EPROM (Erasable Programmable Read-only Memory) [MiniTool Wiki]

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There are different types of ROM in the market, such as EEPROM and flash memory. And this post focuses on EPROM, if you want to know other types of ROM, go to the MiniTool website.

Definition of EPROM

What is EPROM? The EPROM, sometimes called EROM, is an abbreviation for erasable programmable read-only memory. It is a type of programmable read-only memory (PROM) chip that can retain its data when its power is turned off.

Computer memory that can retrieve stored data after the power is turned off and back on is called non-volatile memory. It is a floating-gate transistor array that is individually programmed by an electronic device, and provides a voltage higher than that commonly used in digital circuits.

After programming, the EPROM can be erased by exposing it to a strong ultraviolet light source (such as a mercury-vapor lamp). The EPROM can be easily identified through the transparent fused quartz window on the top of the package, through which the silicon chip can be seen, and can be exposed to ultraviolet light during the erasing

Details of EPROM

Due to the high manufacturing cost of the quartz window, OTP (One Time Programmable) chips were released. Here, the die is installed in an opaque package, so it can’t be erased after programming - this also eliminates the need to test the erase function, thereby further reducing costs.

Thus, OTP versions of EPROMs and EPROM-based microcontrollers are manufactured. However, OTP EPROM (whether it is a separate or part of a larger chip) is increasingly being replaced by small-sized EEPROM, because the cost of small-sized cells is not important, and flash memory is suitable for large-sized.

The programmed EPROM retains its data for at least ten to twenty years, many of which will retain the data after 35 years or more, and can be read indefinitely without affecting the lifetime. The erasing window must be covered with an opaque label to prevent accidental erasure by sunlight or ultraviolet rays in the camera flash.

Old PC BIOS chips were usually EPROMs, and the erasing window was usually covered with a sticker, which contains the name of the BIOS publisher, BIOS revision, and copyright notice. Usually, this label uses aluminum foil as the base paper to ensure that it is opaque to ultraviolet light.

When the wavelength is less than 400 nm, the EPROM starts to be erased. Exposure time for Indoor fluorescent lighting of three years or sunlight of a week may cause erasure. The recommended erasure procedure is an exposure of at least 15 Ws/cm2 under 253.7 nm ultraviolet light, usually with a lamp of about 2.5 cm within 20 to 30 minutes.

The EPROM has a limited but large number of erase cycles. The surrounding silica dioxide accumulates damage in each cycle, making the chip unreliable after several cycles. Compared with other forms of memory, EPROM programming speed is slower.

Since the higher-density part has little exposed oxide between the layers of interconnects and gate, ultraviolet erasing becomes less practical for very large memories. Even the dust in the package can prevent certain batteries from being erased.

Application of EPROM

For a large number of parts (a few or more), mask-programmed ROMs are the lowest cost device. However, since the artwork of the IC mask layer must be changed to store the data on the ROM, the mask-programmed ROM takes many weeks to make.

At first, people thought that EPROM was too expensive for mass production and was limited to development. It soon became apparent that it is economical to use EPROM parts for small batch production, especially when considering the advantages of fast firmware upgrades.

Before these EEPROM and flash memory eras, some microcontrollers used an on-chip EPROM to store their programs. These microcontrollers include certain versions of Intel 8048, Freescale 68HC11, and the “C” version of the PIC microcontroller.

Like EPROM chips, such microcontrollers had a windowed (expensive) version for debugging and program development. The same chip was produced in an opaque OTP package (which is cheaper). When the bare die of such a chip is exposed to light, moving from a windowed part for development to a non-windowed part for production can also change the behavior in unexpected ways.

Pros and Cons of EPROM

Pros

  • EPROM is non-volatile, so it can retain its memory even without power. Therefore, no external memory is required.
  • EPROM is very effective.
  • EPROM is reprogrammable, that is, the data in EPROM can be erased and reprogrammed.

Cons

  • Transistors used in EPROM possess a higher resistance.
  • The EPROM requires ultraviolet light to erase data. This cannot be done using electrical signals.
  • It is impossible to erase specific data bytes in EPROM. The entire data is deleted.
  • The static power consumption of EPROM is very high.
  • It takes some time to erase the data in EPROM. This is different from EEPROM that can erase data immediately.
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