M. Amer Iqbal Qureshi
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| Logic circuits
Have you ever wondered what electronics within some digital integrated circuit, microcontroller or processor 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- meaning 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 Fig. 8 High and Low Nibbles of a Byte Fig. 9 Logical AND gate with its Truth Table Teach Yourself PIC Microcontrollers | www.electronicspk.com | 15 Boolean algebra. As some program instructions used by the microcontroller perform the same way as logic gates but in 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). That’s all! Schematic symbol of AND gate is shown in the figure on the right. Additionally, the table shows mutual dependence between inputs and output. In case the gate has more than two inputs, the principle of operation is the same: a logic one (1) will appear on its output only in case all inputs are driven to logic one (1). Any other combination of input voltages will result in logic zero (0) on its output. When used in a program, logic AND operation is performed by the program instruction, which will be discussed later. For the time being, it is enough to remember that logic AND in a program refers to the corresponding bits of two registers. tải về 3.06 Mb. Chia sẻ với bạn bè của bạn:
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