Microprocessor notes 4th semester, Pin Diagram and internal architecture

Unit: 2 – Microprocessor Notes 

Pin Diagram and internal architecture of 8085 microprocessor:
lPin Diagram of 8085 Microprocessor:
Explain the pin diagram of 8085 Microprossor:
Power Supply

Vcc: +5V Power Supply
Vss: Ground
Clock Frequency Signals

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: It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address.
Data Bus

AD0-Ad7: These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.During the opcode 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 : 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 microprocessor. When pulse goes high it indicates that AD0-AD7 are address. When it is low it indicates that the contents are data.

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 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
Opcode fetch
Memory read
Memory write
I/O read
I/O write
Interrupt Acknowledge
Interrupt and Externally initiated operation:
Que:What is interrupt and types of interrupt in 8085 microprocessor?
Que:What is interrupt explain the types of interrupts?
Que:What is meant by interrupts in 8085 microprocessor?
Que:How many different interrupt types are available in the microprocessor?

Ans: Interrupt is a mechanism by which an I/O or an instruction can suspend the normal execution of processor and get itself serviced. Generally, a particular task is assigned to that interrupt signal. In the microprocessor based system the interrupts are used for data transfer between the peripheral devices and the microprocessor.

There are 5 interrupt signals, i.e. TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR.

Interrupt are classified into following groups based on their parameter −

1. Vector interrupt − In this type of interrupt, the interrupt address is known to the processor. For example: RST7.5, RST6.5, RST5.5, TRAP.

2. Non-Vector interrupt − In this type of interrupt, the interrupt address is not known to the processor so, the interrupt address needs to be sent externally by the device to perform interrupts. For example: INTR.

3. Maskable interrupt − In this type of interrupt, we can disable the interrupt by writing some instructions into the program. For example: RST7.5, RST6.5, RST5.5.

4. Non-Maskable interrupt − In this type of interrupt, we cannot disable the interrupt by writing some instructions into the program. For example: TRAP.

5. Software interrupt − In this type of interrupt, the programmer has to add the instructions into the program to execute the interrupt. There are 8 software interrupts in 8085, i.e. RST0, RST1, RST2, RST3, RST4, RST5, RST6, and RST7.

6. Hardware interrupt − There are 5 interrupt pins in 8085 used as hardware interrupts, i.e. TRAP, RST7.5, RST6.5, RST5.5, INTA.

lInternal Architecture Of 8085 Microprocessor:


Operations of 8085 Microprocessor

The main operation of ALU is arithmetic as well as logical which includes addition, increment, subtraction, decrement, logical operations like AND, OR, Ex-OR, complement, evaluation, left shift or right shift. Both the temporary registers as well as accumulators are utilized for holding the information throughout in the operations then the outcome will be stored within the accumulator. The different flags are arranged or rearrange based on the outcome of the operation.
Flag Registered in 8085 microprocessor
In 8085 microprocessor, the flags register can have a total of eight flags. Thus a flag can be represented by 1 bit of information. But only five flags are implemented in 8085. And they are:
Carry flag (Cy),
Auxiliary carry flag (AC),
Sign flag (S),
Parity flag (P), and
Zero flag (Z).
The respective position of these flag bits in flag register has been show the below figure. The positions marked by “x” are to be considered as don’t care bits in the flags register. The user is not required to memorize the positions of these flags in the flags register.

Control and Timing Unit

The control and timing unit coordinates with all the actions of the microprocessor by the clock and gives the control signals which are required for communication among the microprocessor as well as peripherals.

Special Purpose Registers

These registers are classified into four types namely program counter, stack pointer, increment or decrement register, address buffer or data buffer.
Decoder and Instruction Register

As an order is obtained from memory after that it is located in the instruction register, and encoded & decoded into different device cycles.Permitted couples are BC, DE & HL, and the short term W & Z registers are used in the processor & it cannot be utilized with the developer.

Program Counter
This register is used to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location.
Stack Pointer
 It is used as a memory pointer. It points to a memory location in read/write memory, called the stack. It is always incremented/decremented by 2 during push and pop operation.

Increment or Decrement Register

The 8-bit register contents or else a memory position can be increased or decreased with one. The 16-bit register is useful for incrementing or decrementing program counters as well as stack pointer register content with one. This operation can be performed on any memory position or any kind of register.

Address-Buffer & Address-Data-Buffer

Address buffer stores the copied information from the memory for the execution. The memory & I/O chips are associated with these buses; then the CPU can replace the preferred data by I/O chips and the memory.

Address Bus and Data Bus

The data bus is useful in carrying the related information that is to be stock up. It is bi-directional, but address bus indicates the position as to where it must be stored & it is uni-directional, useful for transmitting the information as well as address input/output devices.

Timing & Control Unit

The timing & control unit can be used to supply the signal to the 8085 microprocessor for achieving the particular processes. The timing and control units are used to control the internal as well as external circuits. These are classified into four types namely control units like RD’ ALE, READY, WR’, status units like S0, S1, and IO/M’, DM like HLDA, and HOLD unit, RESET units like RST-IN and RST-OUT.

8085 Microprocessor Instruction Set

The instruction set of 8085 microprocessor is nothing but instruction codes used to achieve an exact task, and instruction sets are categorized into various types namely control, logical, branching, arithmetic, and data transfer instructions.

Addressing Modes of 8085

The addressing modes of 8085 microprocessor can be defined as the commands offered by these modes which are utilized for denoting the information in different forms without altering the content. These classified into five groups namely immediate, register, direct, indirect and implied addressing mode.

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