7.12 One-Shots

Chapter 7.12 One-Shots

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 7. DIGITAL CIRCUITS152insensitive to the slope or time spent in the high or low state.DELAYD∆tD∆tDFigure 7.24: A slow or delayed gate can be used to convert a level change into a short pulse.Both types of dynamic triggering are represented on a schematic diagram by a specialsymbol near the clock input (figure 7.25). In addition to the clock and data inputs most ICflip-flop packages will also include set and reset (or mark and erase) inputs. The additionalinputs allow the flip-flop to be preset to an initial state without using the clocked logic inputs.JKCSRQQ00↓ 11no change01↓ 110110↓ 111011↓ 11togglexxx0110xxx1001c)b)a)JJJKKKQQQNQNQNQCCCSRFigure 7.25: The schematic symbols for a) a positive edge-triggered JKFF, b) a negative(falling) edge-triggered JKFF and c) a negative edge-triggered JKFF with set and resetinputs.7.12One-ShotsThe one-shot (also called a monostable multivibrator) is essentially an unstable flip-flop.When a one-shot is set by an input clock or trigger pulse, it will return to the reset stateon its own accord after a fixed time delay. Hence a one-shot is able to generate a pulseof a particular width following an input pulse. One-shots are often used in pairs with theoutput of the first used to trigger the second. Unfortunately the time relationship betweenthe signals becomes excessively interdependent and it is better to generate signal transitionssynchronized with the circuit clock.