EET 1131 Unit 2 Digital Signals and Switches Read Kleitz, Sections 2-1 to 2-5, and Section 2-12. Skim the rest of Chapter 2. Homework #2 and Lab #2 due next week. Quiz next week. -Handouts: Quiz 1, period/freq practice sheet; relay for demo; null-modem cables -Collect HW and Lab. -Do Quiz #1

Digital Signal In digital circuits, the logic level at any point usually changes as time passes. This changing level is called a digital signal (or digital waveform).

Graph of a Digital Signal We can describe a digital signal by graphing the voltage (or logic level) versus time.

Oscilloscope For slow-changing signals, you could graph a digital signal by hand, using a logic probe and a wristwatch. But usually we use an oscilloscope to do this work for us.

Periodic Digital Signal A periodic digital signal is one that repeats itself at regular intervals. Which one of these signals is periodic?

Cycle In a periodic signal, each repetition is called a cycle. How many cycles are shown in the diagram below?

Period The time required for one cycle is called the signal’s period. The symbol for period is tp. (Some books use T.) Period is measured in seconds, abbreviated s. Example: If a signal repeats itself every 3 seconds, we’d write tp = 3 s

Frequency A signal’s frequency is the number of cycles that occur in one second. The symbol for frequency is f. Frequency is measured in hertz, abbreviated Hz. Some old-timers say “cycles per second” instead of “hertz.” Example: If a signal repeats itself 20 times every second, we’d write f = 20 Hz

Period and Frequency Period and frequency are the reciprocal of each other: f = 1 / tp tp = 1 / f What are the period and frequency of this signal? Do 1st half of frequency/period practice sheet.

Large and Small Numbers Engineers and technicians often deal with very large or very small numbers. Example: a system’s clock signal might have a frequency of 750,000 Hz and a period of 0.00000133 s. It’s not convenient to write or discuss numbers using so many zeroes. Instead we use engineering prefixes, which are abbreviations for certain powers of 10.

Engineering Prefix Game You must memorize these prefixes. To practice, play the Metric Prefix matching game on my Games page.

Using Engineering Prefixes Whenever you have a number that’s greater than 1000 or less than 1, you should use these prefixes. Examples: Instead of writing 750,000 Hz, write 750 kHz (pronounced “750 kilohertz”). Instead of writing 0.00000133 s, write 1.33 s (pronounced “1.33 microseconds”).

Calculator’s Exponent Key Scientific calculators have an exponent key (usually labeled EE, EXP, or E) that lets you easily enter numbers with engineering prefixes. Examples: To enter 750 k, press 750 EE 3. To enter 1.33 , press 1.33 EE −6.

Calculator Modes Most scientific calculators also have an engineering mode, which forces the answer always to be displayed with one of the engineering powers of 10. Learn how to use this feature of your calculator. It will save you from making mistakes. Do 2nd half of frequency/period practice sheet.

Function Generator To produce a periodic digital signal, you could use a switch that you flip up and down by hand at regular intervals. But usually we use a function generator to do this work for us.

Trainer Function Generator Regular Output, controlled by all four knobs. You’ll use this in other courses. No matter which one of these you use, you must also use the GROUND connection. Demo measuring a waveform on the scope, explaining how the RANGE switch and FREQUENCY knob determine the frequency. Also demo Fluke 45 as freq counter. TTL Mode Output, controlled by the FREQUENCY and RANGE knobs. In this course we’ll always use this one.

The Function Generator Does Not Replace the Power Supply When we build circuits on the breadboard, we must power each DIP by providing +5 V to the DIP’s power pin. We never connect a DIP’s power pin to the function generator. We connect every DIP’s power pin to the trainer’s power supply instead.

Digital Waveforms Digital waveforms change between the LOW and HIGH levels. A positive going pulse is one that goes from a normally LOW logic level to a HIGH level and then back again. Digital waveforms are made up of a series of pulses.

Timing Diagrams A timing diagram (or waveform diagram) is used to show the relationship between two or more digital waveforms. -Number the clock cycles, and ask them whether A, B, C are high or low during particular cycles.

