8085 –Microprocessor Pin diagram
8085 is a microprocessor having 40 pins and works on single power supply
categories of signals are written as follows:
Power supply
Clock Signals
Interrupt Signal
Address and data bus
Control signal
Status signal
Serial i/o port
DMA(Direct Memory Access) Request signal
Other externally initiated signal
Description of pins:
1. Powersupply:
power supply simply give the microprocessor the required supply of power to operate the various operations (arithmetic and logical) as like other ICs we need Vcc and Vss which power is varies from +5v power supply to zero or we can say that ground respectively.
2. Clock Signal:
The X1 and X2 are clock signal pin they are designed for fetching
the clock signals to the microprocessor.
for giving the clock frequency to the microprocessor we use Crystal
oscillator or LC oscillator due to its stability respective to other
oscillator.-
3. Interrupt Signal:
Pin 6 to 11:- These are used for interrupt signals and also external devices are connected here which requests the microprocessor to perform a particular task.
There are different pins for hardware interrupts-
TRAP, RST7.5, RST 6.5, RST5.5 and INTR ,this pins are used for input.
INTA (active low output) is called Interrupt acknowledgement.It is an active low output signal,when microprocessor receives high INTR SIGNAL it goes low.
4. Address and data bus:
Address bus: These are the buses which are use to transfer the address in communication system of microprocessor.
In microprocessor we have 16 bit address bus from A0to A15.
Which are further divided into two different sets according to their use
Data bus: The data busses as the name suggested are use to to transfer data from one periphery to another according to the demand of data .

AD0-AD7:-These address and data bus are multiplexed.It can be used to carry
the lower order 8 bit address and data also. These lines are demultiplexed using the Latch.
When the opcode fetch operation occurs, in the first clock cycle then the lines deliver the lower order address bus A0-A7.
For IO/M read or write, data bus D0-D7 is used and also CPU can read or write data through these lines.
A8-A15:- These are address bus to address the memory location.
5. Control signal:
ALE – Address Latch Enable.
It is an active high signal and its goes high during first clock cycle.
It goes high it indicates that AD0-AD7 are address, otherwise contents are data
RD – Read memory
It is an active low control signal.
It goes low when read operation take place.
WR – Write memory or IO device
It is an active low control signal.
It goes low when write operation take place.
IO/M (output) – Select memory
This status signal indicates that the read or write operation relates to whether the memory or I/O device.
It goes high(1) to indicate an I/O operation.
It goes low(0) for memory operations.
6. Status signal:
S1 and S0 are two status signals which are use to specify the various current operation of the microprocessor . Which are primitively fitted in the microprocessor, as a previously written or stored data to perform various action on various set of command . For referencing see the below table .
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7. Serial I/O port:
There are two serial ports
1:SID(Serial input data line)
2: SOD(Serial output data line)
function of above mention are to simply perform
Serial data communication.
8.DMA (Direct Memory Access) Request signal:
DMA as name suggested is a direct memory access pin .
this technique is use for increasing rate of transferring of data.
In DMA controller transfer of data take place directly from primary memory to various peripherals.
HOLD signal indicates that some other device demands for the use of address and data bus
HLDA signal stands for the”hold acknowledge signal”. which is use for the acknowledgment of holding of signal by microprocessor .
9.Other externally initiated signal:
RESET IN (INPUT):
The Reset in signal is basically a active low signal, which is use to perform reseting the microprocessor for the given input of task by initiallising counter with zero.It is also use for reseting of interrupts enable flip-flop and instruction register.
RESET(OUTPUT):
The main purpose of reset signal is to set the processor to its initial state after completion of the given task . It is also use for resetting the other periphery of the system.
Ready(input):
Ready is a externally initiated signal . It work is to synchronize the input and output devices of the system with the microprocessor. Ready signal is invoke by the processor after the completion of present task to processes the data.