--- title: "Combinational vs sequential; latch as bounce eliminator" subject: "Digital Logic" chapter: 6 key: "semester_3/digital_logic:latch-bounce-eliminator" source: https://www.bctengineer.com/q/semester_3/digital_logic/latch-bounce-eliminator site: BCT Engineering --- # Combinational vs sequential; latch as bounce eliminator Digital Logic · Chapter 6: Flip Flops - Asked in: 2076 Asoj, 2+4 marks - Video: [Latches and Flip-Flops 1 - The SR Latch](https://www.youtube.com/watch?v=-aQH0ybMd3U) — Computer Science Lessons - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/latch-bounce-eliminator - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-6 ## Question Differentiate between combination and sequential circuit. Explain briefly how latch can be used as bounce eliminator. ## Short answer A NAND S'-R' latch with an SPDT switch eliminates contact bounce. When the switch moves, the first contact sets or resets the latch; subsequent bounces leave S'=R'=1 (hold), so output makes a single clean transition. ## Answer #### Combinational vs sequential circuit | Point | Combinational circuit | Sequential circuit | |---|---|---| | Output depends on | Present inputs only | Present inputs and past state | | Memory | No memory element | Has memory (latches, flip-flops) | | Feedback | No feedback path | Output fed back to input | | Clock | Not needed | Usually needed (synchronous) | | Described by | Truth table | State table / state diagram | | Examples | Adder, MUX, decoder | Flip-flop, register, counter | #### Latch as a switch bounce eliminator **Contact bounce:** when a mechanical switch is thrown, the metal contact hits the new terminal and bounces several times for a few milliseconds before it settles. A logic input connected directly to the switch therefore sees a burst of 0-1-0-1 pulses instead of one clean change. A counter would count every bounce. A **NAND S'-R' latch** with a single-pole double-throw (SPDT) switch removes these pulses: ```text +5V +5V | | [R1] [R2] | | S' <---+---o A B o---+---> R' \ / \___o____/ switch arm | (touches A or B) GND +---------------+ S' ----->| NAND S'-R' |---> Q R' ----->| latch |---> Q' +---------------+ ``` **How it works** 1. With the switch at position A, S' = 0 and R' = 1 (pulled up by R2). The latch is **set**: Q = 1. 2. When the switch is moved towards B, it first leaves A. Now S' = R' = 1 (both pulled up): the latch is in the **hold** state and Q stays 1. 3. The first time the contact touches B, R' = 0 and the latch **resets**: Q = 0. 4. The contact now bounces: it leaves B (R' = 1, hold, Q stays 0) and touches B again (reset again, Q stays 0). The bounce never reaches A, so S' never becomes 0. 5. Hence Q makes a **single clean transition** from 1 to 0 at the first contact. Moving the switch back to A gives a single clean 0-to-1 transition in the same way. ```text R' (at B) ‾‾‾‾‾|_|‾|_|‾|________ Q ‾‾‾‾‾|________________ first touch: one clean edge ``` The latch works because its "hold" condition (S' = R' = 1) ignores the open-contact gaps during bouncing, while the first contact already set the final state. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a sequential circuit differ from a combinational circuit? A. It has no feedback B. It never needs a clock C. Output depends on present inputs only D. Output depends on present inputs and past state; it has memory Answer: D. Sequential circuits have memory elements, such as flip-flops, so past state matters. (Key point: Comparison) 2. Why is bounce a problem for a counter? A. It counts every bounce pulse B. It slows the clock C. It resets the counter D. It changes the supply Answer: A. Bounce makes a burst of 0-1 pulses, each counted as a separate event. (Key point: Bounce problem) 3. Why does the NAND S'-R' latch ignore bounce contact gaps? A. It uses the clock B. S' = R' = 1 is the hold state C. The latch resets at every gap D. S' = R' = 0 is the hold state Answer: B. While the contact is open, both pulled-up inputs are 1, so the latch holds. (Key point: Latch hold) 4. Moving the switch from A to B, what happens at the first touch of B? A. The latch toggles on each bounce B. Q' stays 0 C. R' = 0, so the latch resets (Q = 0) and bounce afterwards does not change it D. S' = 0, so Q = 1 Answer: C. R'=0 resets Q to 0; bounces return to hold, and the contact never reaches A. (Key point: First contact) ## Related in Digital Logic - [Combinational vs sequential; BCD-to-decimal decoder](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-to-decimal-decoder) — Digital Logic, chapter 4 - [Define combinational logic circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acombinational-circuit-define) — Digital Logic, chapter 3 - [Full subtractor using decoder; combinational circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afull-subtractor-decoder-combinational) — Digital Logic, chapter 4 - [Two-input sequential circuit using T](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Atwo-input-sequential-circuit-t) — Digital Logic, chapter 9 - [Problems in asynchronous sequential design](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aasynchronous-sequential-problems) — Digital Logic, chapter 9 ## Related in other subjects - [Combinational vs sequential; JK flip-flop](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomb-vs-seq-jk) — Basic Electronics Engineering, chapter 6 - [Sequential vs binary search](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Asequential-vs-binary-search) — Data Structure and Algorithm, chapter 8 - [Ground bounce, decoupling and crosstalk](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-decoupling-crosstalk) — Instrumentation II, chapter 7 - [Ground bounce and crosstalk with reduction ways](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-crosstalk-reduce) — Instrumentation II, chapter 7