--- title: "8255 mode 0 DIP switches and LEDs" subject: "Instrumentation II" chapter: 2 key: "semester_5/instrumentation_ii:ppi-mode0-dip-leds" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-mode0-dip-leds site: BCT Engineering --- # 8255 mode 0 DIP switches and LEDs Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System - Asked in: 2079 Baisakh, 2+2+1+3 marks - Video: [Control Word and Modes of 8255: BSR Mode, IO Modes, and Detailed Mode 0, 1, & 2](https://www.youtube.com/watch?v=la-EoWUcSp4) — Engineering Funda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-mode0-dip-leds - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2 ## Question 8255 is to be operated in mode 0. Port A and port C upper are designed as output for LEDs and port B and port C lower as input ports for DIP switches. Address line A15 is connected with CS of 8255 through an inverter. (a) Draw the complete mapping diagram. (b) Determine the port addresses. (c) Determine the control word. (d) Write a program to read the DIP switches and display the reading from port B at port A and C lower at port C upper. ## Short answer Port A=8000H, B=8001H, C=8002H, Control=8003H (A15=1). Control word 83H sets A and C upper output, B and C lower input. Program reads port B switches, outputs to port A LEDs; reads port C lower switches, rotates to upper nibble, masks F0H, outputs to port C upper LEDs. ## Answer #### (a) Mapping diagram (memory mapped, 8085) A15 is connected to CS' through an inverter, so the 8255 is selected when A15 = 1; other lines A14-A2 are not decoded (partial decoding). A1, A0 select the registers. ```text A15 --[NOT]--> CS' A1 -> A1, A0 -> A0, RD' -> RD', WR' -> WR', RESET OUT -> RESET AD0-AD7 -> D0-D7 PA0-PA7 -> 8 LEDs PC4-PC7 -> 4 LEDs PB0-PB7 <- 8 DIP switches PC0-PC3 <- 4 DIP switches ``` #### (b) Port addresses With unused lines taken as 0: Port A = **8000H**, Port B = **8001H**, Port C = **8002H**, Control = **8003H** (images exist at every address with A15 = 1). #### (c) Control word Port A out (D4 = 0), C upper out (D3 = 0), B in (D1 = 1), C lower in (D0 = 1), mode 0 (D6 D5 D2 = 0), D7 = 1: 1000 0011 = **83H**. #### (d) Program ```text MVI A,83H STA 8003H ; initialise 8255 LOOP: LDA 8001H ; read DIP switches of port B STA 8000H ; display on port A LEDs LDA 8002H ; read port C (lower nibble = switches) RLC RLC RLC RLC ; move lower nibble to upper nibble ANI F0H ; clear lower nibble STA 8002H ; display on port C upper LEDs JMP LOOP ``` Writing to the lower half of port C has no effect because it is an input, so only the upper LEDs change. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. With only A15 connected through an inverter to CS', what address range does the 8255 respond to? A. Any address with A15 = 1, so it has images B. Only 8000H-8003H C. Any address with A15 = 0 D. Only 0000H-0003H Answer: A. A14-A2 are not decoded, so images exist at every address with A15 = 1. (Key point: Partial decoding) 2. Port A out, C upper out, B in, C lower in, mode 0. Control word? A. 93H B. 82H C. 8BH D. 83H Answer: D. D7=1, D4=0, D3=0, D1=1, D0=1 gives 1000 0011 = 83H. (Key point: Control word) 3. Why does the program rotate left four times before writing to port C? A. To clear the carry flag B. To switch port C to input C. To read port A D. To move the lower-nibble switch value into the upper nibble that drives the LEDs Answer: D. The switches are on PC0-PC3 but the LEDs are on PC4-PC7. (Key point: Nibble move) ## Related in Instrumentation II - [8255 port addresses from decoder figure](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-port-addresses-decoder-fig) — Instrumentation II, chapter 2 - [8255 mode 1 keyboard and printer with decoding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-keyboard-printer-mode1-fig) — 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 - [8255 interface with switches and 7-segment](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-switch-sevenseg) — Instrumentation II, chapter 2 - [8255 mode 2 operation with timing diagram](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-mode2) — Instrumentation II, chapter 2 - [ADC at 80H with 8255A: display on port B LEDs](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc-8255-80h-leds) — Instrumentation II, chapter 2 ## Related in other subjects - [Address decoding for 8 LEDs output port](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-output-port-8-leds) — Microprocessor, chapter 4 - [Address decoding for input and output ports](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-input-output-port-block-decoder) — Microprocessor, chapter 4 - [Address decoding for input port at 8050H](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-input-port-lda-8050h) — Microprocessor, chapter 4 - [Blink 8 LEDs at port 2](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-blink-leds-port2) — Embedded System, chapter 9, asked 2×