--- title: "One Time Programmable ROM operation" subject: "Embedded System" chapter: 4 key: "semester_6/embedded_system:otp-rom-storing" source: https://www.bctengineer.com/q/semester_6/embedded_system/otp-rom-storing site: BCT Engineering --- # One Time Programmable ROM operation Embedded System · Chapter 4: Memory - Asked in: 2075 Baisakh, 2+2 marks; 2078 Chaitra (asked 2 times) - Video: [One Time Programmable Memory](https://www.youtube.com/watch?v=PeoiZECN2sI) — Sai Vidya Institute of Technology - SVIT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/otp-rom-storing - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-4 ## Question Explain the operation of storing data in One Time Programmable ROM. Why can't it be reprogrammed? ## Short answer An OTP ROM stores data by selectively blowing fuse links at crosspoints using high voltage pulses. A blown fuse creates a permanent open circuit, changing the stored logic value. Since the fuse is physically destroyed, the change is irreversible, so the memory cannot be reprogrammed. ## Answer An **OTP ROM (One-Time Programmable ROM, or PROM)** is a ROM that the user can program once after manufacture, with a special device called a PROM programmer. #### Operation of storing data - The ROM is made with a transistor (or diode) at every crosspoint of a word line and bit line, each connected in series with a **fuse link** (typically nichrome or polysilicon). Blank, every fuse is intact, so every cell stores 1 (or 0 depending on design). - To store data, the PROM programmer applies a high current/voltage pulse (about 12-21 V) to the selected word and bit lines of every cell that must hold the opposite value. - The pulse blows (melts) the fuse, opening that connection permanently. A cell with a blown fuse reads one logic value, and an intact cell reads the other. - The programmer repeats this for each location until the pattern is stored. ```text word line --+---------+ | | [fuse] [fuse blown] | X transistor transistor --> bit line cell = 1 (intact) cell = 0 (blown) ``` (In an anti-fuse OTP, a programming voltage creates a conducting link instead.) #### Why it cannot be reprogrammed Blowing a fuse is a physical, irreversible change: the metal link is destroyed and cannot be rebuilt. A cell can only change from the "unprogrammed" value to the "programmed" value, never back. Therefore the contents are permanent, and a mistake means discarding the chip. This permanence makes it suitable for stable, low-volume production code. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In an OTP (PROM) ROM, what is placed in series with the transistor/diode at each crosspoint? A. A fuse link B. A floating gate C. A capacitor D. A flip-flop Answer: A. Each cell has a fuse link (nichrome or polysilicon) in series with its transistor or diode; a blank chip has every fuse intact. (Key point: Fuse link at each cell) 2. How does a PROM programmer store a value opposite to the blank value in a cell? A. Exposes the cell to UV light B. Applies a high current/voltage pulse that blows the fuse C. Charges a capacitor in the cell D. Applies a low-voltage pulse that reverses the fuse Answer: B. A high current/voltage pulse (about 12-21 V) melts the fuse, opening the connection so the cell reads the opposite logic value. (Key point: Programming by blowing fuses) 3. Why can an OTP ROM not be reprogrammed? A. The programmer lacks enough voltage B. The quartz window is missing C. Blowing a fuse is an irreversible physical change that cannot be rebuilt D. The data is held in volatile capacitors Answer: C. The metal link is destroyed when blown, so a cell can only go from unprogrammed to programmed value, never back. (Key point: Why OTP cannot be reprogrammed) ## Related in Embedded System - [Count number of 1s in ROM data](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-count-ones-asm) — Embedded System, chapter 9, asked 2× - [Count number of 0s in ROM data](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-count-zeros-asm) — Embedded System, chapter 9, asked 2× - [UV-EPROM storing and erasing](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Auv-eprom-store-erase) — Embedded System, chapter 4 - [Datapath operation and instruction cycles](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Adatapath-operation-instruction-cycles) — Embedded System, chapter 2 ## Related in other subjects - [ROM and BJT memory cell storing 1 or 0](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Arom-bjt-memory-cell) — Digital Logic, chapter 4 - [One-way vs two-way drainage time](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aone-way-two-way-drainage-time) — Soil Mechanics, chapter 9 - [Data logger block diagram and operation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adl-block) — Instrumentation II, chapter 5, asked 6× - [TST time slots for non-blocking operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atst-time-slots-numerical) — Telecommunications, chapter 4