Pin Diagram and Pin description of 8085 Microprocessor

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– A_₈
- 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 f_crystal= 2f₈₀₈₅where f_crystal= 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 – +V_CC, V_SS