Microprocessor 8085 pin diagram

In this article I am going to discuss about microprocessor 8085 pin diagram and their functions.

Microprocessor 8085 pin diagram is shown below

Pin	Name	Pin	Name

1	X1	21	A₈
2	X2	22	A₉
3	Reset Out	23	A₁₀
4	SOD	24	A₁₁
5	SID	25	A₁₂
6	TRAP	26	A₁₃
7	RST 7.5	27	A₁₄
8	RST 6.5	28	A₁₅
9	RST 5.5	29	S₀
10	INTR	30	ALE
11	INTA	31	(WR)’
12	AD₀	32	(RD)’
13	AD₁	33	S₁
14	AD₂	34	IO/M̅
15	AD₃	35	Ready
16	AD₄	36	(Reset In)’
17	AD₅	37	CLK (out)
18	AD₆	38	HLDA
19	AD₇	39	HOLD
20	Vss	40	V_CC

The pins of 8085 microprocessor can be categorised into following groups –

Clock Signals

The 8085 microprocessor has three clock signal pins that is X₁, X₂, CLK OUT.

X₁, X₂ – These two pins are connected to a crystal ( RC, LC N/W ). The frequency of internal clock generator is set by using these pins.
CLK OUT – The devices connected with the microprocessor use this signal as a system clock.

Serial I/O Signal

The 8085 microprocessor has two serial signals known as SOD & SID. They are used for serial communication.

SID ( Serial Input Data ) – the accumulator gets loaded by data on this line when a RIM instruction is executed.
SOD ( Serial output Data ) – The serial output data is set / reset as instructed by SIM.

Interrupts

The microprocessor 8085 has five interrupt signals.

They are – TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR.

Externally Initiated Signal

INTA – Interrupt acknowledgement signal.

(RESET IN)’ – This signal reset the microprocessor. It sets the program counter to zero.

RESET OUT – This signal resets all the connected devices.

READY – This signal indicates that the microprocessor is ready to send and receive data. When ready is low, CPU waits for it to go high.

HOLD – This signal indicates that some other device is waiting / requesting to use address and data busses.

HLDA – HOLD Acknowledge. This signal indicates that the CPU has received the request made by hold and it will relinquish it in next clock cycle.

Data Bus

Pin 12 to pin 19 is data bus that is AD₀ to AD₇. It carries 8 bit least significant address and data bus.

Address Bus

Pin 21 to pin 28 is address bus that is A₈ to A₁₅. It carries 8 bit most significant memory I/O address.

Power Supply

The 8085 microprocessor has 2 power supply pins. Pin 40 ( V_CC ) indicates +5V power supply and pin 20 ( V_SS ) indicates ground signal.

Control and Status Signal

There are three control signal and three status signal.

The control signals are RD, WR & ALE.

(RD)’ – This signal controls the read operation. When this signal is low, the selected memory or IO device is ready to read.

(WR)’ – This signal controls the write operation. When this signal is low, the data on the data bus is written in the selected memory input / output location.

ALE – It stands for Address Latch Enable signal. This signal is a positive going pulse generated when microprocessor starts a new operation. The high pulse indicates address and the low pulse indicates data.

The three status signal are IO/M, S₁ & S₀.

IO/M

This signal is used by the microprocessor to differentiate between IO ( Input Output ) and memory operations. When this signal is high it indicates IO operation and when this signal is low it indicates memory operation.

S₁ & S₀

S₁ & S₀ signals are used to identify the type of current operation.

Author

Akash Sharma

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