Architecture Block diagram and components Of 8051 Microcontroller


Architecture Block diagram and components Of 8051 Microcontroller 

Block Diagram of 8051

We had seen Architecture Block diagram and components Of 8051 Microcontroller today we will see Some more details of  Block diagram and components Of 8051 Microcontroller.

What is a microcontroller?

A complete computer system manufactured on a single chip is called a microcontroller.

The features of a microcontroller are similar to that of a microprocessor like registers, ALU, program counter, flags, stack pointer, etc. Along with these common features, a microcontroller has some additional features such as a clock circuit, internal RAM, ROM, serial and parallel I/O counter.

Using a microcontroller provides a great advantage that a program is stored in ROM that has an important role in managing the system's operations and functions. Since the program written into ROM is fixed that is, it cannot be altered or changed; as a result the operation of the system remains constant (unchanged) regardless of the instructions given to it. Hence, a microcontroller can be thought of a device containing on-chip program memory. A microcontroller can also be referred as a 'microcomputer'.

Now let us see the architecture and block diagram of 8051 microcontroller

Major components of Intel 8051 microcontroller

The 8051 microcontroller is an 8-bit microcontroller. Let us see the major components of 8051 microcontroller and their functions.

An 8051 microcontroller has the following 12 major components:

1. ALU (Arithmetic and Logic Unit)
2. PC (Program Counter)
3. Registers
4. Timers and counters
5. Internal RAM and ROM
6. Four general purpose parallel input/output ports
7. Interrupt control logic with five sources of interrupt
8. Serial date communication
9. PSW (Program Status Word)
10. Data Pointer (DPTR)
11. Stack Pointer (SP)
12. Data and Address bus.

8051 microcontroller-1

Now let us see the functions of each of these components

1. ALU

All arithmetic and logical functions are carried out by the ALU. 

Addition, subtraction with carry, and multiplication come under arithmetic operations.

Logical AND, OR and exclusive OR (XOR) come under logical operations.

2. Program Counter (PC)

A program counter is a 16-bit register and it has no internal address. The basic function of program counter is to fetch from memory the address of the next instruction to be executed. The PC holds the address of the next instruction residing in memory and when a command is encountered, it produces that instruction. This way the PC increments automatically, holding the address of the next instruction.

3. Registers

Registers are usually known as data storage devices. 8051 microcontroller has 2 registers, namely Register A and Register B. Register A serves as an accumulator while Register B functions as a general purpose register. These registers are used to store the output of mathematical and logical instructions. 

The operations of addition, subtraction, multiplication and division are carried out by Register A. Register B is usually unused and comes into picture only when multiplication and division functions are carried out by Register A. Register A also involved in data transfers between the microcontroller and external memory.

8051 microcontroller also has 7 Special Function Registers (SFRs). They are:

1. Serial Port Data Buffer (SBUF)
2. Timer/Counter Control (TCON)
3. Timer/Counter Mode Control (TMOD)
4. Serial Port Control (SCON)
5. Power Control (PCON)
6. Interrupt Priority (IP)
7. Interrupt Enable Control (IE)

4. Timers and Counters

Synchronization among internal operations can be achieved with the help of clock circuits which are responsible for generating clock pulses. During each clock pulse a particular operation will be carried out, thereby, assuring synchronization among operations. For the formation of an oscillator, we are provided with two pins XTAL1 and XTAL2 which are used for connecting a resonant network in 8051 microcontroller device. In addition to this, circuit also consists of four more pins. They are,

Internal operations can be synchronized using clock circuits which produce clock pulses. With each clock pulse, a particular function will be accomplished and hence synchronization is achieved. There are two pins XTAL1 and XTAL2 which form an oscillator circuit which connect to a resonant network in the microcontroller. The circuit also has 4 additional pins -

1. EA: External enable
2. ALE: Address latch enable
3. PSEN: Program store enable and
4. RST: Reset.

Quartz crystal is used to generate periodic clock pulses.

5. Internal RAM and ROM


A code of 4K memory is incorporated as on-chip ROM in 8051. The 8051 ROM is a non-volatile memory meaning that its contents cannot be altered and hence has a similar range of data and program memory, i.e, they can address program memory as well as a 64K separate block of data memory.


The 8051 microcontroller is composed of 128 bytes of internal RAM. This is a volatile memory since its contents will be lost if power is switched off. These 128 bytes of internal RAM are divided into 32 working registers which in turn constitute 4 register banks (Bank 0-Bank 3) with each bank consisting of 8 registers (R0 - R7). There are 128 addressable bits in the internal RAM.

