Microcomputer & Microprocessor A microcomputer is a small
computer, making use of microprocessor as the central processing
unit (CPU) and usually has a word length of 4, 8, or 16 bits. A
microprocessor is a digital electronic component with transistors
on a single semiconductor integrated circuit (IC). Microprocessors
have replaced conventional digital logic in a wide variety of
systems and, therefore, micro- computers are mostly used as
component of electronic systems. Microprocessor is processing
device of every computing device. It is like an artificial brain.
It needs to communicate with outer world. To communicate with
external world, Microprocessor make use of buses. A bus is a group
of conducting lines that carries data, address and control
signals.
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Digital Watches ATM Personal Computers Home Security &
Control devices Video games CD Players Calculators Examples of
devices with microprocessor
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Microprocessor 8085 A common way of categorizing
microprocessors is by the number of bits that their ALU can work at
a time. Microprocessor 8085 can read or write or perform arithmetic
and logical operations on 8-bit data at a time. The 8085 was one of
the first (1978) of the 8-bit Microprocessors where all the
processing elements for a computer were contained on a single chip.
Based on the 8080, the 8085's instruction set was almost identical
but the major changes were that the electronics required to design
an 8085 system were much simpler (one rather than 3 power supplies
and internal clock and bus logic). The 8085 is a typical of that
generation of 8-bit Microprocessor which was able to put all the
processing unit onto one chip but still requiring surrounding
supporting chips to create a complete system.
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Block Diagram / Architecture of Microprocessor 8085
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1.Bus A bus is a collection of conducting path which is used to
transfer signal from one functional unit to another functional
unit. The microprocessor 8085 has three types of buses. 1.Address
bus 2.Data bus 3.Control bus 2.ALU ALU stands for arithmetic and
logical unit. The ALU of microprocessor 8085 is 8-bit
microprocessor. The ALU is responsible to perform all arithmetic
and logical operation like addition, subtraction, comparison,
etc.
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Block Diagram / Architecture of Microprocessor 8085 3.Registers
i.Accumulator: An 8-bit register used in the arithmetic and logical
operations. It stores the result of the operation carried out by
the ALU. ii.General Purpose Register: B, C, D, E, H and L are 8-bit
general purpose registers used to hold data. These registers can
also be used for 16-bit operations in pairs. The default pairs are
BC, DE and HL.
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Block Diagram / Architecture of Microprocessor 8085 iii.Stack
Pointer (SP): A 16-bit register used to store the 16-bit address of
stack memory. It is used as a memory pointer. It points to a memory
location in R/W memory called stack. iv.Program Counter (PC): A
16-bit register which holds the address of the next instruction to
be executed. Consider that an instruction is being executed by
processor. As soon as the ALU finished executing the instruction,
the processor looks for the next instruction to be executed. So,
there is a necessity for holding the address of the next
instruction to be executed in order to save time. This is taken
care by the program counter. Microprocessor increments the program
whenever an instruction is being executed, so that the program
counter points to the memory address of the next instruction that
is going to be executed.
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Block Diagram / Architecture of Microprocessor 8085
v.Instruction Register (IR): It is an 8-bit register used to hold
current instruction which the microprocessor is executing.
vi.Address Latch: It increments/ decrements the address before sent
to the address buffer
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Block Diagram / Architecture of Microprocessor 8085 vii.Flag
Register: An 8-bit register which can store maximum 8-bit data. The
flags are mainly associated with arithmetic and logic operations. A
flag is actually a latch which can hold some bits of information.
It alerts the processor that some event has taken place. There are
5 types of flags: 1. Carry flag 2. Parity flag 3. Auxiliary flag 4.
Zero flag 5. Sign flag
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Flag Registers 1.Sign flag: Sign flag shows whether the output
of operation has positive sign or negative sign. A value 0 is
returned for positive sign and 1 is returned for negative sign.
2.Parity flag: If the result of the latest operation is having even
number of 1s, then this flag will be set. Otherwise this will be
reset to 0. This is used for error checking. 3.Auxiliary flag: This
flag is not accessible to programmer. This flag will be used by the
system during BCD operations. 4.Zero flag: Zero flag shows whether
the output of the operation is 0 or not. If the value of Zero flag
is 0 then the result of operation is not zero. If it is zero the
flag returns value 1. 5.Carry flag: If the result of the latest
operations exceeds 8-bits then this flag will be set. Otherwise it
be reset.
