--- title: "Operation of a digital clock" subject: "Digital Logic" chapter: 8 key: "semester_3/digital_logic:digital-clock-operation" source: https://www.bctengineer.com/q/semester_3/digital_logic/digital-clock-operation site: BCT Engineering --- # Operation of a digital clock Digital Logic · Chapter 8: Counters - Asked in: 2078 Kartik, 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/digital-clock-operation - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-8 ## Question Explain the operation of digital clock with neat and clean diagram. ## Short answer A digital clock receives a 1 Hz signal from a frequency divider (e.g., 50 Hz AC shaped and divided by 50). Seconds and minutes are mod-60 counters (mod-10 units, mod-6 tens). Hours use mod-10 units and flip-flop/reset logic for 12- or 24-hour display. BCD outputs drive 7-segment displays via 7447 decoders. ## Answer A **digital clock** displays hours, minutes and seconds in decimal digits. It works by counting a precise 1 Hz signal with a chain of mod-10 and mod-6 counters, decoding each counter's BCD output and showing it on seven-segment displays. #### Block diagram ```text AC 50 Hz -> [Shaper] -> [÷10] -> [÷5] -> 1 Hz | +--------------------------------------+ v SECONDS (00-59) [÷10 units] -----> [÷6 tens] -------+ 1 pulse/min | | | [7447 dec] [7447 dec] | [7-seg disp] [7-seg disp] | +---------------------------------+ v MINUTES (00-59) [÷10 units] -----> [÷6 tens] -------+ 1 pulse/h | | | [7447 dec] [7447 dec] | [7-seg disp] [7-seg disp] | +---------------------------------+ v HOURS (01-12) [÷10 units] -----> [tens FF] <--- reset/load logic | | [7447 dec] [decoder] [7-seg disp] [7-seg disp] ``` #### Operation 1. **Pulse shaping:** the 50 Hz AC mains is stepped down by a transformer and converted to a clean square wave by a Schmitt trigger (wave-shaping circuit). 2. **Frequency divider:** a ÷10 counter followed by a ÷5 counter divides 50 Hz down to 1 Hz (1 pulse per second). With 60 Hz mains a ÷60 is used; quartz clocks use a 32.768 kHz crystal and a 15-stage binary divider. 3. **Seconds counter:** the units counter (mod-10) counts 0–9. Each 9 → 0 change clocks the tens counter (mod-6), which counts 0–5. At 59 → 00 the tens counter resets and sends 1 pulse per minute onward. 4. **Minutes counter:** same as the seconds counter, giving 00–59 and 1 pulse per hour. 5. **Hours counter:** a mod-10 units counter and a tens flip-flop with reset logic so that the hours go 01 → 12 → 01 (12-hour clock); a 24-hour clock clears at 24. 6. **Decoder/drivers:** each BCD output drives a BCD-to-7-segment decoder (7447) that lights the right segments of each display. The accuracy of the clock depends only on the accuracy of the 1 Hz time base. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the role of the Schmitt trigger at the input? A. Divide the frequency by 50 B. Convert the stepped-down AC mains into a clean square wave C. Drive the displays D. Count the seconds Answer: B. The mains sine wave is turned into a clean square wave that the counters can use. (Key point: Pulse shaping) 2. What happens when the seconds go from 59 to 00? A. The tens counter resets and sends 1 pulse per minute to the minutes counter B. The hours counter resets C. The 1 Hz clock stops D. The decoder clears Answer: A. The tens counter (mod-6) resets and its carry provides one pulse per minute. (Key point: Seconds counter) 3. What decides the accuracy of a digital clock? A. The accuracy of the 1 Hz time base B. The number of displays C. The decoder type D. The mod-6 counter Answer: A. All counters just count pulses, so the clock is only as accurate as the 1 Hz source. (Key point: Accuracy) 4. How is 50 Hz reduced to 1 Hz? A. ÷10 followed by ÷5 B. ÷6 followed by ÷10 C. ÷60 D. ÷5 followed by ÷5 Answer: A. 10 x 5 = 50, giving 1 pulse per second. (Key point: Divider chain) ## Related in Digital Logic - [Seconds section of a digital clock](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-clock-seconds-section) — Digital Logic, chapter 8 - [24-hour digital clock](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-clock-24hr) — Digital Logic, chapter 8 - [12-hour digital clock](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-clock-12hr) — Digital Logic, chapter 8 - [Digital clock with decoders for h, min, s](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-clock-hour-minute-second) — Digital Logic, chapter 8 - [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14× - [SIPO with 1010 data up to 9th clock](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Asipo-1010-ninth-clock) — Digital Logic, chapter 7 ## Related in other subjects - [Digital clock state diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Adigital-clock-state-diagram) — Object Oriented Analysis and Design, chapter 2 - [Data logger block diagram and operation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adl-block) — Instrumentation II, chapter 5, asked 6× - [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4× - [Block diagram of digital communication system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ablock-diagram-digital-comm) — Data Communication, chapter 1, asked 8×