INTRODUCTION TO DIGITAL CONCEPT

Transcription

1 COURSE / CODE DIGITAL SYSTEM FUNDAMENTALS (ECE 421) DIGITAL ELECTRONICS FUNDAMENTAL (ECE 422) INTRODUCTION TO DIGITAL CONCEPT Digital and Analog Quantities Digital relates to data in the form of digits, meaning data are expressed in digits. The information is conveyed in the form of arrangements of discrete digits. Analog relates to data in the form of continuously variable physical quantities. The information content of an analog signal lies in the value of some constituent parameter, such as its amplitude, frequency, or phase. Below is the comparison chart between analog and digital technologies. ANALOG DIGITAL Signal Analog signal is a continuous signal which represents physical Digital signals are discrete time signals generated by digital modulation. measurements. Representation Uses continuous range of values to represent information. Uses discrete or discontinuous values to represent information Example Human voice in air, analog electronic devices. Computers, CDs, DVDs, and other digital electronic devices. Technology Analog technology records waveforms as they are. Samples analog waveforms into a limited set of numbers and records them. Data Transmissions Subjected to deterioration by noise during transmission and write/read cycle. Can be noise-immune without deterioration during transmission and write/read cycle. Response to noise More likely to get affected reducing accuracy. Less affected since noise response are analog in nature. Flexibility Analog hardware is not flexible. Digital hardware is flexible in implementation. Uses Can be used in analog devices only. Best suited for audio and video Best suited for Computing and digital electronics. transmission. Memory Stored in the form of wave signal. Stored in the form of binary bit. Cost Low cost and portable. Cost is high and not easily portable. Errors Analog instruments usually have a scale which is cramped at lower end and give considerable observational errors. Digital instruments are free from observational errors like parallax and approximation errors. Binary Digits (Bits) A bit (short for binary digit) is the smallest unit of data in a digital device (e.g. computer). A bit has a single binary value, either 0 or 1. Some terms used to describe the grouping of binary digits and their range: Bit (1 bit) values range from 0 to 1 Nibble (4 bits) values range from 0 to 15 0 => => => => => => => => => => => => => => => => Mohd Uzir Kamaluddin / Aug 2016 page 1

2 Byte (8 bits) values range from 0 to 255 Word (16 bits) values range from 0 to 65,535 Logic Level In digital circuits, a logic level is one of a finite number of states that a digital signal can have. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. In positive logic, a logic 0 is called LOW, and a logic 1 is called HIGH. TTL Logic Levels TTL is a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast with resistor transistor logic (RTL) and diode transistor logic (DTL)). A majority of systems rely on 5V TTL Logic Levels and it relies on circuits built from bipolar transistors (BJT) to achieve switching and maintain logic states. For any logic family, there are a number of threshold voltage levels to know: VOH Minimum OUTPUT Voltage level a TTL device will provide for a HIGH signal. VIH Minimum INPUT Voltage level to be considered a HIGH. VOL Maximum OUTPUT Voltage level a device will provide for a LOW signal. VIL Maximum INPUT Voltage level to still be considered a LOW. Comparison of various logic levels Not all digital devices use the same logic levels, but in fact there are many different digital technologies available and they all uses different voltages for their logic. CMOS Complementary Metal Oxide Semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Mohd Uzir Kamaluddin / Aug 2016 page 2

3 Basic Logic Operations Binary based numbers, because of their limited range of symbols, give rise to especially simple rules and manipulations. Addition = = = = 10 (0 carry 1) Subtraction 0-0 = = 11 (Borrow 1 from higher column) 1-0 = = 0 Multiplication 0 x 0 = 0 0 x 1 = 0 1 x 0 = 0 1 x 1 = 1 Basic Logic Functions The design of efficient digital circuitry requires an ability by the engineer to manipulate binary system variables. The algebra governing these mathematical manipulations is known as Boolean algebra, and turns out to be that applicable to the laws of logic. Logic may be loosely defined as the science of argument. Because a logic argument is normally true or false; such arguments may be represented by binary circuits. The output of such a circuit is 2-valued, and may hence indicate the truth or non-truth of the argument as a function of its inputs; which in this case is the truth or falsity of the component statements. In essence, there are three Boolean operators or connectives, known as AND, OR, NOT. All logic functions may be described in terms of these three operators. Mohd Uzir Kamaluddin / Aug 2016 page 3

4 The AND operator Switch representation of AND function. The OR operator Switch representation of OR function. The NOT operator Switch representation of NOT function. From the above three basic functions, the following functions is derived. Mohd Uzir Kamaluddin / Aug 2016 page 4

5 The NAND operator Switch representation of NAND function. The NOR operator Switch representation of NOR function. Digital Integrated Circuit Most logic circuits are constructed using Digital IC, or integrated circuit, components. Such components are actually networks of interconnected components manufactured on a single wafer of semiconducting material. Integrated circuits providing a multitude of pre-engineered functions are available at very low cost, benefitting students, hobbyists and professional circuit designers alike. Most integrated circuits provide the same functionality as discrete semiconductor circuits at higher levels of reliability and at a fraction of the cost. Circuits using TTL technology require a regulated power supply voltage of 5 volts, and will not tolerate any substantial deviation from this voltage level. Exercises Choose the correct answers in the following questions. 1. What is the difference between digital and analog techniques? a) Digital quantities can take on any value over a continuous range. b) Digital quantities can take on discrete value over a range. c) Actually, they are indifferent, only digital is a new technology invented in 1980s. d) None of the above. 2. Choose the one which involves digital quantities: a) Ten-Position switch. b) Current meter Mohd Uzir Kamaluddin / Aug 2016 page 5

6 c) Temperature. d) Radio volume control in 80s. 3. Which following is not an advantage of digital technique? a) Digital system is easier to design. b) Accuracy and precision are greater. c) Digital circuits are less affected by noise d) Digital quantities are equivalent to real-world physical quantities. 4. What is the largest decimal number that can be represented using 8 bits? a) 128 b) 255 c) 256 d) Which of the following range is the not used in voltage assignment in digital system: a) 0.4V - 1.2V b) 0.8V - 2V c) 0.8V - 2.4V d) 1V - 2.4V Mohd Uzir Kamaluddin / Aug 2016 page 6