Pin Diagram of 8085
Functions of Pins
The signals from the pins can be grouped as follows
Power supply and Clock Signals
Vss: Ground Signal
X1,X2: Crystal or R/C network or LC network connections to set the frequency of internal clock generator.
CLK OUT: Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.
PINs from A8 - A15.
It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address
PINs from AD0 - AD7.
They are multiplexed pins.
These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.
Control and Status Signal
ALE (Address Latch Enable) : This signal helps to capture the lower order address presented on the multiplexed address / data bus. When the Signal is 1 it indicates AD0-AD7 as address else data.
RD : Indicates that the selected data from memory or I/O is to be read and data bus is ready for accepting data.
WR : indicates that the data on the data bus is to be written into the selected memory location or I/O device.
Inerrupt and externally initiated signals
INTA : Interrupt Acknowledgment signal.
RESET IN : Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA Flip-flops and makes address, data and control lines tri-stated.
RESET OUT : Active hig signal used to indicate that CPU is being reset.
Also it can be used as system reset.
READY : This indicates that CPU is ready to send or receive data.
HOLD : indicates that another Master is requesting the use of the Address and Data Buses.
HLDA : indicates that the CPU has received the Hold request and that it will withdraw the buses in the next clock cycle
Serial I/O signals
SID(Serial Input Data) : Active high, the data is loaded into accumulator whenever a RIM instruction is executed.
SOD(Serial Output Data) : Active high, The output SOD is set/reset as specified by the SIM instruction.