--- title: "Seconds section of a digital clock" subject: "Digital Logic" chapter: 8 key: "semester_3/digital_logic:digital-clock-seconds-section" source: https://www.bctengineer.com/q/semester_3/digital_logic/digital-clock-seconds-section site: BCT Engineering --- # Seconds section of a digital clock Digital Logic · Chapter 8: Counters - Asked in: 2072 Chaitra, 6 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-seconds-section - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-8 ## Question How does second section of a digital clock work? Explain its working principle using block diagram. ## Short answer The seconds section is a mod-60 counter: a mod-10 units counter (e.g., 7490) followed by a mod-6 tens counter, clocked by 1 Hz pulses. Each counter's BCD output drives a 7447 decoder to 7-segment displays. When the tens counter resets at 6, it provides a carry pulse to the minutes section. ## Answer The **seconds section** of a digital clock counts 1 Hz pulses from 00 to 59 and sends one pulse per minute to the minutes section. It is a **mod-60 counter** made of a mod-10 (units) counter followed by a mod-6 (tens) counter, with decoders and displays. #### Block diagram ```text 1 Hz pulses (from ÷50 divider of 50 Hz mains) | v +-----------+ Q3 (falls 9->0) +-----------+ | MOD-10 |------------------>| MOD-6 |--> 1 pulse | units | | tens | per minute | (7490) | | (reset at | to minutes +-----------+ | 0110) | | | | | BCD +-----------+ +-----------+ | | | | 7447 | +-----------+ | decoder | | 7447 | +-----------+ +-----------+ |a..g |a..g [7-seg units] [7-seg tens] ``` #### Working principle 1. **1 Hz input:** the 50 Hz mains is shaped into pulses and divided by 50 (÷10 then ÷5) to give 1 pulse per second. 2. **Units counter (mod-10):** a decade counter such as 7490 counts 0, 1, …, 9 on successive seconds. On the 10th pulse it returns to 0; at this moment its MSB $Q_3$ goes from 1 to 0. 3. **Tens counter (mod-6):** the falling edge of the units $Q_3$ clocks the tens counter, so it advances once every 10 seconds: 0, 1, …, 5. It is a 3-bit ripple counter reset at 6 (0110) by a NAND gate of $Q_2$ and $Q_1$ (or a 7492 wired as ÷6). 4. **Carry to minutes:** when the display shows 59 and the next pulse arrives, units go 9 → 0 and tens go 5 → 6 → 0. The falling of the tens counter's $Q_2$ at this reset gives one pulse per minute, which clocks the minutes units counter. 5. **Decoding and display:** each counter's BCD outputs drive a BCD-to-7-segment decoder/driver (7447 for common-anode displays). The decoder lights segments a–g to show the digit, so the seconds read 00, 01, …, 59, 00. #### Count sequence | Pulses | Tens (BCD) | Units (BCD) | Display | |---|---|---|---| | 0 | 000 | 0000 | 00 | | 9 | 000 | 1001 | 09 | | 10 | 001 | 0000 | 10 | | 59 | 101 | 1001 | 59 | | 60 | 000 | 0000 | 00 (+1 minute) | The minutes section is built exactly the same way; only the hours section uses different (mod-12 or mod-24) logic. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the seconds section consist of? A. A mod-10 units counter followed by a mod-6 tens counter, forming mod-60 B. A single mod-12 counter C. A mod-6 units and mod-10 tens counter D. Two mod-10 counters Answer: A. Units count 0-9 and tens count 0-5, giving 60 seconds. (Key point: Structure of seconds section) 2. What clocks the tens counter? A. The 1 Hz pulse directly B. The falling edge of the units counter's Q3 (9 to 0) C. The decoder output D. The minutes counter Answer: B. When units go from 9 to 0, Q3 falls 1 to 0 and advances the tens counter once every 10 s. (Key point: Clocking the tens) 3. When does the tens counter reset, and what is produced? A. At 6 (0110); a NAND of Q2 and Q1 gives a reset and one pulse per minute B. At 5 (0101); one pulse per second C. At 9 (1001); one pulse per hour D. At 10; one pulse per 10 seconds Answer: A. At 6 (Q2 = Q1 = 1) the counter resets to 0, and the fall of Q2 gives one pulse per minute for the minutes section. (Key point: Tens reset) 4. What is the role of the 7447 in the seconds section? A. It counts the pulses B. It converts BCD to 7-segment signals a-g C. It resets the counter D. It divides 50 Hz by 50 Answer: B. The decoder/driver lights the correct segments to show each BCD digit. 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