Specifications
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is therefore called the least significant bit (LSB). Since eight zeros and units of one byte can be combined in 256 different ways, the largest
decimal number which can be represented by one byte is 255 (one combination represents zero).
A nibble is referred to as half a byte. Depending on which half of the byte we are talking about (left or right), there are “high” and “low” nibbles.
Fig. 0-8 High and Low nibbles
Logic Circuits
Have you ever wondered what electronics within some digital integrated circuits, microcontrollers or processors look like? What do the circuits
performing complicated mathematical operations and making decisions look like? Do you know that their seemingly complicated schematics
comprise only a few different elements called “logic circuits” or “logic gates”?
The operation of these elements is based on the principles established by British mathematician George Boole in the middle of the 19th century-
even before the first bulb was invented! In brief, the main idea was to express logical forms through algebraic functions. Such thinking was soon
transformed into a practical product which far later evaluated in what today is known as AND, OR and NOT logic circuits. The principle of their
operation is known as Boolean algebra. As some program instructions used by the microcontroller perform the same way as logic gates except in
the form of commands, the principle of their operation will be discussed here.
AND Gate
A logic gate “AND” has two or more inputs and one output. Let us presume that the gate used in this case has only two inputs. A logic one (1) will
appear on its output only in case both inputs (A AND B) are driven to logic one (1).
The table shows mutual dependence between inputs and output.
When the gate has more than two inputs, the principle of operation is the
same: a logic one (1) will appear on its output only if case all inputs are driven
to logic one (1). Any other combination of input voltages will result in a logic
zero (0) at its output.
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