《移动信息工程导论》第2章二进制数字、逻辑电平和数字波形.ppt

Binary digit, logic levels, and digital waveforms A bit (short for binary digit) is the smallest unit of data in a digital system (e.g. a computer). A bit has a single binary value, either 0 or 1. The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Digital Data in digital systems 1GB = 210MB = 220KB = 230B = 233b The memory size of a computer is 1GB. How many bits does it have? In digital circuits, two voltage levels are used to represent the two bits. How to represent bits in digital circuits? Positive logic: ‘1’ is represented by HIGH ‘0’ is represented by LOW Negative logic: ‘1’ is represented by LOW ‘0’ is represented by HIGH Digital waveforms How to transmit a digital data? ——by using waveforms Figure: Ideal pulses. The rising and falling edges are assumed to change in zero time. Figure Logic level ranges of voltage for a digital circuit. Logic levels: the voltages used to represent a ‘1’ and a ‘0’. Logic Level This figure answers the question how to represent the binary logic with voltages. For example, for CMOS circuits, 2V-3.3V HIGH 0V-0.8V LOW * Digital waveforms In practice, voltage transitions never occur instantaneously. Period Frequency is the rate at which the pulse repeats itself and is measured in hertz(Hz). It is the reciprocal of the period. Duty cycle（占空比）is the ratio of the pulse width to the period. * Waveforms Characteristics Repeat itself at fixed interval. * A Digital Waveform Carries Binary Information Each bit in a sequence occupies a defined time interval called a bit time. In digital systems, all waveforms are synchronized with a basic timing waveform which is called the clock. Each change in level of waveform occurs at the leading edge of the clock. * Timing Diagrams Timing diagram is a graph of digital waveforms showing time relationship of two or more waveforms and how each waveform changes in relation to the others. Figure 1–12 Illustrati