--- title: "Display 99 to 00 on seven segment" subject: "Embedded System" chapter: 9 key: "semester_6/embedded_system:8051-seven-segment-99-00" source: https://www.bctengineer.com/q/semester_6/embedded_system/8051-seven-segment-99-00 site: BCT Engineering --- # Display 99 to 00 on seven segment Embedded System · Chapter 9: Microcontrollers in Embedded Systems - Asked in: 2070 Magh, 8 marks - Video: [Counting from 0 to 99 using 8051 microcontroller with 7 segment display](https://www.youtube.com/watch?v=7drkBVTWSkM) — Embedotronics Technologies - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/8051-seven-segment-99-00 - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-9 ## Question Write 8051 program and draw circuit diagram to display number from 99 to 00 in seven segment display. The program should write in both assembly and C. ## Answer #### Circuit Two common-cathode seven-segment displays: units digit on Port 1, tens digit on Port 2, each segment through a 330 ohm resistor. ```text 8051 Tens display Units display P2.0-P2.6 --[330R x7]--> a b c d e f g P1.0-P1.6 --[330R x7]------------------> a b c d e f g common cathode common cathode | | GND GND ``` The segment code for each digit (gfedcba, common cathode) is: 0=3F, 1=06, 2=5B, 3=4F, 4=66, 5=6D, 6=7D, 7=07, 8=7F, 9=6F. The count is kept as packed BCD in R0 and decremented using `ADD A,#99H` followed by `DA A` (adds 99H, which equals subtracting 1 in BCD). #### Assembly program ```asm ORG 0000H MAIN: MOV R0,#99H ; BCD count, start at 99 SHOW: MOV DPTR,#SEGTAB MOV A,R0 ANL A,#0FH ; units digit MOVC A,@A+DPTR MOV P1,A ; units display on Port 1 MOV A,R0 SWAP A ANL A,#0FH ; tens digit MOVC A,@A+DPTR MOV P2,A ; tens display on Port 2 ACALL DELAY ; about 0.5 s MOV A,R0 JZ MAIN ; after 00 start again from 99 ADD A,#99H ; A = A + 99H, then DA A gives BCD A - 1 DA A MOV R0,A SJMP SHOW DELAY: MOV R5,#4 D1: MOV R6,#250 D2: MOV R7,#250 D3: DJNZ R7,D3 DJNZ R6,D2 DJNZ R5,D1 RET SEGTAB: DB 3FH,06H,5BH,4FH,66H,6DH,7DH,07H,7FH,6FH ; common cathode codes END ``` Checked in a simulator: the sequence 99, 98, ... 90, 89, ... 00 is displayed, then restarts at 99. #### C program ```c #include unsigned char code seg[10] = {0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F}; void delay_ms(unsigned int ms) { unsigned int i, j; for (i = 0; i < ms; i++) for (j = 0; j < 120; j++); /* about 1 ms at 12 MHz */ } void main(void) { signed char n; /* must be signed: loop ends at -1 */ while (1) { for (n = 99; n >= 0; n--) { P2 = seg[n / 10]; /* tens digit on Port 2 */ P1 = seg[n % 10]; /* units digit on Port 1 */ delay_ms(500); } } } ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For a common-cathode display, how is a segment lit? A. Drive its pin low B. Drive its pin high (1) C. Connect to +5 V directly D. Remove the resistor Answer: B. The cathodes are grounded, so a 1 on a segment pin lights it. (Key point: Common cathode) 2. How does the assembly program get the segment pattern for a digit? A. MOVC A,@A+DPTR from the SEGTAB table B. Calculates it with MUL C. Reads it from P1 D. Uses DA A Answer: A. The digit is the index into SEGTAB read by MOVC A,@A+DPTR. (Key point: Segment code lookup) 3. How is the packed BCD count decremented? A. ADD A,#99H followed by DA A B. SUBB A,#1 C. DEC A only D. INC A then DA A Answer: A. Adding 99H and applying DA A is equivalent to subtracting 1 in BCD. (Key point: BCD decrement) 4. Why is n declared as signed char in the C program? A. To save memory B. Because the loop ends when n becomes -1, so n >= 0 fails C. For P2 D. For delay Answer: B. With unsigned n, n >= 0 is always true and n-- would wrap, so the loop would never end. (Key point: C loop variable) ## Related in Embedded System - [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× - [Seven segment display and clock counter](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aseven-segment-clock-design) — Embedded System, chapter 9 - [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 - [8051 minimum circuit and 75% duty pulse](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-min-config-pwm-75) — Embedded System, chapter 9 - [8051 pin diagram; ports 1 and 2](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-pin-diagram-ports-1-2) — Embedded System, chapter 9 - [Precise 5 ms delay in 8051](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-5ms-delay) — Embedded System, chapter 9 ## Related in other subjects - [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 - [Seven-segment "b" and "d" segments](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ab-d-segments) — Digital Logic, chapter 4 - [Seven-segment "e" segment](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ae-segment-seven-segment) — Digital Logic, chapter 4