At first in the 80's Intel introduced their microcontroller family MCS 8051 to the market. Although this family had quite limited capabilities by today's notions, it quickly captivated the world and became the standard for what is today understood as 'microcontroller'. The most significant cause for such a success can be found in the cleverly chosen configuration which can satisfy a diversity of needs, yet allowing for continuous upgrades (in form of new controllers). In a brief period of time, a decent amount of software has been developed for 8051, making further changes of the hardware core simply uneconomical. Consequently, there is a variety of MCUs available today, basically just the upgraded 8051 models. What exactly makes this microcontroller so special and universal that it is still manufactured by all the major companies, just under a different label.
8051 configuration is intended to satisfy the needs of programmers developing the controlling devices and instruments. This is one part of its key to success: there is nothing missing, yet there is no lavishness; it is meant for the average user. The other clue can be found in the organization of RAM, Central Processor Unit (CPU), and ports - all of which maximally utilize the available resources and allow further upgrades.
Microcontrollers and Microprocessors
What is the difference between a microprocessor and microcontroller? By microprocessor is meant the general-purpose microprocessors such as Intel's x86 family (8086, 80286, 80386, 80486, and the Pentium) or Motorola's 680x0 fami¬ly (68000, 68010, 68020, 68030, 68040, etc.). These microprocessors contain no RAM, no ROM, and no I/O ports on the chip itself. For this reason, they are com¬monly referred to as general purpose microprocessors.
A system designer using a general-purpose microprocessor such as the Pentium or the 68040 must add RAM, ROM, I/O ports, and timers externally to make them functional. Although the addition of external RAM, ROM, and I/O ports makes these systems bulkier and much more expensive, they have the advan¬tage of versatility such that the designer can decide on the amount of RAM, ROM, and I/O ports needed to fit the task at hand. This is not the case with microcon¬trollers. A microcontroller has a CPU (a microprocessor) in addition to a fixed amount of RAM, ROM, I/O ports, and a timer all on a single chip. In other words, the processor, RAM, ROM, I/O ports, and timer are all embedded together on one chip; therefore, the designer cannot add any external memory, I/O, or timer to it. The fixed amount of on-chip ROM, RAM, and number of I/O ports in microcon-trollers makes them ideal for many applications.
Criteria for choosing a Microcontroller
The first and foremost criterion in choosing a microcontroller is that it must meet the task at hand efficiently and cost effectively. In analyzing the needs of a microcontroller-based project, we must first see whether an 8-bit, 16-bit, or 32-bit microcontroller can best handle the computing needs of the task most effectively. Among other considerations in this category are:
- Speed. What is the highest speed that the microcontroller supports?
- Packaging. Does it come in a 40-pin DIP (dual inline package) or a QFP (quad flat package), or some other packaging format? This is important in terms of space, assembling, and prototyping the end product.
- Power consumption. This is especially critical for battery-powered prod-ucts.
- The amount of RAM and ROM on chip.
- The number of I/O pins and the timer on the chip.
- How easy it is to upgrade to higher-performance or lower power con¬sumption versions.
A brief history of the 8051
In 1981, Intel Corporation introduced an 8-bit microcontroller called the 8051. This microcontroller had 128 bytes of RAM, 4K bytes of on-chip ROM, two timers, one serial port, and four ports (each 8-bits wide) all on a single chip. At the time it was also referred to as a "system on a chip." The 8051 is an 8-bit proces¬sor, meaning that the CPU can work on only 8 bits of data at a time. Data larger than 8 bits has to be broken into 8-bit pieces to be processed by the CPU. The 8051 has a total of four I/O ports, each 8 bits wide. Although the 8051 can have a maximum of 64K bytes of on-chip ROM, many manufacturers have put only 4K bytes on the chip.
The 8051 became widely popular after Intel allowed other manufacturers to make and market any flavors of the 8051 they please with the condition that they remain code-compatible with the 8051. This has led to many versions of the 8051 with different speeds and amounts of on-chip ROM marketed by more than half a dozen manufacturers. It is important to note that although there are different flavors of the 8051 in terms of speed and amount of on-chip ROM, they are all compatible with the original 8051 as far as the instruc¬tions are concerned. This means that if you write your program for one, it will run on any of them regardless of the manufacturer.
Block Diagram of 89C51
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