One Time Programmable ROM operation

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.

 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.