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Machine Cycles & Timing of 8085 The timing and control unit
generates timing signals for the execution of instruction and
control of peripheral devices. Timing Diagram: A graphical
representation. It represents the execution time taken by each
instruction in a graphical format. The execution time is
represented in T-states. Instruction Cycle: The time required to
execute an instruction is called an instruction cycle. Machine
Cycle: The time required to access the memory or input/output
devices is called machine cycle. T-State: The machine cycle and
instruction cycle takes multiple clock periods. A portion of an
operation carried out in one system clock period is called as
T-State.
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Machine Cycles & Timing of 8085
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A bus is a collection of conducting path which are used to
transfer signal from one unit to another. The microprocessor 8085
has the following types of buses. They are : address bus, data bus
and control bus 8085 System Bus Address bus: used to specify a
physical memory address. This can include primary memory (e.g. RAM
and ROM) secondary memory (e.g. hard disk drives) and any other
connected devices. Address bus is unidirectional. This is because
the address is an identification number used by the microprocessor
to identify or access a memory location or I/O device. it is an
output signal from the microprocessor. Control bus: carries
commands from and returns status signals to the microprocessor. The
control unit inside a CPU manages the internal control bus internal
to the CPU and the external control bus. Data bus: connects the
microprocessor (CPU) with other devices mapped onto the system. The
data bus is bi-directional. This is because the microprocessor has
to fetch (read) the data from memory or input device fro processing
and after processing, it has to store (write) the data to memory or
output device.
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Bus Structure of 8085 Microprocessor
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Pin Diagram of Microprocessor 8085
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Pin Description of Microprocessor 8085 The pins on the chip can
be grouped into 6 groups: 1.Address Bus. 2.Data Bus. 3.Control and
Status Signals. 4.Power supply and frequency. 5.Externally
Initiated Signals. 6.Serial I/O ports. 1.Address Bus: The 8085
microprocessor has 8 signal line, A 15 - A 8 which are
unidirectional & used as a high order address bus. 2.Data Bus:
The signal line AD 7 - AD 0 are bidirectional for dual purpose.
They are used as low order address bus as well as data bus.
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Pin Description of Microprocessor 8085 3.Control signal and
Status signal: Control Signal RD bar - It is a read control signal
(active low). It is active when memory read the data. WR bar - It
is a write control signal (active low). It is active when written
into selected memory. Status signal ALE (Address Latch Enable) -
When ALU is high, 8085 microprocessor uses address bus. When ALU is
low, 8085 microprocessor uses data bus. IO/M bar - This is a status
signal used to differentiate between i/o and memory operation. When
it is high, it indicates an i/o operation and when it is low, it
indicates memory operation. S 1 and S 0 - These status signal,
similar to i/o and memory bar, can identify various operation, but
they are rarely used in small system.
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Pin Description of Microprocessor 8085 4.Power Supply and
Frequency signal: V cc - +5v power supply. V ss (GND) - ground
reference. X 1, X 2 - A crystal is connected at these two pins. The
frequency is internally divided by two operate system at 3-MHz, the
crystal should have a frequency of 6-MHz. CLK out - This signal can
be used as the system clock for other devices.
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Pin Description of Microprocessor 8085 5.Externally initiated
signal: INTR(i/p) Interrupt request. INTA bar (o/p) It is used as
acknowledge interrupt. TRAP(i/p) This is non-maskable interrupt and
has highest priority. HOLD(i/p) It is used to hold the executing
program. Indicates that another Master is requesting the use of the
Address and Data buses. The CPU, upon receiving the Hold request
will relinquish the use of buses as soon as the completion of the
current machine cycle. Processor can regain the buses only after
the Hold is removed. HLDA(o/p) Hold acknowledge. Indicates that the
CPU has received the Hold request and that it will relinquish the
buses in the next clock cycle.
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Pin Description of Microprocessor 8085 5.Externally initiated
signal: READY(i/p) - This signal is used to delay the
microprocessor read or write cycle until a slow responding
peripheral is ready to accept or send data. RESET IN bar - sets the
program counter is set to zero, the bus are tri-stated, & MPU
is reset. RESET OUT - This signal indicate that MPU is being reset.
