Transcript Basic Digital Signals 1

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