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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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Figure 2-1
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Clock Waveform Timing
• Periodic clock waveform– repetitive form– specific time interval– successive pulses identical
• Period
• Frequency
• f = 1/tp and tp = 1/f
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Engineering Notation
• See Table 2-1• giga• mega• kilo• milli• micro• nano• pico
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Serial Representation
• Single electrical conductor
• Slow– one bit for each clock period– telephone lines, intracomputer
• COM ports
• Plug-in cards
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Serial Representation
• Several standards– V.90, ISDN, T1, T2, T3, USB, Ethernet,
10baseT, 100baseT, cable, DSL
• COM - 115 kbps
• USB - 12 Mbps
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
William KleitzDigital Electronics with VHDL, Quartus® II Version
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)
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Parallel Representation
• LPT1– 8-bit parallel– 115 kBps
• SCSI– 16-bit parallel– 160 MBps
• Bps - BYTES per second
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Figure 2-17
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Figure 2-24
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
A Transistor as a Switch
• Bipolar transistor– input signal at one terminal– two other terminals become short of open
• Types– NPN– PNP
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
A Transistor as a Switch
• 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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
A Transistor as a Switch
• PNP– negative voltage base to emitter– ON– positive voltage or 0 V from base to emitter– OFF
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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.
William KleitzDigital Electronics with VHDL, Quartus® II Version
The TTL Integrated Circuit
• 7404– hex inverter– six complete logic circuits– single silicon chip– 14 pins– 7 on a side
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
The TTL Integrated Circuit
• DIP - dual-in-line package– NC - not physically or electrically connected
• Pin Configuration– see Figure 2-39
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Figure 2-39
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
The CMOS Integrated Circuit
• Complementary Metal Oxide Semiconductor– low power consumption– battery-powered devices– slower switching speed than TTL– sensitive to electrostatic charges
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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.
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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.
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
William KleitzDigital Electronics with VHDL, Quartus® II Version
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.
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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.
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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William KleitzDigital Electronics with VHDL, Quartus® II Version
Summary
• SMD-style ICs are gaining popularity over the through-hole style DIP ICs because of their smaller size and reduced manufacturing costs.
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
William KleitzDigital Electronics with VHDL, Quartus® II Version
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