EPROM (erasable programmable read-only memory)

What is EPROM (erasable programmable read-only memory)?

EPROM (erasable programmable read-only memory) is memory that does not lose its data when the power supply is cut off. The data can be erased and the chip reprogrammed by shining an intense ultraviolet (UV) light through a window designed into the memory chip. This process is known as burning and requires a specific device called a PROM programmer, which is plugged in to an EPROM burner.

A programmed EPROM can retain its data for 10 to 20 years. Some EPROMs can retain data for even longer. The data is retained even when power is cut off and can be retrieved after the power is turned back on, making it a type of non-volatile memory.

To erase and reprogram the EPROM, UV light is required. The light clears the data on the chip and readies it for reprogramming by focusing on a quartz crystal window at the top of the EPROM. This window is transparent, and the silicon chip is visible through it, which permits exposure to UV light during erasure.

In general, ordinary room lighting does not contain enough UV light to cause data erasure on the EPROM. However, bright sunlight contains a lot of UV light, which can cause erasure. To protect the chip from such accidental erasures, which may also be possible via bright camera flashes or X-rays, the window is covered with an opaque label.

EPROM history

EPROM technology was first developed at Intel in 1971 by Dov Frohman. He based his invention on the idea that the floating gate of a metal-oxide semiconductor field-effect transistor (MOSFET) could be used for the cell of a reprogrammable ROM. This idea was initially proposed by two researchers at Bell Labs in 1967. By building on this idea, Frohman designed the first EPROM, a 2,048-bit chip called the Intel 1702, which could only store up to 256 bytes of information.

For years, PC manufacturers used EPROMs to design BIOS chips. They covered the erasing window with an adhesive, foil-backed label containing details about the BIOS, including its publisher name and revision. EPROMs were also used to store lookup tables (LUTs) for applications where a set of known input values has one output value.

EPROM operation

An EPROM consists of a large array of floating gate MOSFETs underneath a control gate transistor. The transistors are separated by a thin insulating oxide layer. To program the EPROM, a high voltage is applied to the control gate, which causes electrons to move through the oxide layer into the floating gate. The floating gate impedes the subsequent operations of the control gate.

As long as the floating gate is linked to the control gate, the cell's value is 1. To change the value to 0, a process called tunneling is required, which alters the placement of electrons in the floating gate. When the electrical charge is applied to the floating gate, it enters the floating gate and then drains to ground.

The excited electrons get trapped on the other side of the oxide layer and become negatively charged. They also act as a barrier between the control gate and the floating gate. If the electron flow through the gate is greater than 50% of the charge, the cell value is 1. But, when the charge drops below 50%, the cell value changes to 0.

Types of EPROMs

There are two main types of EPROM technology:

1. UV-EPROM. A UV-EPROM is programmed at a high voltage. As the name indicates, it is erased by shining UV light through the window on the chip. Most UV-EPROM chips can store data for 20 or so years and can accommodate a high number of read cycles.
2. One-time programmable (OTP) EPROM. An OTP EPROM is similar to a UV-EPROM with one notable exception: It does not have a window for UV light and is, therefore, not readily erasable. However, in some cases, OTP EPROMs can be erased using X-rays.

EPROM, EEPROM and flash memory

EPROM replaced ROM and PROM. After about 30 years of continued use, EPROMs were eventually replaced by electrically erasable programmable read-only memory (EEPROM) and flash memory.

EEPROM is essentially an updated version of EPROM technology -- the key difference is that EEPROM chips are electrically erasable. EEPROMs require a high voltage for reprogramming, which can be generated on the chip by a charge pump. On these chips, writes require much more power than reads. Moreover, write times can be slow, and the memory itself supports a limited number of write cycles. Nonetheless, EEPROMs are extensively used in modern-day computers along with flash memory.

One reason for their wide application is that an EEPROM and flash can be erased and reprogrammed in place on the circuit board itself. In contrast, EPROM chips were programmed on an external programming device and then placed on the computer's circuit board. Further, in an EEPROM, the data is erased using electrical signals, which is much more convenient than using the UV light that's required to erase and reprogram EPROMs.

Flash memory is also a popular choice for use in modern computers. Unlike both EPROMs and EEPROMs, which can only write or erase 1 byte at a time, flash chips are designed for multibyte operations. This means they can both write and erase entire blocks of data in one operation, speeding up the erasure or reprogramming process and minimizing the damage to transistors. Damage is a common problem with EPROMs and, to a lesser extent, EEPROMs due to multiple erasure and reprogramming cycles.

EPROM applications

Older microcontrollers used an on-chip EPROM to store smaller programs, such as BIOS and LUT code. These microcontrollers were used mainly for program development and debugging. Some microcontrollers also used EPROMs to store other types of data.

Older PCs and some video game applications also used EPROMs. But, today, EPROMs are largely replaced by faster and electrically programmable EEPROMs and flash memory.

Advantages and drawbacks of EPROM

EPROM production can be economical in smaller volumes. Since it is reprogrammable, data can be erased and reprogrammed multiple times so the chip can be reused for different applications. Because EPROMs are non-volatile, there is no risk of data loss if the power supply is switched off. Finally, EPROMs are a convenient option for testing and debugging applications and for upgradeable programs designed for repeated use.

The biggest drawback of an EPROM is that it requires UV light or X-rays to erase the data. Another problem is that it is not possible to erase only certain bytes of data -- byte by byte -- on the EPROM. Rather, the entire chip must be erased and reprogrammed. Also, the erasure and reprogramming process can take quite a lot of time, depending on the quantity of data.

EPROMs can consume fairly high amounts of power, and their transistors have high resistance. Finally, EPROMs can cost more than older ROMs or PROMs.

See also: volatile memory, memory management, NOR flash memory, cache memory, swap file, NAND flash memory, memory dump, microcontroller, flash storage, solid-state drive, non-volatile dual in-line memory module and persistent storage.