8085 microprocessor Architecture and pin discription

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Intel 8085
Microprocessor
Architecture, Features & Signals
Introduction
It is invented in 1976.
It is an 8-bit microprocessor.
It has 16 address lines, therefore it can access 2^16 = 64K bytes of memory.
It has 8 Data lines and 16 address lines, out of which 8 address lines are
multiplexed with 8 data lines.
It provides one Accumulator, one Flag register, 6 General purpose registers(B, C,
D, E, H and L) and two special purpose registers (PC, SP).
The clock frequency of 8085 microprocessor is 3MHz.
8085 microprocessor provides on chip clock generator, therefore there is no
need of external clock generator, but it requires external tuned circuit like LC,
RC or crystal.
It is available in 40 pin dual in line (DIP) package.
It requires a +5volts of power supply.

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It generates 8 bit I/O address, hence it can access 2^8 = 256 input ports and
256 output ports.
8085 has five hardware interrupts: TRAP, RST 5.5, RST 6.5, RST 7.5, and INTR.
The hardware interrupt capability of 8085 microprocessor can be increased by
providing external hardware.
8085 microprocessor has capability to share its bus with external bus
controller (Direct Memory Access controller); for transferring large amount of
data from memory to I/O and vice versa.
It provides two serial I/O lines which are SOD and SID; it means, serial
peripherals can be interfaced with 8085 microprocessor directly.
Pin diagram of 8085 Microprocessor

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Pin Details of 8085 Microprocessor
Address & data lines:
Address bus:
8085 has 16-bit address bus AD0-AD7 and A8-A15. In this lower
address bus is multiplexed with data bus. A8-A15 lines are
unidirectional and AD0-AD7 lines are bidirectional.
Data bus:
AD7-AD0 is 8-bit bidirectional data bus. It is multiplexed with
lower order address bus.
ALE:
Address latch enable. It is used to de-multiplex AD0-AD7. It is
connected to strobe input of latch which is used to separate
address and data bus lines. It is issued in first T-state.
Address & data lines:
Address bus:
8085 has 16-bit address bus AD0-AD7 and A8-A15. In this lower
address bus is multiplexed with data bus. A8-A15 lines are
unidirectional and AD0-AD7 lines are bidirectional.
Data bus:
AD7-AD0 is 8-bit bidirectional data bus. It is multiplexed with
lower order address bus.
ALE:
Address latch enable. It is used to de-multiplex AD0-AD7. It is
connected to strobe input of latch which is used to separate
address and data bus lines. It is issued in first T-state.
Continue…

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Control & Status Signals:
RD: Read control signal is issued to memory or IO device to read
data from it.
WR: Write control signal is issued to memory or IO device to write
data into it.
IO/M: It is a signal which is used to distinguish between IO
operation and memory operation. It is also used in generating
memory and IO, read and write control signals.
Continue…
S1,S0: These are status signals. Depending
on the value on these lines, the type of
operation being performed by the
processor can be determined. Below table
shows that information.
Continue…
Power Supply and clock signals:
Vcc: +5v power supply line
Vss: Electrical ground signal.
X1, X2: Crystal is connected between these pins. The frequency is
internally divided by 2. The systems operate generally at 3MHz.
Hence 6 MHz clock signal needs to be connected between X1, X2
lines.
CLK (out): It is the clock output signal from processor, which can be
used to clock other peripherals in the microprocessor based system.
READY: This is used when the processor is reading or writing data to
a slow peripheral. When this signal goes low processor inserts wait
states, until it goes high.

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Reset Signals:
RESET IN: When low signal is applied on this pin, 8085
resets and the microprocessor boots from 0000h location
in memory i.e. PC is loaded with 0000h location.
RESET OUT: When processor is reset, this signal goes high.
This pin is connected to reset input of other peripherals. So
when processor is reset, other peripherals are also reset.
Serial IO lines:
SID: serial input data, used to receive serial data.
SOD: serial output data, used to send serial data
Continue…
Interrupt Signals:
INTR: interrupt request is general purpose interrupt signal. The interrupting
device needs to send the vector address also.
INTA : is interrupt acknowledging signal. This signal indicates that processor has
accepted the interrupt.
RST7.5, RST6.5, RST5.5: These are external vectored interrupts. When these
interrupt occurs, processor vectors to a specific location.
TRAP: It is a non-mask able interrupt.
DMA signals:
HOLD: This line is used by DMA controller to request microprocessor for system
bus. When this line goes high microprocessor completes its current bus cycle and
issues system bus to DMA controller.
HLDA: HOLD acknowledging signal. Processor acknowledges DMA request using
this signal.

