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Multivibrators and the 555 Timer
Chapter 14 Multivibrators and the 555 Timer William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Multivibrators Changes between two digital levels Three types
continuous, free-running on demand Three types bistable (S-R flip-flop) astable monostable (one shot) William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Capacitor Charge and Discharge Rates
RC circuit See Figure 14-1 charging curve discharging curve Exponentially changing - time constant () Solving the equation for t - See Equation 14-2 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-1 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Equation 14-2 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Astable Multivibrators
Single Schmitt Inverter and an RC circuit See Figure 14-5 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-5 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Monostable Multivibrators
See Figure 14-8 block diagram waveforms Built from NAND gates See Figure 14-9 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-8 Figure 14-9 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-12 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-15 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-16 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-17 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-20 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-23 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Monostable Operation of the 555 IC Timer
See Figure 14-24 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-24 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 14-28 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Figure 14-28 William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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Summary The 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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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