The signal can be used to reset other devices. RST 7.5, RST 6.5,
RST 5.5 (Request interrupt) - It is used to transfer the program
control to specific memory location. They have higher priority than
INTR interrupt.
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Pin Description of Microprocessor 8085 6.Serial I/O ports: The
8085 microprocessor has two signals to implement the serial
transmission which are SID (serial input data) and SOD (serial
output data).
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Interfaces between memory and I/O devices In every
microprocessor, the memory is the central part of microprocessor.
Microprocessor need to access memory quite frequently to read
instructions and data stored in memory, and the memory interfacing
circuit enables that access. Any application of a microprocessor
based system requires the transfer of data between external
circuitry to the microprocessor and vice versa. User can give
information to the microprocessor using keyboard and user can see
the result or output information form the microprocessor with the
help of display device. The transfer of data between keyboard and
microprocessor, and between microprocessor and display device is
called input/output data transfer or I/O data transfer. An
interface: involves designing a circuit that will match the memory
requirements with the microprocessor signal. is a concept that
refers to a point of interaction between components, and is
applicable at the level of both hardware and software.
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Interfaces between memory and I/O devices This allows a
component (such as a graphics card or an Internet browser) to
function independently while using interfaces to communicate with
other components via an input/output system and an associated
protocol. Memory has certain signal requirements to read from and
write into memory. Similarly, microprocessor initiates the set of
signals when it wants to read from and write into memory. There are
two types of interfacing in context of the 8085 processor. Memory
Interfacing I/O Interfacing
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Interfaces between memory and I/O devices
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Memory Interfacing While executing an instruction, there is a
necessity for the microprocessor to access memory frequently for
reading various instruction codes and data stored in the memory.
The interfacing circuit aids in accessing the memory. Memory
requires some signals to read from and write to registers.
Similarly the microprocessor transmits some signals for reading or
writing a data. But what is the purpose of interfacing circuit
here? The interfacing process involves matching the memory
requirements with the microprocessor signals. The interfacing
circuit therefore should be designed in such a way that it matches
the memory signal requirements with the signals of the
microprocessor. For example for carrying out a READ process, the
microprocessor should initiate a read signal which the memory
requires to read a data. In simple words, the primary function of a
memory interfacing circuit is to aid the microprocessor in reading
and writing a data to the given register of a memory chip.
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I/O Interfacing Keyboard and displays are used as communication
channel with outside world. So it is necessary that we interface
keyboard and displays with the microprocessor. This is called I/O
interfacing. In this type of interfacing we use latches and buffers
for interfacing the keyboards and displays with the microprocessor.
But the main disadvantage with this interfacing is that the
microprocessor can perform only one function. It functions as an
input device if it is connected to buffer and as an output device
if it is connected to latch. Thus the capability is very limited in
this type of interfacing.
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8085 Interfacing Pins 8085 Higher Address Bus Lower
Address/Data Bus ALE READY A 15 A 8 AD 7 AD 0
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Microprocessor Operation Fetch Decode Execute
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Fetch Execute Cycle Fetch Cycle Fetch the Instruction from Main
Memory The CPU presents the value of the program counter on the
address bus. The CPU then fetches the instruction from main memory
via the data bus into the Instruction Register. Decode the
Instruction The instruction decoder (ID) interprets and implements
the instruction Execute Cycle Get Data from Main Memory Fetch
required data from main memory to be processed and placed into
registers. Execute the Instruction Control signals are initialized
to process the data using ALU and write the result back to a
register. Store Results The result generated by the operation is
stored in the main memory, or sent to an output device.
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Semiconductor Memories Memory refers to any device that stores
information for later use. The memory of a computer can be divided
into two categories: primary and secondary memory. Primary memories
are semiconductor memories. Semiconductor memory is a device used
for storing digital information that is made up by using integrated
circuit technology. Semiconductor memory technology is an essential
element of today's electronics. Indeed as processors have become
more popular and the number of microprocessor controlled items has
increased so has the requirement for semiconductor memory. With the
rapid growth in the requirement for semiconductor memories there
have been a number of technologies and types of memory that have
emerged. Names such as ROM, RAM, EPROM, EEPROM, DRAM, SRAM, SDRAM,
and the very new MRAM can now be seen in the electronics
literature.
