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About Non-Volatile Memory

Word Origin and Invention Origin

Non-volatile memory is memory that can be unplugged and still retain its data. Computer hard drives and camera memory cards are both examples of non-volatile memory, as are all of the chips covered here.

  • PROM (Programmable Read Only Memory)
  • EPROM (Erasable Programmable Read Only Memory): Sometimes this acronym is incorrect. Some EPROM chips are additionally designated OTP, which stands for One Time Programmable … these are not erasable, but they use the same memory mechanism as regular EPROM.
  • EEPROM (Electronically Erasable Programmable Read Only Memory): These chips are all electronically erasable.
  • FLASH: A form of EEPROM, but newer technology. Can be written to (programmed) using common circuit voltages.

What does the name EPROM mean in detail?

“Erasable” means that the data stored on the chip is erasable. EPROM chips utilize the semiconductor sensitivity to light for their method of erasure. EEPROMs on the other hand, utilize a newer electronic method of erasure.

“Programmable” means that we can add (write) data to the memory. All of these chips are electronically programmable, which means that we use electronic signals to write data to them. Typically (excluding FLASH) these chips require a special, higher voltage (12+ volts) for programming and erasure, and thus require a special device, often called an eprommer, for such tasks.

“Read Only Memory” means that the the chip memory can be read from, but not written to by the circuitry that uses it.

Simple Explanation

These chips all are designed to use some means to store a large number of bits for later recall. A bit is always either a 1 or a 0, and everything computers do is based upon them. Bits are the basic building block of all computer memory.

Once a bit is stored, it then can be accessed and used by a computer system. These chips are used to store all manner of data encoded in bits: programs, pictures, files, audio, … the list is practically unending.

Kinds and their Uses

The acronym names of these chips have all arisen chronologically as the industry has grown, so one must understand that the meanings have changed over time and the terminologies have not been carefully standardised and controlled but have instead been driven by the industry.

PROM: Programmable Read Only Memory, though the concept applies to all of these chips, this specifically refers to chips that use tiny internal fuses to store data. Think of the bits in this way: intact-fuse = 1; blown-fuse = 0. Quite simple really. To store data, these chips require a special programming device that knows the correct voltage and current needed to blow the internal memory fuses, the process is irreversible.

Some of these chips use antifuses that operate in the reverse of a fuse. Blowing a fuse removes a connection, while blowing an antifuse creates a connection (hence the name).

Kinds:

  • Fuse – uses a fuse array to store data
  • Antifuse – uses an antifuse array to store data
  • Serial – Only one bit can be accessed at a time. Enables smaller packages and fewer pins.
  • Parallel – Output an entire data byte (8 bits) at once, and thus have faster average data access.

Advantages:

  • Reliability
  • Stores data permanently
  • Moderate price
  • Built using integrated circuits, rather than discrete components.
  • Fast: speed is between 35ns and 60ns.

Programming:

  • Requires special programming device
  • Irreversible

Erasure:

  • Impossible

EPROM: Erasable Programmable Read Only Memory chips store their data in a special transistor matrix that responds to strong UV light. EPROMs are easily recognizable for their clear window through which the memory unit is visible. When we erase an EPROM, we are simply exposing the data storage area to light (specifically, the 253.7nm UV wavelength). After programming, the window should be covered with a sticker to prevent accidental data loss.

Kinds:

  • NMOS – Older design
  • CMOS – Newer design. Generates less heat and can be programmed with lower voltages than NMOS
  • OTP (One Time Programmable) – Same thing as the others, but lacking the clear UV window. These chips can not be erased, but are cheaper.
  • Serial – Only one bit can be accessed at a time. Enables smaller packages and fewer pins
  • Parallel – Output an entire data byte (8 bits) at once, and thus have faster average data access.

Programming:

  • Requires special programming device
  • Limited number of redo's (chip will wear out after a while)

Erasure:

  • Erases entire memory at once
  • Requires concentrated UV light
    • Best results with 253.7nm wavelength for 20 to 30 minutes, with the lamp at a distance of about 1 inch (this is a germicidal light that kills cells, take precautions to not expose yourself to it!)
    • Best accomplished with a special EPROM erasing device.
  • Memory can be corrupted accidentally in sunlight, especially over extended periods of time.
    • Place a protective sticker over the window once programming is complete.

Alternate Names:

  • UV EPROM
  • OTPROM (One Time Programmable ROM)
  • OTP EPROM

Part Number Prefixes (partial list):

  • 27
  • 27C

EEPROM: Electronically Erasable Programmable Read Only Memory chips store data in a transistor matrix akin to that used by EPROMs, but can be erased electronically. These chips tend to be more expensive than EPROMs.

