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

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.

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