chapter 2 digital electronic signals and switches copyright ©2006 by pearson education, inc. upper...

Chapter 2

Digital Electronic Signals and Switches

Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

William KleitzDigital Electronics with VHDL, Quartus® II Version

Digital Signals

• See Figure 2-1(a)

• Timing Diagram– voltage versus time– shows logic state

• Interpretation if not exactly 0V or 5V

• Use the oscilloscope to view




Figure 2-1




Clock Waveform Timing

• Periodic clock waveform– repetitive form– specific time interval– successive pulses identical

• Period

• Frequency

• f = 1/tp and tp = 1/f




Engineering Notation

• See Table 2-1• giga• mega• kilo• milli• micro• nano• pico




Serial Representation

• Single electrical conductor

• Slow– one bit for each clock period– telephone lines, intracomputer

• COM ports

• Plug-in cards




Serial Representation

• Several standards– V.90, ISDN, T1, T2, T3, USB, Ethernet,

10baseT, 100baseT, cable, DSL

• COM - 115 kbps

• USB - 12 Mbps




Parallel Representation

• Separate electrical conductor for each bit

• Expensive

• Very fast

• Inside a computer

• External Devices– Centronics printer interface (LPT1)– SCSI (Small Computer Systems Interface)




Parallel Representation

• LPT1– 8-bit parallel– 115 kBps

• SCSI– 16-bit parallel– 160 MBps

• Bps - BYTES per second




Switches in Electronic Circuits

• Make and break a connection• Manual switch of electromechanical relay• Semiconductor devices

– diodes– transistors

• Manual Switches - Ideal resistances– ON - 0 ohms– OFF - infinite




A Relay as a Switch

• Electromechanical Relay– contacts– external voltage to operate– magnetic coil energizes

• NC - normally closed• NO - normally open• Total isolation

– triggering source– output




A Relay as a Switch

• Disadvantages– several milliamperes of current to operate– slower - several milliseconds vs. micro or nano

• Energized relay coil

• Replace source with clock oscillator

• Timing diagrams

• See Figure 2-17




Figure 2-17




A Diode as a Switch

• Semiconductor• Current flow in one direction only• Forward-biased

– anode more positive than cathode– current flow

• Reverse-biased– anode equal or more negative than cathode– no current flow




A Diode as a Switch

• Analogous to a water check valve

• Not a perfect short– See Figure 2-24

• 0.7 V across its terminals




Figure 2-24




A Transistor as a Switch

• Bipolar transistor– input signal at one terminal– two other terminals become short of open

• Types– NPN– PNP





• NPN– positive voltage from base to emitter– collector-to-emitter junction short– ON– negative voltage or 0 V from base to emitter– collector-to-emitter junction open– OFF





• PNP– negative voltage base to emitter– ON– positive voltage or 0 V from base to emitter– OFF




The TTL Integrated Circuit

• Transistor-transistor logic• Inverter

– takes digital level input– complements it to the output

• Transistor Saturation• Transistor Cutoff• TTL Integrated Circuit

– totem-pole outputCopyright ©2006 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458All rights reserved.



• 7404– hex inverter– six complete logic circuits– single silicon chip– 14 pins– 7 on a side





• DIP - dual-in-line package– NC - not physically or electrically connected

• Pin Configuration– see Figure 2-39




Figure 2-39




The CMOS Integrated Circuit

• Complementary Metal Oxide Semiconductor– low power consumption– battery-powered devices– slower switching speed than TTL– sensitive to electrostatic charges




Surface-Mounted Devices

• SMD– reduced size and weight– lowered cost of manufacturing circuit boards– soldered directly to metalized footprint– special desoldering tools and techniques– chip densities increased– higher frequencies




Surface-Mounted Devices

• SO (small outline)– dual-in-line package– gull-wing format– lower-complexity logic

• PLCC (plastic leaded chip carrier)– square with leads on all four sides– J-bend configuration– more complex logic




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.




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.




Summary

• Electromechanical relays are capable of forming shorts and opens in circuits requiring high current values but not high speed.

• Diodes are used in digital circuitry whenever there is a requirement for current to flow in one direction but not the other.




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 and CMOS integrated circuits are formed by integrating thousands of transistors in a single package. They are the most popular ICs used in digital circuitry today.




Summary

• SMD-style ICs are gaining popularity over the through-hole style DIP ICs because of their smaller size and reduced manufacturing costs.




chapter 2 digital electronic signals and switches copyright ©2006 by pearson education, inc. upper...

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