chapter 14 multivibrators and the 555 timer william kleitz digital electronics with vhdl, quartus®...

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)




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




Figure 14-1




Equation 14-2




Astable Multivibrators

• Single Schmitt Inverter and an RC circuit

• See Figure 14-5




Figure 14-5




Monostable Multivibrators

• See Figure 14-8– block diagram– waveforms

• Built from NAND gates– See Figure 14-9




Figure 14-8

Figure 14-9




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




Figure 14-12




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




Figure 14-15




Figure 14-16




Figure 14-17




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




Figure 14-20




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




Figure 14-23




Monostable Operation of the 555 IC Timer

• See Figure 14-24




Figure 14-24




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



Figure 14-28




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.




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.




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.




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.




chapter 14 multivibrators and the 555 timer william kleitz digital electronics with vhdl, quartus®...

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