--- title: "8255 interface with switches and 7-segment" subject: "Instrumentation II" chapter: 2 key: "semester_5/instrumentation_ii:ppi-switch-sevenseg" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-switch-sevenseg site: BCT Engineering --- # 8255 interface with switches and 7-segment Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System - Asked in: 2074 Asoj, 8 marks - Video: [Interfacing 7-Segment Display with 8086 Microprocessor by using 8255PPI](https://www.youtube.com/watch?v=gVSazQGbGc8) — NAVEEN TECHNOLOGIES - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-switch-sevenseg - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2 ## Question A microprocessor kit has an onboard 8255. Interface to the 8255 eight single-pole-double-throw (SPDT) switches numbered S0 to S7 and a seven segment common anode LED display. Draw the complete circuit setup. Define clearly the functions of all ports, write a program to initialize 8255, detect a switch closure, and display the value of the switch number on the LED display. ## Short answer Port A outputs segment codes to a common-anode 7-segment display, port B reads eight SPDT switches (closure gives 0). The 8255 is initialised with control word 82H. The program polls port B, complements the bits, rotates right to find the switch number, and outputs the corresponding code from a table: C0H, F9H, A4H, B0H, 99H, 92H, 82H, F8H for 0–7. ## Answer #### Assumptions 8085 kit with the 8255 I/O-mapped at **80H-83H**: Port A = 80H, Port B = 81H, Port C = 82H, Control = 83H (A1 and A0 select the register; CS' comes from a decoder of A7-A2, selected when A7 = 1 and A6-A2 = 0). A switch is called "closed" when it is thrown to the GND side. #### Circuit ```text Each SPDT switch Sn: one throw -> +5 V, other throw -> GND, common terminal -> PBn (S0 -> PB0, S1 -> PB1, ... S7 -> PB7) PA0..PA6 --[330 ohm each]--> segments a..g of the common anode display Common anode pin ----------> +5 V 8255: D0-D7 <-> AD0-AD7, RD', WR', A0, A1 from the 8085, RESET <- RESET OUT, CS' <- address decoder ``` A segment glows when its port pin is **0** (common anode). #### Port functions | Port | Mode | Direction | Function | |---|---|---|---| | A | 0 | Output | 7-segment code (PA0=a ... PA6=g, PA7=dp) | | B | 0 | Input | Switches S0-S7 (+5 V side reads 1, closed (GND) side reads 0) | | C | - | Unused | - | Control word: D7=1 (mode set), A mode 0, A output, C upper output, B mode 0, B input, C lower output = 1000 0010 = **82H**. #### Display codes (common anode, dp off) 0 = C0H, 1 = F9H, 2 = A4H, 3 = B0H, 4 = 99H, 5 = 92H, 6 = 82H, 7 = F8H. #### Program ```text MVI A,82H OUT 83H ; initialise 8255 START: IN 81H ; read switches CMA ; closed switch becomes 1 ORA A ; set flags, clear carry JZ START ; no switch closed MVI C,00H ; C = switch number NEXT: RAR ; shift lowest bit into carry JC FOUND INR C JMP NEXT FOUND: LXI H,TABLE MVI B,00H DAD B ; HL = TABLE + number MOV A,M ; get 7-segment code OUT 80H ; display switch number JMP START TABLE: DB C0H,F9H,A4H,B0H,99H,92H,82H,F8H ``` Example: if S3 is closed, the port reads 1111 0111. After CMA it is 0000 1000. The first three RAR instructions leave carry = 0 (each is followed by INR C), and the fourth sets carry, so C = 3 and the code B0H shows "3". If more than one switch is closed, the lowest-numbered one is displayed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What control word sets port A output, port B input in mode 0 with port C outputs? A. 82H B. 92H C. 80H D. 8BH Answer: A. D7=1, A out, B in, C out gives 1000 0010 = 82H. (Key point: Control word) 2. How does a segment glow on a common anode display? A. When its port pin is 1 B. When its port pin is 0 C. When port C is 1 D. When pin is floating Answer: B. The common anode is at +5 V, so a low pin sinks current. (Key point: Common anode) 3. What is the common anode code for digit 3? A. B0H B. A4H C. F9H D. 99H Answer: A. Digit 3 = B0H in the table. (Key point: Display code) 4. Why is CMA used after reading the switches? A. A closed switch reads 0, so CMA makes it 1 to detect with RAR B. To clear the accumulator C. To enable interrupts D. To select the 7-segment table Answer: A. Inversion makes the closed switch a 1 bit. (Key point: Switch detection) ## Related in Instrumentation II - [8255 mode 0 DIP switches and LEDs](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-mode0-dip-leds) — Instrumentation II, chapter 2 - [8-channel ADC interface with 8255 and 8085](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc8-8255-problem) — Instrumentation II, chapter 4 - [8255 at A0A0H: interface and control words](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-a0a0h-control-words) — Instrumentation II, chapter 2 - [Double handshake data transfer with 8255-PPI](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adouble-handshake-8255) — Instrumentation II, chapter 2 - [PCI bus; 8255 printer interface at 5500H](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Apci-printer-5500h) — Instrumentation II, chapter 2 - [Keyboard and printer interface in 8255 mode 1](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Akeyboard-printer-mode1) — Instrumentation II, chapter 2 ## Related in other subjects - [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 - [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 - [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× - [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