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Quick differences between different types of ROMs

ROM (Read Only Memory)

  • As the name suggests, Once programmed cannot be reprogrammed.
  •  If the data needs to be updated throw away the programmed chip and start over with a new chip.
  • Pros:
  • less costly when produced in huge volumes
  • Robust, power efficient.
  • Cons:
  • slower

PROM (Programmable Read Only Memory)

  • This also can be programmed only once.
  • This is field programmable (ie., can be programmed anywhere).
  • Economical even when produced in small quantities.

EPROM (Electronically Programmable Read Only Memory)

  • Electronically Programmable.
  • Inexpensive per chip.
  • EPROM eraser is not selective, it erases entire chip.
  • To erase it is to be placed under the UV laser for several minutes.
  • Caution: EPROM kept under the UV laser more than required time might over erase the chip making it un-usable.

EEPROM (Electrically erasable programmable read-only memory)

  • The changes for EPROM cannot be made incrementally. For any change it erases whole chip and re-writes.
  • EEPROM can erase the targeted cells through re-writing only required cells. This way it removes this biggest drawback of EPROM.
  • EEPROM can be changed 1 Byte at a time but it is very slow.
  • Flash memory overcomes the the above limitation of slowness.
  • Flash memory erases in terms of blocks of size 512 Bytes thus improving the overall performance.





Flash and EEPROM programming from Mcirochip

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