About 8051

The base of the embbeded system is Intel microcontroller 4004 which is nibble based. The development becomes quicker after the invention of the microcontroller of 8051 by Intel.
The Intel 8051 is Harvard architecture, single chip microcontroller (µC) which was developed by Intel in 1980 for use in embedded systems. Today 8051-compatible devices manufactured by more than 20 independent manufacturers viz: Atmel, Infineon Technologies (formerly Siemens AG), Maxim Integrated Products (via its Dallas Semiconductor subsidiary), NXP (formerly Philips Semiconductor), Winbond, ST Microelectronics, Silicon Laboratories (formerly Cygnal), Texas Instruments ,Cypress Semiconductor.
Intel's original 8051 family was developed using NMOS technology, but later versions used CMOS technology and were less power-hungry than their NMOS predecessors.

The microcontroller contains mainly CPU, RAM, ROM, I/O, interrupt logic, timer, etc.
8-bit data bus
16-bit address bus
On-chip RAM - 128 bytes ("Data Memory")
On-chip ROM - 4 kB ("Program Memory")
Four byte bi-directional input/output port
UART (serial port)
Two 16-bit Counter/timers
Two-level interrupt priority
Power saving mode
A particularly useful feature of the 8051 core is the inclusion of a Boolean processing engine which allows bit-level Boolean logic operations to be carried out directly and efficiently on internal registers and RAM.

The 8051 UART can be configured to use a 9th data bit that can provide addressable communications in an RS-485 multi-point communications environment.

8051 based microcontrollers typically include one or two UARTs, two or three timers, 128 or 256 bytes of internal data RAM (16 bytes of which are bit-addressable), up to 128 bytes of I/O, 512 bytes to 64 kB of internal program memory, and sometimes a quantity of extended data RAM (ERAM) located in the external data space. The original 8051 core ran at 12 clock cycles per machine cycle, with most instructions executing in one or two machine cycles. With a 12 MHz clock frequency, the 8051 could thus execute 1 million one-cycle instructions per second or 500,000 two-cycle instructions per second. Enhanced 8051 cores are now commonly used which run at six, four, two, or even one clock per machine cycle, and have clock frequencies of up to 100 MHz, and are thus capable of an even greater number of instructions per second.

Common features included in modern 8051 based microcontrollers include built-in reset timers with brown-out detection, on-chip oscillators, self-programmable Flash ROM program memory, boot loader code in ROM, EEPROM non-volatile data storage, I²C, SPI, and USB host interfaces, PWM generators, analog comparators, A/D and D/A converters, RTCs, extra counters and timers, in-circuit debugging facilities, more interrupt sources, and extra power saving modes