chapter 14 multivibrators and the 555 timer william kleitz digital electronics with vhdl, quartus®...
TRANSCRIPT
Chapter 14
Multivibrators and the 555 Timer
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Multivibrators
• Changes between two digital levels– continuous, free-running– on demand
• Three types– bistable (S-R flip-flop)– astable– monostable (one shot)
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Capacitor Charge and Discharge Rates
• RC circuit
• See Figure 14-1– RC circuit– charging curve– discharging curve
• Exponentially changing - time constant ()
• Solving the equation for t - See Equation 14-2
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-1
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Equation 14-2
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Astable Multivibrators
• Single Schmitt Inverter and an RC circuit
• See Figure 14-5
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-5
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Monostable Multivibrators
• See Figure 14-8– block diagram– waveforms
• Built from NAND gates– See Figure 14-9
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-8
Figure 14-9
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
IC Monostable Multivibrators
• 74121– nonretriggerable– connect RC components for proper pulse width– two active-LOW trigger inputs– one active-HIGH trigger input– See Figure 14-12
• block diagram
• function table
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-12
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Retriggerable Monostable Multivibrators
• 74123– new timing cycle each time new trigger applied– See Figure 14-15
• comparison waveforms
– See Figure 14-16• logic symbol
• function table
– See Figure 14-17• component selection chart
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-15
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-16
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-17
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Astable Operation of the 555 IC Timer
• One shot or astable oscillator
• Voltage divider
• Comparators
• S-R flip-flop
• Discharge transistor
• See Figure 14-20
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-20
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Astable Operation of the 555 IC Timer
• 50% Duty Cycle Astable Oscillator– RA cannot = 0 ohms
– RA = RB and short RB with a diode
– See Figure 14-23
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-23
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Monostable Operation of the 555 IC Timer
• See Figure 14-24
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-24
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Crystal Oscillators
• Quartz crystal• Size and shape determine specific frequency• Accurate to more than five significant digits• Integrated circuit packages or use external
quartz crystal• 74S124
– voltage controlled oscillator
• See Figure 14-28William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 14-28
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• Multivibrator circuits are used to produce free-running clock oscillator waveforms or to produce a timed digital level change triggered by an external source.
• Capacitor voltage charging and discharging rates are the most common way to produce predictable time duration for oscillator and timing operations.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• An astable multivibrator is a free-running oscillator whose output oscillates between two voltage levels at a rate determined by an attached RC circuit.
• A monostable multivibrator is used to produce an output pulse that starts when the circuit receives an input trigger and lasts for a length of time dictated by the attached RC circuit.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• The 74121 is an IC monostable multivibrator with two active-LOW and one active-HIGH input trigger sources and an active-HIGH and an active-LOW pulse output terminal.
• Retriggerable monostable multivibrators allow multiple input triggers to be acknowledged even if the output pulse from the previous trigger had not expired.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• The 555 IC is a general-purpose timer that can be used to make astable and monostable multivibrators and perform any number of other timing functions.
• Crystal oscillators are much more accurate and stable than RC timing circuits. They are used most often for microprocessor and digital communication timing.
William KleitzDigital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.