Microprocessor and Application (8085)

Theory and Application of
Microprocessor 8085

Theory and Application of
Microprocessor
By: Sundas Shabbir

Recommended books
1. Theory and problems of microprocessor
fundamentals by schaum outline series.
2. The intel microprocessors by Barry B.
brey & C. R. Sarma
3. Other sources
– internet

Introduction
• Brain of the computer
• silicon chip that contains a CPU
• First microprocessors emerged in the early 1970.
• A microprocessor is designed to perform
arithmetic and logic operations.
• It is the most expensive component found inside
the system.
• Intel is world leader in Microprocessor.

Definition
• A microprocessor may be briefly defined as a
(VLSI) chip that performs a task of the central
processing unit of a microcomputer or other
automatic control systems.

Definition (2)
• Microprocessor is multipurpose, programmable logic
device that reads the binary instruction from storage
device called memory, accepts the binary data as input
and processes data according to those instructions, and
provides the results as output.

• Microprocessor is broadly divided into three parts.
• Arithmetic logic Unit :
– Arithmetic logic unit, the part of a computer that performs all arithmetic
computations, such as addition and multiplication, and all comparison
operations. The ALU is one component of the CPU .
• Registers:
– A register is one of a small set of data holding places that are part of a
computer processor . A register may hold a computer instruction , a
storage address, or any kind of data.
• Control Unit :
– The control unit is the circuitry that controls the flow of information
through the processor, and coordinates the activities of the other units
within it.

Common microprocessor
characteristics
• Power connections: microprocessors require a 5-v Dc regulated
power supply.
• Bit size: the bit size is sometimes referred to as its word size
commonly classified as 4, 8, 16 or 32 bit units.
• Data lines: Mp transfer data or instructions b/w the MPU and
memory or I/O via a bidirectional data bus.
• Address lines: older processors use 16 bit address buses, and
newer MPUs have 16, 20 or more bits in an address bus.

• Control lines: communicate with ALU and memory by
generating control signals, e.g
– R/W lines
– I/O lines
– interrupt lines.
• Internal registers: Registers work under the direction of the
control unit to accept, hold, and transfer instructions or data and
perform arithmetic or logical comparisons at high speed. Few of
the registers are :
– Program counter
– Accumulator
– Flag register
– General purpose register
– Stack pointer

• Addressing modes: a technique used to fetch the desired operand
during the execution of an instruction. Following are the few
types of modes:
– Register addressing mode
– Register indirect addressing mode
– Immediate addressing mode
– Inherent addressing
– Index addressing mode

Evolution of Intel Mp
• 4004
• 8008
• 8080
• 8085
• 8086/8088
• 80186
• 80286
• 80386
• 80486
• Pentium
• Pentium pro
• P-11
• P-111
• P-4 and so on..
Find the specifications of these
processors???

What to study
• when learning about a new microprocessor , one must
study the following:
– Microprocessor architecture
– Pin functions
– Control signals
– Addressing modes
– Instruction set
• The Mp we will discuss :
– 8085
– 8086
– 6800
– 6502 and many other acc. To availability of time 

8085 INTRODUCTION
The features of INTEL 8085 are :
• It is an 8 bit processor.
• It is a single chip N-MOS device with 40 pins.
• It has multiplexed address and data bus.(AD0-AD7).
• It works on 5 Volt dc power supply.
• The maximum clock frequency is 3 MHz while
minimum frequency is 500kHz.
• It provides 74 instructions with 5 different addressing
modes.

8085 INTRODUCTION
• It provides 16 address lines so it can access 2^16 =64K
bytes of memory.
• It generates 8 bit I/O address so it can access 2^8=256
input ports.
• It provides 5 hardware interrupts:TRAP, RST 5.5, RST
6.5, RST 7.5,INTR.
• It provides Acc ,one flag register ,6 general purpose
registers and two special purpose registers(SP,PC).
• It provides serial lines SID ,SOD.So serial peripherals
can be interfaced with 8085 directly.

Microprocessor and Application (8085)

• It has 40 pins and uses +5V for power. It can run at a
maximum frequency of 3 MHz.
• The pins on the chip can be grouped into 6 groups:
1. Power supply and frequency.
2. Serial I/O ports.
3. Address Bus.
4. Data Bus.
5. Control and Status Signals.
6. Externally Initiated Signals.
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Power supply and Frequency signals
• The microprocessor operates on a singles +5V power supply
connected to Vcc at pin number 40.
• The ground is connected to Vss at pin number 20.
• There are 3 pin in the frequency control group 1. X1 2. X2.
3. CLK OUT.
(i) X1&X2: Pin number 1&2 are the inputs from the clock
generating circuit. It is use to synchronize the operations of
the 8085 microprocessor.
• The microprocessor has operates at 3MHz frequency.
(ii) CLK OUT: Pin number 37, It is an output pin. This signal is
generated by microprocessor. It can be used as the system
clock for other devices.

