• 8 bit based Central Processing Unit
  • In-build Clock circuit and Oscillators
  • 32 bit I/O lines
  • 64Kb External data memory, 64Kb external program  memory
  • Two 16bit Timer/Counter
  • Five interrupt based on two priority
  • Full duplex serial port
  • P0-port does not have pull-up resistor.
  • Harvard architecture
  • CISC

 Block Diagram of 8051


  •  8 bit register also called as A regiester or Acc register

B- Register

  • Its moslt using Multiply and division operations

Program Status Word (PSW)

  • This is Special function  register, can also address bit or byte method
  • It contains the programming status.
  • It is mainly using for select the particular register bank form four register banks

Stack Pointer

  • 8 bit Special function register.
  • Stack may be in any location of On chip memory of RAM
  • Stack pointer values will increment when execute the PUSH and CALL instructions

Data Pointer

  • 16 Bit register (DPH-8bit and DPL-8bit)


I/O Ports 0-3 Latch and Drivers

Four different I/O Ports(P0,P1,P2,P3.

  • Each Port have 8 pins (32 pins).
  • All port pins are working Bi-directional method.
  • Each Ports are contains separate Latch, Output Driver and Input Buffer.
  • Port Pin as a INPUT Mode
    • pin = 1 (input) //define
  • Port Pin as a OUTPUT Mode
    • pin = 0 (output) //define

Serial Data Buffer

  • It contains Two buffer register
  • Transmit buffer register
    • parallel in serial out
  • Receiver buffer  registers
    • serial in parallel out

Timer register

  • It has Two 16 bit registers namely TH0, TL0 and TH1, TL1
  • Its mainly using for choosing timer and counter

Control Registers

  • IP
  • IE
  • TMOD
  • TCON
  • SCON
  • PCON

Timer and Control Unit

  • It gives all timing and control of micro-controller


  • Its using for generate a signal for controller

Instruction register

  • This  decode the next execute instruction’s op-code

EPROM and Program address register

RAM Address registers


SFR Register Banks

 Pin Diagram


  • Vcc – +5v supply Voltage.
  • Vss – Ground Pin
  • Reset – Using for reset the micro-controller

EA(Bar) /Vp-p

  • High – Accessing Internal Program Memory
  • Low  – Accessing External Program Memory

Note :Should gives 21volts DC supply, while programming EPROM .in Internal

XTAL1 and XTAL2 – Using Crystal Oscillator

Types of Memory

  • Code Memory
  • Internal RAM
  • External RASM
  • Special Function Registers(SFRs)
  • Bit Memory

Data sheet

Development Tool (Compiler, Assembler, Linker, Debugger)


8051 Interview Questions

8051 Projects

See also