# Chapter 2 Digital Electronic Signals and Switches 1.

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Chapter 2 Digital Electronic Signals and Switches 1

Objectives Describe the parameters of digital-versus- time waveforms. Convert between frequency and period for a periodic waveform. Sketch the timing waveform for a binary string in either serial or parallel form. Discuss the applications of manual switches and electromechanical relays in electric circuits. 2

Objectives (Continued) Explain the characteristics of diodes and transistors when forward and reverse biased. Calculate output voltage in circuits containing diodes or transistors used as digital switches. Perform I/O timing analysis in circuits containing relays or transistors. Explain the operation of a common-emitter transistor circuit used as a digital inverter. 4

2-1 Digital Signals Timing diagram –Voltage is measured on the y-axis; time on the x-axis 0 and 1 correspond to 0 V and 5 V Can be viewed using an oscilloscope. 5

2-2 Clock Waveform Timing Periodic waveform are –Repetitive –Have a specific time interval –Successive pulses are identical Period – the time from one edge to the corresponding edge on the next cycle –t p = 1/f Frequency – reciprocal of waveform period – f = 1/t p 6

Engineering Notation 8

Solving for Period and Frequency Example 2-1 If a clock waveform has a period of 2 µs, its frequency is f = 1/t p = 1/2 µs = 500 kHz Example 2-2 A 4.17 MHz waveform has a period of t p = 1/f = 1/4.17 MHz = 240 ns

2-3 Serial Representation Uses a single electrical conductor -> inexpensive Slow since only one bit for each clock period –Used for telephone lines, computer-to-computer networks Computer COM ports are most often used for serial communications Ethernet plug-in cards are for serial networking 10

Figure 2.5 Serial communication between computers.

Figure 2-6 Serial Representation of a Binary Number A serial representation of the binary number 01101100 is illustrated below. –Note that there is one bit per clock period and the bit-to-bit change occurs on the falling edge. 11

2- 4 Parallel Representation Separate electrical conductor for each bit -> Expensive Very fast Inside a computer –Data bus External Devices –Centronics printer interface (LPT1) –SCSI (Small Computer Systems Interface) 12

Figure 2.7 Original parallel communication between a computer and a printer.

Figure 2.8 Parallel representation of the binary number 01101100.

2-5 Switches in Electronic Circuits Make or break connections between conductors –Manual switches –Electromechanical relays Semiconductor devices –Diodes –Transistors Manual Switches - ideal resistances: –ON - 0  –OFF -   15

Switches and Logic Levels A double-throw switch can be used to produce a high (1) or a low (0) at an output. –This circuit produces a high (1) at the output. –This circuit produces a low (0) at the output. 15

Figure 2.14 Physical representation of an electromechanical relay (a) normally closed (NC) relay; (b) normally open (NO) relay; (c) photograph of actual relays.

Figure 2.15 Symbolic representation of an electromechanical relay

2.7 A Diode as a Switch A diode is a semiconductor device that allows current to flow in one direction but not the other. Figure 2.22 Diode in a series circuit: (a) forward biased and (b) reverse biased. Forward biased: a diode whose Anode voltage is more positive than its Cathode. Reversed biased: its anode voltage is equal to or more negative than its cathode. Thus not allow current flow.

Figure 2.23 Water system check valve. A diode is analogous to a check valve in a water system.

Figure 2.24 Diode voltage versus current characteristic curve. A diode is not a perfect short in the forward- biased condition. –No current flows until V forw reaches a 0.7v cut- in voltage. As V rev becomes more negative, current flow is still practically zero.

Figure 2.25 Forward-biased diode in an electric circuit (a)original circuit (b)equivalent circuit showing the diode voltage drop and V out = 5 – 0.7 = 4.3 V.

