Chapter 14 Multivibrators and the 555 Timer William

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 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 • 74 S 124 – 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 07458 All rights reserved.

Figure 14 -28 William Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital 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 Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright © 2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.