8085 pin functions

8085 pin functions.

the 8085 pin configuration and functional pin diagram of 8085 respectivly.
the signals of 8085 can be classified in to seven groups according to their functions.

1. power supply and frequency signals.

2. data bus and address bus.

3. contol bus.

4. interrupt signals.

5. serial input output signals.

6. DMA signals.

7. reset signals.

1. Power Supply and Frequency Signals

i)Vcc: it requires a single+5 V power supply.

ii)Vss: ground reference.

iii)X1 and X2(Input):

A tuned circuit like LC, RC or crystal is connected at theset two pins.The internal clock generator divides oscillator frequency by 2,therefore, to operate a system at 3 MHz, the crystal of the tuned circuit must have a frequency of 6 MHz.

iv)CLK OUT(Output):

This signal is used as a system clock for other devices.Its frequency is half the oscillator frequency.

2. Data Bus and Address Bus

A)ADo to AD7(Input/Output):

The 8-bit data bus(D0-D7)is multiplexed with a lower half(A0-A7)of the 16-bit address bus. During first part of the machine cycle

(T1), smaller 8 bits of memory address or I/O address appear on the bus. During remaining part of the machine cycle(T2 and T3) these lines are used as a bi-directional data bus.

B)A7 to A15(Output):

Thus The upper half of the 16-bit address appears on the address lines.

This lines are exclusively used for the most significant of the 16-bit address lines.

3 Control and Status Signals

A)ALE(Address Latch Enable)(Output):

We know that the AD0 to AD7 lines are multiplexed and lower half of the address(A0-A7)is available only during T1, of the machine cycle.These lower half of address is also necessary during T2 and T3 of machine cycle to access specific location in memory or I/O port.This means that the lower half of the address be latched in T1 of the machine cycle, so that it is available throughout the machine cycle.The latching of lower half of the address bus is done by using external latch and ALE signal from 8085.

B)RD and WR (Output):

The low on RD indicates that the data must be read from the selected memory location or Input output port via data bus. the low on WR indicates that the data must be written into the selected memory location or I/O port via data bus.

C)IO/M, S0 and S1(Output):

IO/M indicates whether I/O operation or memory operation it is carried out. S1 and S0 indicate that the type of machine cycle in progress.

D)READY(Input):

It is used by the microprocessor to sense whether a peripheral is ready or not for data transfer.,If not, the processor waits. It is used to synchronize slower peripherals to the microprocessor.

4 Interrupt Signals

Hardware Interrupts (Input):
The 8085 have a five hardware interrupt signals: RST 5.5, RST 6.5 ,RST 7.5, TRAP and INTR. The microprocessor recognizes interrupt requests on this lines at the end of the current instruction execution.

INTA(Output):
The INTA(Interrupt Acknowledge) signal is used to indicate that the processor has acknowledged an INTR interrupt.

5 Serial I/O Signals

A)SID (Serial I/P Data)(input):
This input signal is used to accept serial data bit from the external device.

B)SOD(Serial O/P Data)(Output):
These is an output signal which enables the
transmission of the serial data bit by bit to the external device.

6. DMA signal

A)HOLD(Input):
This signal is indicate that the another master is requesting for the use of address bus,data bus and control bus.

B)HLDA(Output):
Thus the active high signal is used to acknowledge HOLD request.

7. Reset Signals

A)RESET IN(Input):A low on this pin
1)Sets the program counter to zero(0000H)and clears the INTE fiag.
2)Resets the interrupt enable and HLDA flip-flops.
3)Tri-states the data bus,address bus and control bus.
(Note:Only during RESET is active).
4)Affects the contents of processor's internal registers randomly.

B)RESET OUT(Output):This active high signal indicates that processor is being
reset.These signal is synchronized to the processor clock and it can be used to reset other devices connected in the system.