - AD7: 8-bit data bus (D0 - D7) is multiplexed with the lower half of 16-bit address bus (A0 - A7).

• A8 - A15: The upper half of 16-bit address bus appears on these pins.

Control and status signals:

• ALE: ALE stands for Address Latch Enable. The lower half of address bus (A0 - A7) is available only during T1 of machine cycle. This is also necessary during T2 and T3 of machine cycle. Therefore the lower half of address bus must be latched to T1 of machine cycle so that it is available throughout the machine cycle. This important function is done through ALE signal.

• Read and Write signals: These two signals are active low signals. Read signal indicates that selected I/O or memory device is to be read and data is available on data bus. Write signal indicates that data available on data bus is to be written into selected I/O or memory device.

• IO/M, S0 and S1 : IO/M states whether IO operations or memory operation is to be carried out. S0 and S1 indicates the type of machine cycle.

• READY: It is used to sense whether the input or output device is ready for data transfer or not. If it is not ready then the processor waits.

Interrupt signals:

8085 microprocessor has five hardware interrupts:

• TRAP
• RST 7.5
• RST 6.5
• RST 5.5
• INTR

There is also an active low signal called Interrupt Acknowledge (INTA). This signal is used to acknowledge the INTR interrupt.

Serial I/O signals:

• SID: SID stands for Serial Input Data. It is used to accept the data serially bit by bit from external device.

• SOD: SOD stands for Serial Output Data. It is used to transmit the data serially bit by bit to external device.

DMA signal:

• HOLD: It indicates that another user is requesting for the use of address bus, data bus and control bus.

• HLDA: It is an active high signal. It is used to acknowledge HOLD request.

Reset signals:

• RESET IN: A low on this pin sets the program counter to zero, resets the interrupt enable and HLDA FFs, tristates data, address and control bus, and also affects the contents of internal registers.

• RESET OUT: A high on this pin indicates that processor is being reset.