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Semiconductor Memories RAM - Random Access Memory: As the names
suggest, the RAM or random access memory is a form of semiconductor
memory technology that is used for reading and writing data in any
order as required. It is used for such applications as the computer
or processor memory where variables and other stored and are
required on a random basis. Data is stored and read many times to
and from this type of memory. ROM - Read Only Memory: A ROM is a
form of semiconductor memory technology used where the data is
written once and then not changed. In view of this it is used where
data needs to be stored permanently, even when the power is removed
- many memory technologies lose the data once the power is removed.
PROM: PROM refers to the kind of ROM that the user can burn
information into. In other words, PROM is a user-programmable
memory. For every bit of the PROM, there exists a fuse. PROM is
programmed by blowing the fuses. If the information burned into
PROM is wrong, that PROM must be discarded since its internal fuses
are blown permanently. For this reason, PROM is also referred to as
OTP (one-time programmable). Programming ROM, also called burning
ROM, requires special equipment called a ROM burner or ROM
programmer.
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Semiconductor Memories EPROM (Erasable Programmable Read Only
Memory. ): This form of semiconductor memory can be programmed and
then erased at a later time. EPROM was invented to allow making
changes in the contents of PROM after it is burned. In EPROM, one
can program the memory chip and erase it thousands of times. This
is especially necessary during development of the prototype of a
microprocessor- based project. A widely used EPROM is called
UV-EPROM, where UV stands for ultraviolet. The only problem with
UV-EPROM is that erasing its contents can take up to 20 minutes.
All UV-EPROM chips have a window through which the programmer can
shine ultraviolet (UV) radiation to erase its contents. For this
reason, EPROM is also referred to as UV-erasable EPROM or simply
UV-EPROM.
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Semiconductor Memories EEPROM (Electrically Erasable
Programmable Read Only Memory. ): Data can be written to it and it
can be erased using an electrical voltage. EEPROM has several
advantages over EPROM, such as the fact that its method of erasure
is electrical and therefore instant, as opposed to the 20-minute
erasure time required for UV-EPROM. In addition, in EEPROM one can
select which byte to be erased, in contrast to UV- EPROM, in which
the entire contents of ROM are erased. However, the main advantage
of EEPROM is that one can program and erase its contents while it
is still in the system board. It does not require physical removal
of the memory chip from its socket. In other words, unlike
UV-EPROM, EEPROM does not require an external erasure and
programming device. To utilize EEPROM fully, the designer must
incorporate the circuitry to program the EEPROM into the system
board. In general, the cost per bit for EEPROM is much higher than
for UV-EPROM.
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Semiconductor Memories DRAM: Dynamic RAM is a form of random
access memory. DRAM uses a capacitor to store each bit of data, and
the level of charge on each capacitor determines whether that bit
is a logical 1 or 0. However these capacitors do not hold their
charge indefinitely, and therefore the data needs to be refreshed
periodically. As a result of this dynamic refreshing it gains its
name of being a dynamic RAM. DRAM is the form of semiconductor
memory that is often used in equipment including personal computers
and workstations where it forms the main RAM for the computer.
SRAM: Static Random Access Memory. This form of semiconductor
memory gains its name from the fact that, unlike DRAM, the data
does not need to be refreshed dynamically. It is able to support
faster read and write times than DRAM (typically 10 ns against 60
ns for DRAM), and in addition its cycle time is much shorter
because it does not need to pause between accesses. However it
consumes more power, is less dense and more expensive than DRAM. As
a result of this it is normally used for caches, while DRAM is used
as the main semiconductor memory technology.
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Semiconductor Memories SDRAM: Synchronous DRAM. This form of
semiconductor memory can run at faster speeds than conventional
DRAM. It is synchronised to the clock of the processor and is
capable of keeping two sets of memory addresses open
simultaneously. MRAM: This is Magneto-resistive RAM, or Magnetic
RAM. It is a non-volatile RAM memory technology that uses magnetic
charges to store data instead of electric charges. Unlike
technologies including DRAM, which require a constant flow of
electricity to maintain the integrity of the data, MRAM retains
data even when the power is removed. An additional advantage is
that it only requires low power for active operation. As a result
this technology could become a major player in the electronics
industry now that production processes have been developed to
enable it to be produced.