Read Only Memory (ROM)

• Nonvolatile memory
• Can be read from but not written to
• Uses
  • Store software for general purpose processor
  • Store constant data needed by system
  • Implement combinational circuit as lookup table

Implementation

• Programmed by only connecting select programmable connections

Types

Mask-programmed

• Connected programmed at fabrication
• Shines a light on a set of masks, to etch circuit onto silicon
• Lowest write ability (only once)
• Highest storage performance (bits never changed unless damaged)
• Typically used for final designs of high-volume systems

One Time Programmable (OTP)

• Connections programmed after manufacture by user
  • User provides file of desired contents of ROM
  • each programmable connection is a fuse
  • ROM programmer blows fuses for connections that should not exist
• Very low write ability (typically only once)
• Very high storage performance (would not change unless reconnected to programmer)
• Often used in final products

Erasable Programmable ROM (EPROM)

• Programmable component is a MOS transistor
  • Transistor has floating gate surrounded by an insulator
  • Write 1: Negative charges form channel between source and drain
  • Write 0: Large positive voltage at gate causes negative charges to move out of channel and get trapped in floating gate
  • Erase: Shining UV rays on surface of floating gate causes negative charges to return to channel from floating gate
• Better write ability (can be reprogrammed thousands of times)
• Reduced storage permanence (lasts about 10 years but is susceptible to radiation and electrical noise)
• Typically used during design development

Electrically Erasable Programmable ROM (EEPROM)

• Programmed and erased electronically
  • Typically by using a higher than normal voltage
  • Can program and erase individual words
• Better write ability
• Similar storage to EPROM (about 10 years)
• Far more convenient than EPROM but more expensive

Flash

• Fast erase
  • Large blocks of memory erased as once, rather than once word at a time
  • Blocks typically several thousand bytes large
• Writes to single words may be slower
  • Entire block must be read, modified, and then written back
• Used with embedded systems storing large data items in nonvolatile memory
• Extension of EEPROM
  • Same floating gate principle
  • Same write ability and storage permanence

Composing

• Parallel
  • Increases word size
  • 
• Serial
  • Increases number of words
  • 
• Hybrid
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