8085 microprocessor

UNIT – I
8085 MICROPROCESSOR
PREPARED BY,
MANIKANDAN S
AP / EEE
JCT CET
JCT College of Engg & Tech, Coimbatore

CHAPTERS
 Hardware Architecture
 Pin outs
 Functional Building Blocks of Processor
 Memory organization
 I/O ports
 Data transfer concepts
 Timing Diagram
 Interrupts.

HARDWARE ARCHITECTURE

CPU INTERNAL STRUCTURE
Registers
 Six general purpose 8-bit registers: B, C, D, E, H, L.
 They can also be combined as register pairs to Perform 16-
bit operations: BC, DE, HL.
 Registers are programmable (data load, move, etc.)
 Accumulator Single 8-bit register that is part of the ALU !
 Used for arithmetic / logic operations – the result is always
stored in the accumulator.

Program Counter (PC)
 This is a register that is used to control the sequencing of
the execution of instructions.
 This register always holds the address of the next
instruction.
 Since it holds an address, it must be 16 bits wide.
Stack Pointer (SP)
 The stack pointer is also a 16-bit register that is used to
point into memory.
 The memory this register points to is a special area called
the stack.
 The stack is an area of memory used to hold data that will
be retreived soon.
 The stack is usually accessed in a Last In First Out (LIFO)
fashion.

ARITHMETIC AND LOGIC
UNIT
 In addition to the arithmetic & logic circuits, the ALU includes
the accumulator, which is part of every arithmetic & logic
operation.
 Also, the ALU includes a temporary register used for holding
data temporarily during the execution of the operation. This
temporary register is not accessible by the programmer.

ACCUMULATOR
 A register in which intermediate arithmetic and logic results
are stored.
 Example for accumulator use is summing a list of numbers.
 The accumulator is initially set to zero, then each
number in turn is added to the value in the accumulator.
 Only when all numbers have been added is the result
held in the accumulator written to main memory or to
another, non-accumulator, CPU register.

DATA BUS
 The data bus is 'bi-directional'
 data or instruction codes from memory or Input/output
are transferred into the microprocessor
 the result of an operation or computation is sent out
from the microprocessor to the memory or input/output.
 Depending on the particular microprocessor, the data bus
can handle 8 bit or 16 bit data.

ADDRESS BUS
 The address bus is 'unidirectional', over which the
microprocessor sends an address code to the memory or
input/output.
 The size (width) of the address bus is specified by the
number of bits it can handle.
 The more bits there are in the address bus, the more
memory locations a microprocessor can access.
 A 16 bit address bus is capable of addressing 65,536 (64K)
addresses.

CONTROL BUS
• The control bus is used by the microprocessor to send out
or receive timing and control signals in order to coordinate
and regulate its operation and to communicate with other
devices, i.e. memory or input/output.

THE CONTROL AND STATUS
SIGNALS
 There are 4 main control and status signals. These are:
ALE: Address Latch Enable. This signal is a pulse that
become 1 when the AD0 – AD7 lines have an
address on them. It becomes 0 after that. This signal
can be used to enable a latch to save the address bits
from the AD lines.
RD: Read. Active low.
WR: Write. Active low.
IO/M: This signal specifies whether the operation is
a memory operation (IO/M=0) or an I/O operation
(IO/M=1).
S1 and S0 : Status signals to specify the kind of
operation being performed .Usually un-used in small
systems.

FREQUENCY CONTROL
SIGNALS
 There are 3 important pins in the frequency control group.
 X0 and X1 are the inputs from the crystal or clock
generating circuit.
 The frequency is internally divided by 2.
So, to run the microprocessor at 3 MHz, a clock
running at 6 MHz should be connected to the X0
and X1 pins.
 CLK (OUT): An output clock pin to drive the clock of the
rest of the system.