8051 microcontroller-2

6. Four General Purpose Parallel Input/Output Ports

The 8051 microcontroller has four 8-bit input/output ports. These are:

PORT P0: When there is no external memory present, this port acts as a general purpose input/output port. In the presence of external memory, it functions as a multiplexed address and data bus. It performs a dual role.

PORT P1: This port is used for various interfacing activities. This 8-bit port is a normal I/O port i.e. it does not perform dual functions.

PORT P2: Similar to PORT P0, this port can be used as a general purpose port when there is no external memory but when external memory is present it works in conjunction with PORT PO as an address bus. This is an 8-bit port and performs dual functions.

PORT P3: PORT P3 behaves as a dedicated I/O port

7. Interrupt Control

An event which is used to suspend or halt the normal program execution for a temporary period of time in order to serve the request of another program or hardware device is called an interrupt. An interrupt can either be an internal or external event which suspends the microcontroller for a while and thereby obstructs the sequential flow of a program.

There are two ways of giving interrupts to a microcontroller – one is by sending software instructions and the other is by sending hardware signals. The interrupt mechanism keeps the normal program execution in a "put on hold" mode and executes a subroutine program and after the subroutine is executed, it gets back to its normal program execution. This subroutine program is also called an interrupt handler. A subroutine is executed when a certain event occurs.

In 8051, 5 sources of interrupts are provided. They are:

a) 2 external interrupt sources connected through INT0 and INT1
b) 3 external interrupt sources- serial port interrupt, Timer Flag 0 and Timer Flag 1.

The pins connected are as follows:

1. ALE (Address Latch Enable) - Latches the address signals on Port P0
2. EA (External Address) - Holds the 4K bytes of program memory
3. PSEN (Program Store Enable) - Reads external program memory
4. RST (Reset) - Reset the ports and internal registers upon start up

8. Serial Data Communication

A method of establishing communication among computers is by transmitting and receiving data bits is a serial connection network. In 8051, the SBUF (Serial Port Data Buffer) register holds the data; the SCON (Serial Control) register manages the data communication and the PCON (Power Control) register manages the data transfer rates. Further, two pins - RXD and TXD, establish the serial network.

The SBUF register has 2 parts – one for storing the data to be transmitted and another for receiving data from outer sources. The first function is done using TXD pin and the second function is done using RXD pin.

There are 4 programmable modes in serial data communication. They are:

1. Serial Data mode 0 (shift register mode) 
2. Serial Data mode 1 (standard UART)
3. Serial Data mode 2 (multiprocessor mode) 
4. Serial Data mode 3

9. PSW (Program Status Word)

Program Status Word or PSW is a hardware register which is a memory location which holds a program's information and also monitors the status of the program this is currently being executed. PSW also has a pointer which points towards the address of the next instruction to be executed. PSW register has 3 fields namely are instruction address field, condition code field and error status field. We can say that PSW is an internal register that keeps track of the computer at every instant.

Generally, the instruction of the result of a program is stored in a single bit register called a 'flag'. The are7 flags in the PSW of 8051. Among these 7 flags, 4 are math flags and 3 are general purpose or user flags.

The 4 Math flags are: 

• Carry (c)
• Auxiliary carry (AC)
• Overflow (OV) 
• Parity (P)

The 3 General purpose flags or User flags are: 

• FO 
• GF 1

10. Data Pointer (DPTR)

The data pointer or DPTR is a 16-bit register. It is made up of two 8-bit registers called DPH and DPL. Separate addresses are assigned to each of DPH and DPL. These 8-bit registers are used for the storing the memory addresses that can be used to access internal and external data/code.

11. Stack Pointer (SP)

The stack pointer (SP) in 8051 is an 8-bit register. The main purpose of SP is to access the stack. As it has 8-bits it can take values in the range 00 H to FF H. Stack is a special area of data in memory. The SP acts as a pointer for an address that points to the top of the stack.

12. Data and Address Bus

A bus is group of wires using which data transfer takes place from one location to another within a system. Buses reduce the number of paths or cables needed to set up connection between components.

There are mainly two kinds of buses - Data Bus and Address Bus

Data Bus: The purpose of data bus is to transfer data. It acts as an electronic channel using which data travels. Wider the width of the bus, greater will be the transmission of data.

Address Bus: The purpose of address bus is to transfer information but not data. The information tells from where within the components, the data should be sent to or received from. The capacity or memory of the address bus depends on the number of wires that transmit a single address bit.

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