Clock circuit

Serial I/O ports
• These signals are used for giving serial input and
output data.
• There are used two pin for serial I/O data .
1. SID: Serial Input Data. Pin number 5
This pin provides serial input data. The serial data on
this pin is loaded into the seventh bit of the
accumulator.
2. SOD: Serial Out Data Pin number 4
This pin provides the serial output data. The serial data
on this pin delivers its output from the seventh bit of
the accumulator.

Address Bus and Data Bus
• 8085 μp consists of 16 signal pins use as address bus.
• Address bus is available from pin number 21 to 28.
• Divide into 2 part: A15 – A8 (upper) and AD7 – AD0
(lower).
– A15 – A8 : Unidirectional, known as ‘high order
address’ bits.
– AD7 – AD0 : bidirectional and dual purpose pins
also known as ‘low order address’ bits.
– To execute an instruction, at early stage AD7 – AD0
uses as address bus and alternately as data bus for
the next cycle.
– The method to change from address bus to data bus
known as ‘bus multiplexingbus multiplexing’.

• The 8 address bits are multiplexed (time shared) with
the 8 data bits. AD0 to AD7
• Pin number 12 to 19 are used for data bus in
microprocessor.
• The data bus is a bidirectional bus. The data bus can
transfer the data from CPU to memory or vice versa.
The data bus also connects the I/O ports and
microprocessor.

Control and Status Signals
• There are 4 main control and status signals.
1. ALE: Address Latch Enable: Pin number 30.
• 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.

2. RD: Read. Active low
• Pin number 32
• The read control signal is used to control the reading
operations of microprocessor.
• indicate that the I/O or memory selected is to be read
and data are available on the bus.

3. WR: Write. Active low.
• Pin number 31
• The write control signal is same as read signal.
• Indicate that the data available on the bus are to be
written to memory or I/O ports.

4. IO/M: Input-output/Memory
• Pin number 34
• This signal specifies whether the operation is a
memory operation (IO/M=0) or an I/O operation
(IO/M=1).

• S1 and S0:
• Pin number 33&29
• S1 & S0 are two status signals.
• These signals are used to indicate the internal operation
of microprocessor.
• Status signals to specify the kind of operation being
performed .

31
Control and Status Signals.

Externally initiated and acknowledge signals
• These are total 11 pins in this group:
1. RESET IN
2. RESET OUT
3. READY
4. HOLD
5. HLDA
6. Types of INTERRUPT

1. RESET
• RESET IN: This is an input signal. Pin number-36
• When the signal on this chip goes low, the program
counter is set to zero.
• RESET OUT: Pin number-3
• This signal is generated by microprocessor in response
of the signal RESET IN when RESET IN is logic 0,
RESET OUT is logic 1,This signal show that
microprocessor in reset.

2. READY
• Pin number-35
• If Ready is high during a read or write cycle, it show that the
memory or peripheral is ready to send or receiving the data.
• If Ready is low, the CPU wait for Ready to go high before
completing the read or write cycle.
• A peripheral may be a LCD display or analog to digital converter
or any other. These peripherals are connected to microprocessor
using the READY pin. If READY is high then the peripheral is
ready for data transfer.

3. HOLD & HLDA
• HOLD:
• Pin number-39
• is an active high signals.
• It means other device is requesting for DMA
operations.

• HOLD:
• This indicates if any other device is requesting the use
of address and data bus. Consider two peripheral
devices. One is the LCD and the other Analog to
Digital converter. Suppose if analog to digital
converter is using the address and data bus and if LCD
requests the use of address and data bus by giving
HOLD signal, then the microprocessor transfers the
control to the LCD as soon as the current cycle is over.
After the LCD process is over, the control is
transferred back to analog and digital converter.

• DMA is a method of transferring data from the
computer's RAM to another part of the computer
without processing it using the CPU.
• DMA can save processing time and is a more efficient
way to move data from the computer's memory to other
devices.
• In order for devices to use direct memory access, they
must be assigned to a DMA channel. Each type of port
on a computer has a set of DMA channels that can be
assigned to each connected device. For example, a PCI
controller and a hard drive controller each have their
own set of DMA channels.

• HLDA: Pin number-38
• It is active high signal.
• This signal acknowledges the HOLD request.
• It’s inactive by the microprocessor after the I/O device
has completed the DMA operation.

Microprocessor and Application (8085)

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