V1 = 5-0.7=4.3v V2 = 0v V3 = 0.7v V4 = 5v

V5 = 5-0.7=4.3v V6 = 0.7v

2.8 A Transistor as a Switch Bipolar transistor is a very commonly used switch in digital electronic circuits. –A three-terminal semiconductor component –An input signal at one terminal generates a short or open between the other two terminals Transistor is made by bombarding pure silicon into N-type and P-type materials –N-type: having one more electron than silicon –P-type: having one less electron than silicon 22

2.8 A Transistor as a Switch Three distinct regions: base, emitter, and collector –Can be a combination of N-P-N-type or P-N-P- type material bonded together 22

NPN transistor Positive base-to-emitter voltage shorts the transistor output (collector-to-emitter) –Transistor is said to be ON Negative voltage (or 0 V) base-to-emitter opens the transistor output –Transistor is said to be OFF 23 V be > 0v <= 0v C E C E

Figure 2.28 NPN transistor switch: (a) transistor ON and (b) transistor OFF.

Example 2.11

Example 2.11 Continued

Example 2.12

Example 2.12 Continued

PNP transistor Positive voltage (or 0 V) base-to-emitter opens the transistor output (collector-to- emitter) –Transistor is said to be OFF Negative base-to-emitter voltage shorts the transistor output –Transistor is said to be ON 24 V be > 0v <= 0v C EC E

2.9 The TTL Integrated Circuit Transistor-Transistor Logic (TTL) is one of the most widely used integrated circuit technology –A combination of transistors, diodes, and resistors integrated in a single package. One basic function of a TTL IC is as a complementing switch or inverter –The output is the complement of the input. (Converts 1s to 0s and vice-versa) 26

Figure 2.35 Common-emitter transistor circuit operating as an inverter. V in equals 1 (+5v), the transistor is turned on (saturation) and V out equals 0 (0v), V in equals 0 (0v), the transistor is turned off (cutoff) and V out equals 1 (≈ 5v) –Assume R load >> R c

Example 2-13

Figure 2.36 Common-emitter calculations. When the transistor is cut off, R c is expected to be small to ensure V out close to 5v. When the transistor is saturated, R c is expected to be large to avoid excessive collector current. Need a variable R c resistance

Figure 2.37 Schematic of a TTL inverter circuit. 26 Q 4 replaces R c, acts in opposite to Q 3. –Such combination is called the totem-pole arrangement. Q 1 (input transistor) drives Q 2. D 1 protects Q 1 from negative voltages at input. D 2 ensures that Q 4 cuts off totally when Q 3 is saturated.

A 7404 TTL IC chip is called 14 pin DIP( Dual-in-line package)

Figure 2.40 Photograph of three commonly used ICs: the 74HC00, 74ACT244, and 74150.

2-11 The CMOS IC Another common IC technology is the CMOS (Complementary metal oxide semiconductor) –Use a complementary pair of metal oxide semiconductor field-effect transistors (MOSFETs) –NOT bipolar transistors, which relies on the contact of two types of semiconductor for its operation, in TTL chips –VS. TTL: Lower power consumption (commonly used in battery-powered devices); slower switching speed.

Figure 2.42 A 4049 CMOS hex inverter pin configuration.

2-12 Surface-Mounted Devices (SMD) Modern electronics depends on the ability to design smaller, more dense components and systems. –SMDs reduce the size of DIP-style logic by 70% –SMDs reduce the weight by 90% –7400 IC in the DIP style:19.23mm X 6.48mm –7400 SMD: 8.75mm X 6.20mm –SMDs significantly lower the manufacture cost

Popular SMD package styles SO (Small Outline) –Dual-in-line package PLCC (Plastic Leaded Chip Carrier) –Leads on all four sides; BGA (Ball Grid Array) –Higher pin counts than PLCC or SO –Connections made through round solder tabs on the bottom of the component. 33

Popular SMD package styles 33 configuration (a) small outline (b) plastic leaded chip carrier (PLCC) c) ball grid array (BGA) (d) photograph of actual SMDs (e) photograph of SMDs mounted on a printed-circuit board

Summary The digital level for 1 is commonly represented by a voltage of 5 V in digital systems. A voltage of 0 V is used for the 0 level. An oscilloscope can be used to observe the rapidly changing voltage-versus-time waveform in digital systems. 35

Summary The frequency of a clock waveform is equal to the reciprocal of the waveform’s speed The transmission of binary data in the serial format requires only a single conductor with a ground reference. The parallel format requires several conductors but is much faster than serial. 36

Summary The transistor is the basic building block of the modern digital integrated circuit. It can be switched on or off by applying the appropriate voltage at its base connection. TTL vs. CMOS circuits (Complete coverage in Chapter 9) 38