FLAG REGISTER

PIN DESCRIPTION
 8085 is a 40 pin IC, The signals from the pins can be
grouped as follows
 1. Power supply and clock signals
2. Address bus
3. Data bus
4. Control and status signals
5. Interrupts and externally initiated signals
6. Serial I/O ports

POWER SUPPLY AND CLOCK
FREQUENCY SIGNALS
 Vcc: + 5 volt power supply
 Vss: Ground.
 X1, X2 : Crystal or R/C network or LC network connections to
set the frequency of internal clock generator. The frequency is
internally divided by two. Since the basic operating timing
frequency is 3 MHz, a 6 MHz crystal is connected externally.
CLK (output)-Clock Output is used as the system clock for
peripheral and devices interfaced with the microprocessor.

ADDRESS BUS
 A8 - A15: (output; 3-state) It carries the most significant 8 bits of the
memory address or the 8 bits of the I/O address.
DATA BUS
• AD0 - AD7 (input/output; 3-state) These multiplexed set of lines used to carry
the lower order 8 bit address as well as data bus.
• During the op code fetch operation, in the first clock cycle, the lines deliver the
lower order address A0 - A7.
• In the subsequent IO / memory, read / write clock cycle the lines are used as
data bus.
• The CPU may read or write out data through these lines.

CONTROL AND STATUS
SIGNALS
ALE (output) - Address Latch Enable.
 It is an output signal used to give information of AD0-AD7
contents.
 It is a positive going pulse generated when a new operation
is started by uP.
 When pulse goes high it indicates that AD0-AD7 are
address.
 When it is low it indicates that the contents are data.
RD (output 3-state, active low)
 Read memory or IO device.
 This indicates that the selected memory location or I/O
device is to be read and that the data bus is ready for
accepting data from the memory or I/O device

WR (Write)
 Write memory or IO device.
 This indicates that the data on the data bus is to be written
into the selected memory location or I/O device.
IO/M (output) - Select memory or an IO device
 This status signal indicates that the read / write operation
relates to whether the memory or I/O device.
 It goes high to indicate an I/O operation.
 It goes low for memory operations.

STATUS SIGNALS

INTERRUPTS AND EXTERNALLY
INITIATED OPERATIONS
 They are the signals initiated by an external device to request
the microprocessor to do a particular task or work.
 There are five hardware interrupts called,
TRAP
RST 7.5
RST 6.5
RST 5.5
INTA
 On receipt of an interrupt, the microprocessor acknowledges
the interrupt by the active low INTA (Interrupt Acknowledge)
signal.

Reset In (Input)
 This signal is used to reset the microprocessor.
 The program counter inside the microprocessor is
set to zero.
 The buses are tri-stated.
Reset Out (Output)
 It indicates CPU is being reset.
 Used to reset all the connected devices when the
microprocessor is reset.

DIRECT MEMORY ACCESS
(DMA)
 When 2 or more devices are connected to a common bus,
to prevent the devices from interfering with each other, the
tri state gates are used to disconnect all devices except the
one that is communicating at a given instant.
 The CPU controls the data transfer operation between
memory and I/O device. Direct Memory Access operation is
used for large volume data transfer between memory and
an I/O device directly.
 The CPU is disabled by tri-stating its buses and the transfer
is effected directly by external control circuits.

 HOLD signal is generated by the DMA controller circuit. On
receipt of this signal, the microprocessor acknowledges the
request by sending out
 HLDA signal and leaves out the control of the buses. After
the HLDA signal the DMA controller starts the direct
transfer of data.
READY (input)
 Memory and I/O devices will have slower response
compared to microprocessors.
 Before completing the present job such a slow peripheral
may not be able to handle further data or control signal
from CPU.
 The processor sets the READY signal after completing the
present job to access the data.
 The microprocessor enters into WAIT state while the READY
pin is disabled.

SINGLE BIT SERIAL I/O
PORTS
 SID (input) Serial input data line
 SOD (output) Serial output data line
 These signals are used for serial communication.

8085 microprocessor

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