8085 Pin Diagram

The following figure shows the pin diagram of the 8085 microprocessor.

8085 pin diagram
8085 Pin diagram
8085 pin diagram

Control and Status SIgnals

ALE (Address Latch Enable)

A positive-going pulse is generated every time the 8085 begins an operation (machine cycle). It indicates that the AD7 – AD0 bits are address bits.

\overline {RD} (Read)

Indicates that the selected I/O or memory device is to be read and data are available on the data bus.

\overline {WR} (Write)

Indicates the data data on the bus are to be written into a selected memory or I/O location.

I0 / \overline M

When it is high, it indicates an I/O operation. When it is low it indicates a memory operation.

S_1 and S_0

Status signals similar to I0 / \overline M. They are rarely used in small systems.

Power Supply and Clock Frequency Signals

V_{cc}

+5V Power Supply.

V_{ss}

Ground Reference.

X_1, X_2

A crystal (or RC, LC network) is connected at these two pins. It is used to give external input to generate clock internally.

CLK (OUT)

Clock Output: Can be used as the system clock for other devices.

Serial I/O Ports

  • 8085 has two signals SID (Serial Input Data) and SOD (Serial Output Data) for serial transmission.
  • In serial transmission, data bits are sent over a single line, one bit at a time

Interrupts and Externally Initiated Signals

INTR (Input)

Interrupt Request.

\overline {INTA} (Output)

Interrupt Acknowledge.

RST 7.7, RST 6.6, RST 5.5 (Restart Interrupts)

Vectored interrupts that transfer the program control to specific memory locations.

TRAP (Input)

Nonmaskable interrupt that has highest priority.

HOLD (Input)

Indicates that a peripheral controller is requesting access the use of the address and data buses.

HLDA (Output)

Signal acknowledges the HOLD request.

READY (Input)

Used to delay the microprocessor Read or Write cycles until a slow-responding peripheral is ready to send or accept data.

\overline {RESET IN}

When this signal goes low, the PC is set to 0 and MPU is RESET.

RESET OUT

Indicates that the MPU is being reset. This can be used to reset other devices.

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