Specifications
mikroElektronika | Free Online Book | PIC Microcontrollers | Introduction: World of Microcontrollers
contains information upon WHICH data it should be done). However, it is not a hard and fast rule, but the most common case. In
microcontrollers with Harvard architecture, the program bus is wider than one byte, which allows each program word to consist of
instruction and data. In other words: one program word- one instruction.
INSTRUCTION SET
Instructions that can be understood by the microcontroller are known as an instruction set. When you write a
program in assembly language, you actually “tell a story” by specifying instructions in the order they should be
executed. The main restriction in this process is the number of available instructions. The manufacturers stick to
one of the two following strategies:
RISC (Reduced Instruction Set Computer)
In this case, the microcontroller recognizes and executes only basic operations (addition, subtraction, copying etc.). All other more complicated
operations are performed by combining these (for example, multiplication is performed by performing successive addition). The constrains are
obvious (try by using only a few words, to explain to someone how to reach the airport in some other city). However, there are also some great
advantages. First of all, this language is easy to learn. Besides, the microcontroller is very fast so that it is not possible to see all the arithmetic
“acrobatics” it performs. The user can only see the final result of all those operations. At last, it is not so difficult to explain where the airport is
if you use the right words. For example: left, right, kilometers etc.
CISC (Complex Instruction Set Computer)
CISC is the opposite of RISC! Microcontrollers designed to recognize more than 200 different instructions can do much and are very fast. However,
one needs to understand how to take all that such a rich language offers, which is not at all easy...
How to make the right choice
Ok, you are the beginner and you have made a decision to go on an adventure of working with the microcontrollers. Congratulations on your
choice! However, it is not as easy to choose the right microcontroller as it may seem. The problem is not a limited range of devices, but the
opposite!
Before you start designing some device based on the microcontroller, think of the following: how many input/output lines will I need for
operation? Should it perform some other operations than to simply turn relays on/off? Does it need some specialized module such as serial
communication, A/D converter etc. When you create a clear picture of what you need, the selection range is considerably reduced, then it is
time to think of price. Is your plan to have several same devices? Several hundred? A million? Anyway, you get the point...
If you think of all these things for the very first time then everything seems a bit confusing. For that reason, go step by step. First of all, select
the manufacturer, i.e. the family of the microcontrollers you can easily obtain. After that, study one particular model. Learn as much as you
need, do not go into details. Solve a specific problem and something incredible will happen- you will be able to handle any model belonging to
that family.
http://www.mikroe.com/en/books/picmcubook/ch0/ (28 of 30)5/3/2009 11:28:39 AM