8085 is a general purpose microprocessor having 40 pins and works on single power supply. To study the pin diagram we group
the signals into 5 categories:
1.Power Supply
2.Clock Signals
3.Interrupt Signals
4.Address and Data bus
5.Control and Status signals
6.Serial I/O Port
7.DMA Request Signals
Power Supply Signal and clock signal:
VCC:-Vcc is to be connected to +5V power supply.
Vss:-Ground reference
X1 and X2:- This pin is used for providing the clock frequency to the microprocessor. Generally Crystal oscillator or LC oscillator is used to generate the frequency. The frequency generated here is internally divided into two.As we know that the basic operating timing frequency of the microprocessor is 3 MHz so
6 MHz frquency is applied.
Serial Input Output port:-
SID and SOD:-These pins are used for serial data communication.
Interrupt Signal:-
Pin 6 to 11:- These pins are used for interrupt signals. Generally and external devices are connected here which requests the microprocessor to perform a particular task.
There are 5 pins for hardware interrupts-
TRAP, RST7.5, RST 6.5, RST5.5 and INTR
INTA is used for acknowledgement. Microprocessor sends the acknowledgement to external devices through the INTA pin.
Address Bus and DATA Buses:-
AD0-AD7:-These are multiplexed address and data bus. So it can be used to carry the lower order 8 bit address as well as the data. Generally these lines are demultiplexed using the Latch.
During the opcode fetch operation, in the first clock cycle the lines deliever the lower order address bus A0-A7.
In the subsequent IO/M read or write it is used as data bus D0-D7. CPU can read or write data through these lines.
A8-A15:- These are address bus used to address the memory location.
Control And Status Signal:-
S0 and S1:-It is used for the status signal in microprocessor.
ALE(Airthmetic Latch Enable):-This signal is used to capture the lower address presented on multiplexed address and data bus.
RD:-This is active low input generally
used for reading operation.
WR:-This is active low input used for writing operation.
IO/M:-This pin is used to select the memory or input-output through which we want to communicate the data.
READY:-As we know that memory and input -output have slower response than microprocessor. So a microprocessor may now be able to handle further data till it completes the present job. So it is in waiting state. As it completes the present job it sets the READY pin. Microprocessor enters into wait state while READY pin is disabled.
RESET IN:-This is active low input. This pin is used to reset the microprocessor. An active low signal applied to this pin reset the program counter inside the microprocessor. The busses are tristated.
RESETOUT:-If we want to reset the external devices connected to the microprocessor then a signal applied to this pin resets the external devices.
DMA Request Signal:
HOLD and HLDA:-HOLD is an active high input signal used by the other controller to request microprocessor about use of address, data and control signal. The HOLD and HLDA signal are used for direct memory access(DMA). DMA controller receives a requests from a device and in turn issues the HOLD signal to the microprocessor.
The processor releases the system bus and then acknowledges the HOLD signal with HLDA signal. The DMA transfer thus begins.
