Waveform 1.1 Basic Digital Waveform Parameters 1 Paul Godin Updated December 2014

Waveform 1.2 Digital Waveform Logic 1 Logic 0 Ideal Waveform

Measuring a Signal ◊A static logic state is measured with a Logic Probe ◊A probe will display the logic state. Typically: ◊Green = Logic Low ◊Red = Logic High Waveform 1.3 H L Logic Probe Vcc

Floating Inputs ◊A floating input is neither a logic high or low. It is not connected to a ground or a Vcc. ◊Floating inputs are an indication of a logic error and will create unpredictable results. Waveform 1.4 H L Logic Probe

Waveform 1.5 Waveform Parameters ◊Periodic: A waveform that recurs at regular intervals, that has repeating cycles. ◊Aperiodic: A waveform that is irregular, that has timing cycles that vary. Periodic Aperiodic Name examples of periodic and aperiodic waveforms

Waveform 1.6 Time Period: Time for one complete cycle. Symbol: T Units: seconds T 1 Cycle

Waveform 1.7 Time Frequency (f) = 1/ T T Frequency applies to periodic signals

Waveform 1.8 Time Time High (t H ) and Time Low (t L ) tHtH tLtL T

Waveform 1.9 Duty Cycle Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse. Typically expressed in percent. T tHtH tLtL What is the approximate Duty Cycle of this waveform?

Waveform 1.10 Exercise 1 What is the approximate duty cycle of the following waveforms?

Waveform 1.11 Exercise 2 ◊What is the pulse width of a signal, given: ◊T = 2 seconds ◊DC = 50% ◊What is the pulse width of a signal, given: ◊T = 4 seconds ◊DC = 75% ◊What is the duty cycle of a signal, given: ◊T = 2 seconds ◊Pw (pulse width) = 0.5 seconds

IC Families

Logic Families ◊Logic families are devices that share the same electrical and other performance properties. ◊Texas Instruments lists over 40 logic families. ◊There are large varieties of IC families to cover a multitude of electronics applications, such as: ◊Low voltage ◊Very low current ◊High speed ◊Etc… Waveform 1.13

Families of IC’s : Transistor-based ◊TTL (Transistor-Transistor Logic) ◊Made of transistors ◊Transistors use current to switch ◊Fast and rugged ◊Supplies more current but needs current to operate (current = power & heat) Waveform 1.14 B C E

Families of IC’s: MOSFET ◊CMOS (Complimentary Metal Oxide Semiconductor) ◊Made of MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) ◊MOSFETs use voltage to switch ◊Very low power (little current) ◊Sensitive to static electricity Waveform 1.15 S G D

Chips ◊The logic devices we will be using are housed in chips. ◊Chips come in a large variety of: ◊sizes ◊packaging styles ◊Logic families ◊Logic functions ◊See Lab 2 Notes for more information on chips used in our labs Waveform 1.16

Waveform 1.17 Voltage ◊The TTL devices used in this course are based on 5 Volt / 0 Volt logic. ◊Other devices may function on different values: ◊Most new logic designs use 3.3 Volt ◊More advanced designs are looking at lower voltages ◊Communication systems may use higher voltages, and may even use negative voltages

Waveform 1.18 Logic Values The input voltage levels must fall within a range. 5 V 2.0 V 0.8 V 0 V Logic Low Logic High Undefined TTL Voltage – Input logic values

Waveform 1.19 Typical Waveform ◊Due to the effects of Inductance, Capacitance, noise, grounding, device properties and other factors, digital signals tend to be electrically less than perfect. These negative effects usually increase with frequency. Typical Waveform Over-shoot Pre-shoot Ringing Rise time Fall time Droop

Waveform 1.20 Logic Values with Typical Waveform 5 V 2.0 V 0.8 V 0 V Logic Low Logic High Undefined

Exercise 3 ◊If an input of a device is not connected to anything what is its logic state? ◊If the output of a 5-Volt “TTL” logic device is 2.8 Volts, what is the logic state of the output? Waveform 1.21

Waveform 1.22 End gmail.com