Pin Diagram of 8085A Microprocessor and working functions of each pin (Study Notes)

Pin Diagram of 8085A Microprocessor 

Diagram:

Pin Diagram of 8085 Microprocessor

Functional of Each Pin (Briefly):

👉A₈ – A₁₅ (Output):

These are address bus and used for the eight most significant bits of the memory address or eight bits i/o address.

👉AD₀ – AD₇ (Both Input and Output):

Thses are time multiplexed address/data bus that is served dual purpose. They are used for the LSB (Least Significant Bits) 8 bits of memory address and data during other time.

👉ALE (Output):

It is abbreviated as Address Latch Enable. It goes high when 8 lower bits of address is latched.

👉IO/M* (Output):

It is a status signal which distinguishes whether the address is for i/o or memory. When it goes high, the address bus is for an i/o device. And when it goes low, the address is for memory location because M is an active low signal.

👉S₀, S₁ (Ouput):

These are status signal send by microprocessor to distinguish various types of operation.

S0	S1	Operation
0	0	HALT
0	1	WRITE
1	0	READ
1	1	FETCH

👉Active Low Signal (RD):

When Microprocessor read data or codes from a memory location or an input devices, it is called read operation. When it goes low, the selected memory or input device is read.

👉Active Low Signal (WR):

When the microprocessor sends a data to the memory location or an output devices, it is called write operation. When it goes low, the data which is on the data bus is written into the selected memory or send to the output devices.

👉Ready (Input):

It is a signal sent by i/o device to the microprocessor. These signal indicates that the i/o device is ready to send or receive data. When ready is high, it indicates that the i/o devices is ready to send or receive data.

👉HOLD(Input) & HLDA (Output):

When another device of the computer system requires data and address buses for data transfer, it send HOLD signal to the microprocessor. 

After receiving HOLD request, the microprocessor sends out a HLDA (Hold Acknowledge) signal to the device. Then, the microprocessor leaves the control over the buses as soon as the current machine cycle is completed.

👉INTR:

It is an interrupt signal sent by an external device to the microprocessor through this pin. An external device inform microprocessor that is ready to transfer data or to initiate certain operation.

👉INTA(Active Low Signal):

INTA* is an interrupt acknowledge signal issued by microprocessor after receiving an interrupt request from an external devices. It is Active Low Signal.

👉RST 5.5, RST 6.5, RST 7.5, TRAP:

When an interrupt is recognized, the next instruction is executed from a fixed location in the memory as given below.

LINE	Location From Which Next Information is Picked Up
TRAP	0024
RST 5.5	002C
RST 6.5	0034
RST 7.5	003C

TRAP has given the highest priority , then RST 7.5, RST 6.5, RST 5.5. Finally, INTRis having lowest priority. 

👉Active Low Signal RESET IN ( Input ):

It resets the program counter to zero. It also resets interrupt enable and HLDA flip-flop.

👉RESET OUT (Output):

It indicates that the CPU is being reset.

👉X_1, X₂ (Input):

These are terminal to be connected to the external crystal oscillator which drives an internal ciculating of the micrprocessr.

👉CLK (output):

It is a clock output for user which can be used for other digital ICs. Its frequency is same at which processor operates.

👉SID (Input):

It is a data-line for serial input. The data on this line is loaded into the seventh bit of the accumulator, when RIM instruction us executed.

👉SOD (output):

It is a data-line for serial output. The seventh bit of accumulator on SOD line when SID instruction is executed. 

👉V_{ss :}

_{The pin is ground pin.} 

👉V_cc 

+5V supply is given to the microprocessor through this pin.