Digital Electronics: Principles, Devices and Applications

Flip-Flops and Related Devices 403+V21(a)(b)+V0Bounce(c)Figure 10.47Switch bounce phenomenon.+V volts for a few milliseconds owing to contact bounce before it finally settles at +V volts. Similarly,when it is moved from position 2 back to position 1, it makes several transitions before coming torest at 0 V. Although this random behaviour lasts only for a few milliseconds, it is unacceptable formany digital circuit applications. A NAND or a NOR latch can solve this problem and provide a cleanoutput transition. Figure 10.48 shows a typical switch debounce circuit built around a NAND latch.The circuit functions as follows.When the switch is in position 1, the output is at a ‘0’ level. When it is moved to position 2, theoutput goes to a ‘1’ level within a few nanoseconds (depending upon the propagation delay of theNAND gate) after its first contact with position 2. When the switch contact bounces, it makes andbreaks contact with position 2 before it finally settles at the intended position. Making of contact