Digital Electronic Signals and Switches

1. Chapter 2 Digital Electronic Signals and Switches

2. Objectives You should be able to: 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 mechanical switches and electromechanical relays in electric circuits.)

3. 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. 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. Clock Waveform Timing Periodic waveform Repetitive Specific time interval Successive pulses are identical Period � the time from one edge to the corresponding edge on the next cycle tp = 1/f Frequency � reciprocal of waveform period f = 1/tp

6. Engineering Notation

7. Discussion Points What does the vertical scale of an oscilloscope represent? What does the horizontal scale of an oscilloscope represent? Describe frequency and period. What is the period of a 75 MHz waveform? What is the frequency of a waveform with a period of 20 ns?

8. Serial Representation

9. Serial Representation Single electrical conductor Slow One bit for each clock period Telephone lines, computer-to-computer COM ports are most often used for serial communications Plug-in cards

10. Serial Representation Standards V.90, ISDN, T1, T2, T3, USB, Ethernet, 10baseT, 100baseT, cable, DSL Standard COM port transmission rate: 115 kbps USB � Different speeds, depending on version � typically in hundreds of Mbps.

11. Parallel Representation

12. 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)

13. Parallel Representation LPT1 8-bit parallel 115 kBps SCSI 16-bit parallel Up to 160 MBps Bps - bytes per second

14. Discussion Points Describe the difference between parallel and serial transmission. What advantage does parallel transmission have over serial transmission? Are there any disadvantages to parallel transmission? How long will it take to transmit two 8 bit binary strings using both serial and parallel if the clock frequency is 25 MHz?

15. Switches in Electronic Circuits Make or break connections between conductors Manual switches Electromechanical relays Semiconductor devices Diodes Transistors Manual Switches - ideal resistances: ON - 0 W OFF - ? ?

16. A Diode as a Switch Semiconductor Current in one direction only Forward-biased Anode more positive than cathode Current Reverse-biased Anode equal or more negative than cathode No current

17. A Diode as a Switch Analogous to a water check valve Not a perfect short 0.7 V across its terminals

18. A Transistor as a Switch Bipolar junction transistor (BJT) Three-terminal component An input signal at one terminal generates a short or open between the other two terminals Terminals: base, emitter, and collector Two types NPN PNP

19. A Transistor as a Switch NPN 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 (collector-to-emitter) Transistor is said to be OFF

20. A Transistor as a Switch PNP Negative base-to-emitter voltage shorts the transistor output (collector-to-emitter) Transistor is said to be ON Positive voltage (or 0 V) base-to-emitter opens the transistor output (collector-to-emitter) Transistor is said to be OFF

21. Discussion Points Name the three pins (leads) of a transistor. Describe how to turn an NPN transistor ON. Describe how to turn a PNP transistor ON. Example 2-11, 2-12

22. The TTL Integrated Circuit Transistor-Transistor Logic (TTL) Inverter The output is the complement of the input. (Converts 1 to 0 and vice-versa) TTL Integrated Circuit Totem-pole output

23. The TTL Integrated Circuit 7404 Hex Inverter Six complete logic circuits on a single silicon chip 14 pins, 7 on a side DIP (dual-in-line package) Pin configuration

24. The CMOS Integrated Circuit Complementary Metal Oxide Semiconductor Low power consumption Useful in battery-powered devices Slower switching speed than TTL Sensitive to electrostatic discharge NC = Not Connected

25. Surface-Mount Devices SMD Reduced size and weight Lowered cost of manufacturing circuit boards Soldered directly to metal footprint Special desoldering tools and techniques Chip densities increased Higher frequencies

26. Surface-Mounted Devices SO (Small Outline) Dual-in-line package; gull-wing format Lower-complexity logic PLCC (Plastic Leaded Chip Carrier) Leads on all four sides; J-bend configuration More complex logic BGA (Ball Grid Array) Higher pin counts than PLCC or SO Connections made through round solder tabs on the bottom of the component.

27. Discussion Points What are some key characteristics of: TTL devices CMOS devices Surface mount devices From a technician�s standpoint, is there a problem troubleshooting and repairing SMD based equipment?

28. 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.

29. 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.

30. 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.

31. 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.

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

33. Review Questions P31 P35 P42 P46 P53