Pin Description

Pin Diagram of 8085 Microprocessor and its description is as follows:-

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Pin-wise description of 8085 is as follows-

Pin No	Pin Name	Description
1,2	X1-X2	A crystal (or RC, LC network) is connected to these two pins. The frequency is internally divided by two; therefore to operate a system at 3 MHz, the crystal should have frequency of 6 Mhz.
3	RESET OUT	This is an active high output signal used to indicate that the microprocessor is reset. This signal is used to reset other devices in system.
4	SOD	Serial Output Data. This is an active high, serial output port pin, used to transfer serial 1 bit data under software control.
5	SID	Serial Input Data. This is an active high, serial input port pin, used to accept serial 1 bit data under software control.
6	TRAP	This is a non-maskable interrupt and has highest priority.
7	RST 7.5	Pin 7-9 are vectored interrupt that transfer the program control to specific memory location. They have higher priority than the INTR interrupt.
8	RST 6.5
9	RST 5.5
10	INTR	Interrupt request. General purpose interrupt.
11	INTA	Interrupt acknowledgement. This is used to acknowledge interrupt.
12-19	AD0-AD7	These pins are multiplexed to be used as address bus as well as data bus.These are signal lines which are bidirectional & they serve dual purpose. They are used as lower order address bus as well as data bus.
20	VSS	It is ground reference.
21-28	A8-A15	These 8 signal lines are unidirectional and used for most significant bits called higher order address bus of a 16-bit address.
29,33	S0	These are status signal, similar to IO/M¯, can identify various operations. IO/M S1 S0 Machine Cycle 0 1 1 Opcode fetch 0 1 0 Memory read 0 0 1 Memory write 1 1 0 I/O Read 1 0 1 I/O Write 1 1 1 Interrupt Acknowledgement
30	ALE	Address Latch Enable ALE signal is used to separate AD0-AD7 ie demultiplex ALE=1 indicates that contents are address. ALE=0 indicates that the contents are data.
31	WR (active low)	Write. This is a Write control signal (active low). This signal indicates that the data on the data bus are to be written into selected memory or I/O location.
32	RD (active low)	Read. This is a Read control signal (active low). This signal indicates that the selected I/O or memory device is to be read and data are available on data bus.
34	IO/M	This is a status signal used to differentiate between I/O and memory location. When it is high. It indicates an I/O operation; when it is low, it indicates a memory operation.
35	READY	If the signal at READY pin is low, the microprocessor enters in to wait state. This signal is primarily used to synchronize slower peripherals with the microprocessor.
36	RESET IN	When the reset pin is activated by an external key all the internal operations are suspended and the program counter is cleared and the program execution begins at zero memory address.
37	CLK OUT	Clock Output. This signal can be used as the system clock for other devices.
38	HLDA	Hold acknowledgement. This pin is used for hold acknowledgement.
39	HOLD	When HOLD pin is activated by an external signal the microprocessor relinquishes control of buses and allows the external pheripheral to use them.
40	VCC	+5V power supply.