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Transcript
Basic Digital Waveform Parameters 1 Paul Godin Updated December 2014 Waveform 1.1 Digital Waveform Ideal Waveform Logic 1 Logic 0 Waveform 1.2 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 Vcc Waveform 1.3 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 Waveform Parameters ◊ Periodic: A waveform that recurs at regular intervals, that has repeating cycles. Periodic ◊ Aperiodic: A waveform that is irregular, that has timing cycles that vary. Aperiodic Name examples of periodic and aperiodic waveforms Waveform 1.5 Time Period: Time for one complete cycle. Symbol: T Units: seconds T 1 Cycle Waveform 1.6 Time Frequency (f) = 1/T T Frequency applies to periodic signals Waveform 1.7 Time Time High (tH) and Time Low (tL) T tH tL Waveform 1.8 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 tH tL tH D.C. 100% tH tL What is the approximate Duty Cycle of this waveform? Waveform 1.9 Exercise 1 What is the approximate duty cycle of the following waveforms? Waveform 1.10 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 Waveform 1.11 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) C B E Waveform 1.14 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 S G D Waveform 1.15 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 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 Voltage Waveform 1.17 Logic Values The input voltage levels must fall within a range. 5V Logic High 2.0 V Undefined 0.8 V Logic Low 0V TTL Voltage – Input logic values Waveform 1.18 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. Over-shoot Ringing Pre-shoot Droop Typical Waveform Waveform 1.19 Logic Values with Typical Waveform 5V Logic High 2.0 V Undefined Logic Low 0.8 V 0V Waveform 1.20 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 End prgodin @ gmail.com Waveform 1.22