Serial and Parallel Data Data can be transmitted by either serial transfer or parallel transfer.

Serial Transmission Uses a single electrical conductor for data, so it’s inexpensive. Slow, since only one bit for each clock period. Used for telephone lines, computer-to-computer networks. Computer COM ports and USB ports are used for serial communications. Ethernet plug-in cards are for serial networking.

Serial Representation of a Binary Number A serial representation of the binary string 01101100 is illustrated below. Note that only one bit is transmitted per clock period.

Parallel Transmission Separate electrical conductor for each bit, so more expensive than serial. Very fast Inside a computer Data bus External Devices Centronics printer interface (LPT1) SCSI (Small Computer Systems Interface)

Serial versus Parallel Transmission Question 1: How long will it take to transmit an 8-bit binary string using serial transmission if the clock frequency is 25 MHz? Question 2: How long will it take to transmit an 8-bit binary string using 8-bit parallel transmission if the clock frequency is 25 MHz?

Ports on a Typical Laptop Computer RS-232 (Serial) USB (Serial) PS/2 Mouse (Serial) Have them ID and distinguish male/female. IEEE 1394 (Serial) Printer (Parallel) VGA (Analog video)

Ports on a Fluke 45 DMM IEEE 488 (“GPIB”) option not installed (Parallel) RS-232 (Serial) -Have them use hyperterminal & serial cable to let PC control meter. Connect serial cable. Meter setup: 2nd Rate, 9600, n, on. Connect meter’s leads to func gen set for half-max amplitude & 5 kHz. Hyperterminal setup: COM1, 9600, 8, N, 1, XON/XOFF. Commands: *IDN?, VAC, FREQ2, VAL?

Ports on a Tektronix TDS2014 RS-232 (Serial) Printer (Parallel) -Have them use hyperterminal & serial cable to let PC control scope. Connect serial cable. Scope setup: Utility> Options> RS232> 19200, Hard flagging, LF/CR, None. Connect probe to compensation terminal. Hyperterminal setup: COM1, 19200, 8, N, 1, HARDWARE. Use Notepad to create text file: *IDN? Autoset execute measurement:immed:source ch1 measurement:immed:type freq measurement:immed:value? (MUST PRESS ENTER AFTER THE LAST COMMAND LINE) Save file & send it from Hypterminal using Transfer>Send Text File IEEE 488 (“GPIB”) (Parallel)

Switches A digital circuit is basically a combination of a huge number of switches. Each switch is either on or off at any given time. Unlike manual switches that require a person to switch them, we need automatic switches that can be turned on or off by the voltage level (HIGH or LOW) present at the switch’s control input.

Electromechanical Relays Before the invention of semiconductor devices in the mid-1900s, the electromechnical relay was the main type of automatic switch. Still used in industrial applications, where large currents are involved. We won’t use any relays in this course. But demo a 12-V relay, connecting one V terminal to GND, +5 V to one of the C terminals, and the NO terminal to a trainer LED. Then touch 12 V to the other V terminal to activate relay.

Electronic Switches Relays are slow, large, and expensive compared to modern electronic switches (diodes and transistors). In past decades, some digital electronics used diodes as switches, but today almost all digital electronics is based on using transistors as switches.

Two Kinds of Transistors In Electronic Devices & Circuits (EET 2201) you’ll study two major classes of transistors: Bipolar Junction Transistors (BJTs) Metal-Oxide Semiconductor Field Effect Transistor (MOSFETs)

Logic Families Two major logic families: TTL (Transistor-Transistor Logic) based on bipolar junction transistors CMOS (Complementary Metal Oxide Semiconductor) based on MOSFETs Within each family are several subfamilies; we’ll study these in Chapter 9. Originally, TTL chips were fast but used lots of power, and CMOS chips used little power but were slow. CMOS chips are sensitive to static discharge, and must be handled carefully. 33

7400 Series and 4000 Series A popular series of TTL chips is the 7400 series that you’ll use in Sinclair’s digital courses: Wikipedia's list A popular series of CMOS chips is the 4000 series: Wikipedia's list 34