Microcontroller manufacturers have been competing for a long time for attracting choosy customers and every couple of days a new chip with a higher operating frequency, more memory and upgraded A/D converters appeared on the market.However, most of them had the same or at least very similar architecture known in the world of microcontrollers as “8051 compatible”. What is all this about? The whole story has its beginnings in the far 80s when Intel launched the first series of microcontrollers called the MCS 051. Even though these microcontrollers had quite modest features in comparison to the new ones, they conquered the world very soon and became a standard for what nowadays is called the microcontroller. The main reason for their great success and popularity is a skillfully chosen configuration which satisfies different needs of a large number of users allowing at the same time constant expansions (refers to the new types of microcontrollers). Besides, the software has been developed in great extend in the meantime, and it simply was not profitable to change anything in the microcontroller’s basic core. This is the reason for having a great number of various microcontrollers which basically are solely upgraded versions of the 8051 family. What makes this microcontroller so special and universal so that almost all manufacturers all over the world manufacture it today under different name? As seen in figure above, the 8051 microcontroller has nothing impressive in appearance: 4 Kb of ROM is not much at all. 128b of RAM (including SFRs) satisfies the user's basic needs. 4 ports having in total of 32 input/output lines are in most cases sufficient to make all necessary connections to peripheral environment. The whole configuration is obviously thought of as to satisfy the needs of most programmers working on development of automation devices. One of its advantages is that nothing is missing and nothing is too much. In other words, it is created exactly in accordance to the average user‘s taste and needs. Another advantages are RAM organization, the operation of Central Processor Unit (CPU) and ports which completely use all recourses and enable further upgrade.