Block Diagram of Microcontroller 8051

Block Diagram of Microcontroller 8051

CPU – The CPU (Central Processing Unit) comprising of ALU (Arithmetic and Logic Units) and Control units

ALU - Arithmetic and Logic Unit performing the arithmetic and logical operations. These operations are multiplication, addition, subtraction, logical AND,OR etc. To do these operations one operand must be in Accumulator, and another may in B register or in general purpose register. The ALU operation results are mostly placed in A register. Some results are placed in B register too.

OSC - Oscillator provides clock for controller operation .Crystal oscillator is used for providing perfect clock and stability. So this microcontroller uses crystal oscillator. For this purpose, the crystals linked to the pins are used.


Microcontroller operation needs some interrupts. Here mainly five interrupts are used. The controller controls the operation interrupts. ie some interrupts may be allowed, some others are disabled and priority assigned and changed. The five interrupts are:


For both Address and data, lower byte address buses are used. The BUS control controls the BUS usage. There are 3 control signals: PSEN , EA, and ALE. The signals like Program Store Enable (PSEN), Address Latch Enable (ALE) and External Access (EA),  are used for external memory interfacing.


The 8051 has 4 kilobyte of inbuilt ROM. It is otherwise called program memory. Usually program-code is stored in ROM. To store program into ROM, programmer is needed. If more area is required in ROM, an external ROM may be connected. Maximum of 64kb ROM memory can be used.


The 8051 has an inbuilt of 128 byte RAM, some version have 256 byte and are used as data memory. If the system needs more memory, external RAM may be connected up to 64kb.In 128 byte RAM chip, 00h to 71Fh are the address range .In this range ,00H to 1FH are the general purpose registers . 20H to 2F are the bit Addressable area and rest of this is byte addressable .This is used as general purpose scratch pad. In 256 byte RAM chip, another 128 bytes are used for Special Function Registers.


There are four IO ports in 8051.These are named as P0, P1, P2, P3.All are bidirectional. Each port have its address, output driver, latch and input buffer. Each port is output by default.


TXD and RXD are utilised for serial port. Each pin has separate buffer registers named as SBUF.


There are two types of counter/timer in 8051.These counter/timers are name as Timer/Counter 0 and Timer/Counter 1. Each one can be used as either Timer or Counter. 16 bit timer register are used for counting in timer operation or counter operation. Clock pulses are counted in Timer operation and external events are counted in counter operation.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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