Kinds:

  • Serial – Only one bit can be accessed at a time. Enables smaller packages and fewer pins
  • Parallel – Output an entire data byte (8 bits) at once, and thus have faster average data access.

Programming:

  • Takes much more time than EPROMs
  • Can often be programmed in blocks to speed up write time
  • Requires a special programming device
  • Limited number of redo's (chip will wear out after too many)

Erasure:

  • Requires special electrical signals (rather than UV light)
    • Often a special erasing device is required
  • Can selectively erase certain memory areas while leaving others untouched.
    • Down to individual bits

Alternate Names:

  • E2PROM
  • E2PROM

Part Number Prefixes (partial list):

  • 24C (serial)
  • 93C (serial)
  • 28C (parallel)

FLASH: The most common EEPROM device, flash uses a logic-gate structure that combines the faster writing method of EPROMs with the electrical erasure of EEPROMs,... and does it all with smaller packages and lower voltages.

Kinds:

  • NOR – Uses a NOR logic-gate structure. First commercial chip came out in 1988, produced by Intel. Many early flash chips were NOR type.
    • Drop-in replacement for older ROM (PROM, EPROM, EEPROM) chips.
  • NAND – Uses a NAND logic-gate structure. Thumb drives, camera memory cards, cellphone memory cards, etcetera all use NAND flash.
    • Writes and erases faster than NOR
    • Lasts 10 times longer
    • Is cheaper
    • Holds more data in the same space as NOR
    • Uses a different data access mechanism than NOR, so is not a drop-in replacement for older ROM chips
  • Comes in serial and parallel versions like other EEPROMs

Programming:

  • Requires no special device. Can be done in-circuit.
  • Takes more time than EPROMs, but less than EEPROMs.
  • Limited number of redo's (chip will wear out after too many)

Erasure:

  • Can selectively erase certain memory areas while leaving others untouched.
    • By block only, individual bits can't be singled out.
  • Requires no special device. Can be done in-circuit.

Part Number Prefixes (partial list):

  • 28F
  • 29C
  • 29F

Choosing Replacement Chips

  1. Determine the part number.
    • This will be stamped on the chip along with “lot code” and often “date code” information, usually on separate lines
  2. Find a datasheet for your particular chip .
    • The datasheet will most often have a section with ordering information and a part number breakdown.
      • The different sections of the part number will describe the specifications of your particular chip
  3. Use the datasheet to verify the following information:
    • Chip technology – this is quite important
      • Flash?, EEPROM?, EPROM? …
      • Along with this, also check the write and erase voltages.
    • Memory size – this is quite important
      • Note that this value can be given in Bits rather than Bytes, so double check
    • The chip housing (called the “package”). This is the chip shape, size, and lead orientation.
      • Memory chips for many years (especially EPROMs) came primarily in DIP packages, but many other packages exist with differing lead locations, sizes, lead types, etcetera. Newer chips especially come in many different “packages.” Make sure you get the right one.
    • The I/O access method
      • Is the chip serial or parallel?
      • Is the chip byte-level addressed (like NOR flash) or block-level addressed (like NAND flash)?
    • The access speed
      • Faster chips are usually fine, but if you don't know the speed constraints of the system the original chip came from, then be sure not to get a slower one.
    • The operating temperature range.
      • If a chip exceeds its designed temperature range, it can malfunction or worse, self destruct.
  4. Find a chip with comparable specs to the old one.
    • In particular, be sure to match:
      • chip technology
      • memory size
      • chip “package”
      • I/O access method
    • Remember: datasheets are handy
  5. When purchasing a new chip the following information is useful:
    • Is the chip refurbished or new?
      • Remember that most non-volatile memories wear out after too much use. Refurbished chips can be a gamble, especially if you expect to run your purchase through a high number of write/erase cycles.
    • Also, the date of manufacture, though often not necessary, can be an interesting piece of information
      • The date of manufacture is often written in a special “date code” format:
        • “YYWW”; where YY is the year and WW is the week
          • For example, a date code of 0350 refers to the 50th week of 2003
    • Quite a few chips will have a date code stamped on them, but even if they don't, they almost always have a “lot code” stamp.
      • Lot codes are manufacturer specific and impossible to read unless you know their specific format.
      • Lot codes are often alpha-numeric and will include information about the date of manufacture, but some, for example, encode the year of manufacture in letters rather than numbers, and there is no easy way to tell.
      • Lot codes often can include a three-digit date code:
        • “YWW”; where Y is the year and WW is the week
          • For example, a date code of 350 refers to the 50th week of … 1983, or 1993, or 2003
          • Note that, in this case, the specific decade is ambiguous