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Architecture of 8085 Microprocessor
Fig:- Architecture of 8085 microprocessor
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The following are the different blocks in the 8085 processor.
ALU:
It is 8-bit ALU. It can perform arithmetic and logical operations on 8-bit
data. If an operation needs to be performed on 16-bit data, it needs to be broken
into two 8-bit parts and each 8-bit operation should be performed on each 8-bit
data. It takes operand inputs from accumulator and a temporary register. Result
of the operation is stored in accumulator. Depending on the result of operation,
flags in flag register values will be changed.
Accumulator
It is a 8-bit register which is used to perform airthmetical and logical
operation. It stores the output of any operation. It also works as registers for i/o
accesses.
Temporary Register
It is a 8-bit register which is used to hold the data on which the
acumulator is computing operation. It is also called as operand register because it
provides operands to ALU.

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Flag register contains five flags, namely S, Z, CY, AC, P flags.
8085 has two 16- bit register PC and SP. PC always consists of address of next
instruction to be executed. SP always points to top of stack. i.e. address of top
memory location of stack. Stack is a data structure. It is used to store return
addresses whenever call to subprograms or an interrupt occurs.
Two temporary registers W, Z are also present. These are used to hold
temporary results during execution. But these are not accessible to the user.
Incrementer and decrementer address latch is for incrementing the PC content
for every fetch cycle.
Register Array:
8085 has six general purpose registers B, C, D,
E, H, L. They can be used as pairs to hold 16-bit
data as BC, DE, HL.
Accumulator is 8-bit register which holds the
results of operations as well as operand on which
some operation needs to be performed.
Continue…
Sign flag (S): when the result of ALU operation is negative sign flag is set. If the
result is positive, then sign flag is reset. i. e. the D7 bit of accumulator is copied into
the sign flag, as D7 anyhow contains sign.
Zero flag (Z): when the result of ALU operation is zero, Zero flag is set. If the result
is non-zero then flag is reset.
Auxiliary carry (AC): If an ALU operation results in carry from lower nibble to
upper nibble (or) bit D3 to bit D4, Auxiliary flag is set. Else it is reset. This flag is
used in BCD arithmetic.
Parity flag (P): If the result contains even number of ones, the flag is set else it is
reset. So the parity flag is odd parity bit.
Carry flag (CY): If the arithmetic operation results in carry, CY flag is set, else it is
reset.
Flag register :

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Instruction Register and Decoding:
Instruction register holds instruction that is fetched from memory. Instruction decoder
decodes the opcode (which is part of fetched instruction present in instruction register).
Instruction register is not accessible to the programmer.
Interrupt Controller:
8085 has 5 external interrupts. TRAP, INTR, RST 5.5, RST 6.5, and RST 7.5. Whenever
processor gets interrupt it finishes current instruction execution and issues INTA (interrupt
acknowledge) signal to the peripheral which raised the interrupt and goes to execute
interrupt service routine. Interrupt controller controls the interrupts.
Serial I/O control:
Serial data can be sent out using SOD pin and serial data can be read from SID pin. It
controls serial IO related operations.
Program Counter
It is a 16 bit register used as memory pointer. It stores the memory address of the next
instruction to be executed. So we can say that this register is used to sequencing the
program. Generally the memory has 16 bit addresses so that it has 16 bit memory. The
program counter is set to 0000H.
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Stack Pointer
It is also a 16 bit register used as memory pointer. It points to the memory location
called stack. Generally stack is a reserved portion of memory where information
can be stores or taken back together.
Timing and Control Unit
It provides timing and control signal to the microprocessor to perform the various
operation. It has three control signal. It controls all external and internal circuits. It
operates with reference to clock signal. It synchronizes all the data transfers.
There are three control signal:
ALE- Arithmetic Latch Enable, It provides control signal to synchronize the
components of microprocessor.
RD- This is active low used for reading operation.
WR-This is active low used for writing operation.
There are three status signal used in microprocessor S0, S1 and IO/M. It
changes its status according the provided input to these pins.

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Thank You
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