--- title: "Seven segment display and clock counter" subject: "Embedded System" chapter: 9 key: "semester_6/embedded_system:seven-segment-clock-design" source: https://www.bctengineer.com/q/semester_6/embedded_system/seven-segment-clock-design site: BCT Engineering --- # Seven segment display and clock counter Embedded System · Chapter 9: Microcontrollers in Embedded Systems - Asked in: 2075 Bhadra, 2+6 marks - Video: [How 7-Segment Display Works | 7 segment Interfacing | 7 Segment operation | 7-Segment Display Basics](https://www.youtube.com/watch?v=-0lZup0CA4g) — Embedded System Creations - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/seven-segment-clock-design - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-9 ## Question What is seven segment display and write its types. Design a circuit with 7 segments display which is used as a counter watch which display second and minute. ## Short answer A seven-segment display uses seven LEDs (a–g) to show digits 0–9; types are common cathode, common anode, and static or multiplexed drive. The counter watch uses four multiplexed common-cathode digits (MM:SS) driven by an 8051: Timer 0 interrupts every 5 ms to refresh one digit via shared segment bus on P1 and NPN transistors on P3.0–P3.3; seconds and minutes increment in the ISR. ## Answer #### Seven-segment display A seven-segment display is a package of seven LEDs (segments a-g, plus a decimal point) arranged in the shape of an 8, used to show the digits 0-9 and some letters. ```text --a-- f| |b --g-- e| |c --d-- .dp ``` **Types:** 1. **Common cathode:** all LED cathodes are joined and grounded; a segment lights when its pin is 1. 2. **Common anode:** all anodes are joined to +5 V; a segment lights when its pin is 0. 3. By drive method: *static* (one decoder/port per digit) and *multiplexed* (the digits share the segment lines and are turned on one at a time). #### Design of a minutes:seconds counter Use four common-cathode digits (MM:SS) with multiplexing, so only 7 + 4 port pins are needed. ```text 8051 P1.0-P1.6 --[330R]--+--- a..g of all 4 digits (parallel) P3.0 -[1k]-NPN-- cathode, digit 0 (seconds units) -- GND P3.1 -[1k]-NPN-- cathode, digit 1 (seconds tens) -- GND P3.2 -[1k]-NPN-- cathode, digit 2 (minutes units) -- GND P3.3 -[1k]-NPN-- cathode, digit 3 (minutes tens) -- GND ``` Each NPN transistor (e.g. BC547) sinks the common-cathode current of one digit; a 1 on P3.x enables that digit. All digits share the segment bus on P1 and are shown one after another, each for 5 ms (so each digit is refreshed every 20 ms and looks steady). **Operation:** Timer 0 interrupts every 5 ms. The interrupt routine puts the pattern of the next digit on P1 and enables that digit. After 200 interrupts (1 s) the seconds count is incremented; at 60 s it resets and the minutes are incremented; at 60 min both reset. ```c #include unsigned char code seg[10] = {0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F}; unsigned char sec = 0, min = 0; /* time */ unsigned char idx = 0; /* digit being refreshed */ unsigned int ticks = 0; /* 5 ms tick counter */ void timer0_isr(void) interrupt 1 /* Timer 0 overflow every 5 ms */ { unsigned char d; TH0 = 0xEC; TL0 = 0x9C; /* reload: 65536 - 5000 + 36 (ISR delay) = EC9CH */ P3 = 0x00; /* blank all digits first */ switch (idx) { /* pick the digit value */ case 0: d = sec % 10; break; case 1: d = sec / 10; break; case 2: d = min % 10; break; default: d = min / 10; break; } P1 = seg[d]; /* segment pattern on Port 1 */ P3 = 1 << idx; /* turn on one digit (P3.0 - P3.3) */ idx = (idx + 1) & 3; /* next digit, 0..3 */ if (++ticks == 200) { /* 200 x 5 ms = 1 s */ ticks = 0; if (++sec == 60) { sec = 0; if (++min == 60) min = 0; } } } void main(void) { TMOD = 0x01; /* Timer 0, mode 1 */ TH0 = 0xEC; TL0 = 0x78; ET0 = 1; EA = 1; /* enable Timer 0 interrupt */ TR0 = 1; /* start timer */ while (1); /* everything is done in the ISR */ } ``` Checked in a simulator: the minutes count increased after 60.0 s. The reload value includes 36 extra counts to compensate for the time between the overflow and the reload in the compiler-generated routine; without it each tick would last about 5036 cycles and the clock would run slow (lose time) by about 0.7 %. A 12 MHz crystal is assumed; a 32.768 kHz RTC chip gives better long-term accuracy. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the difference between common cathode and common anode displays? A. Cathodes joined to ground, lit by 1; anodes joined to +5 V, lit by 0 B. Both light with 1 C. Anode types light with 1 D. Cathode types need no resistor Answer: A. Common cathode lights a segment with 1; common anode lights with 0. (Key point: Display types) 2. Why is multiplexing used for the four-digit MM:SS clock? A. Digits share the segment bus and are lit one at a time, saving port pins B. It makes digits brighter C. Each digit needs its own timer D. It removes the need for transistors Answer: A. Only 7 segment lines plus 4 digit-select lines are needed instead of 28. (Key point: Multiplexing) 3. What is the NPN transistor on each digit used for? A. Generating the clock B. Sinking the common-cathode current of that digit C. Storing the digit D. Decoding BCD Answer: B. A 1 on the P3.x pin turns the transistor on, grounding that digit's common cathode. (Key point: Role of transistors) 4. With a 5 ms Timer 0 interrupt, how many interrupts make 1 second? A. 60 B. 1000 C. 20 D. 200 Answer: D. 1 s / 5 ms = 200 interrupts. (Key point: Counting seconds) ## Related in Embedded System - [BCD counter 0-9999 on seven segment VHDL](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Avhdl-bcd-counter-seven-segment) — Embedded System, chapter 9 - [Display 99 to 00 on seven segment](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-seven-segment-99-00) — Embedded System, chapter 9 - [Display 0 to 9 on seven segment](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-seven-segment-0-9) — Embedded System, chapter 9, asked 2× ## Related in other subjects - [BCD-to-seven-segment decoder, segments a and e](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-seven-segment-a-e) — Digital Logic, chapter 4 - [Seven-segment "b" and "d" segments](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ab-d-segments) — Digital Logic, chapter 4 - [DEMUX vs decoder; seven-segment "b" segment](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ademux-decoder-b-segment) — Digital Logic, chapter 4 - [Seven-segment "a" segment using NOR gates](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aa-segment-nor-gates) — Digital Logic, chapter 4