What does EPROM stand for
EPROM definition & explanation of terms
Programming an EPROM
An EPROM is programmed using an EPROM burner, a special programming device. After successful programming, the content can only be deleted again using UV light. However, the shelf life is not to be regarded as unlimited. Depending on the type, the service life is limited to 100 to 200 deletion processes. EPROMs have a quartz glass window on their housing. UV erasure can only take place through this quartz glass. Since they are not easy to manufacture, EPROMs are relatively expensive to produce. In contrast to the EPROMs that have to be erased several times, there are also versions without a quartz glass window. However, these blocks can only be programmed once. It is called One Time Programmable (OTP).
Structure of an EPROM
As a rule, an EPROM consists of a matrix of several memory cells. One transistor represents one bit in each memory cell. This is a so-called MOSFET transistor, which is provided with the floating gate, an insulated auxiliary electrode. A correspondingly high voltage is applied during programming via the EPROM burner. The floating gate is charged as a result of the tunnel effect. This shifts the control voltage and switches the transistor. The stored data can now be read out at will, since the reading voltage is always below the programming voltage.
If an EPROM is to be erased, the UV radiation causes a large-area ionization of the semiconductor. As a result, the charge applied to the transistors during programming can leave the floating gate again. The result is that the EPROM is in its original state, since the bit pattern has been completely deleted. EPROMs without quartz glass windows, the so-called OTPs, can also be erased. However, X-rays are used for this purpose. However, reprogramming is not possible. The erasing process of an EPROM usually takes between 10 and 30 minutes. The ionization continues for a while even after the UV lamp has been switched off.
Depending on the block used, this can heat up during the deletion, whereby no immediate programming can take place again. The new programming can only be started after the EPROM has cooled down. But there are also EPROM burners that use UV flash technology. Instead of permanent UV radiation, the building blocks are exposed to intense flashes of light, which significantly shortens the deletion process. However, caution is advised here.
Not all manufacturers of EPROM modules guarantee a long data service life with this method. After successful programming, the quartz glass window should be sealed with an opaque film. Usually a small strip of self-adhesive silver foil is used for this. If the quartz glass is exposed, direct sunlight can accidentally extinguish it after approximately 90 days. You should also avoid photographing an exposed EPROM with a conventional flash unit. The lightning can lead to temporary corruption of data and in the worst case to failure of the computer. As is usual with most integrated circuits, the pin assignment of an EPROM is also standardized.
In-circuit simulations help develop EPROM programs
Unfortunately, most EPROMs are not rewritable indefinitely. A simulation program is particularly useful when it comes to developing programs for EPROMs. An example would be the simulation via the conventional USB connection. These devices do not use a real EPROM, but can simulate the functionality up to 4 Mbit. This is also called in-circuit simulation. The respective program code is loaded into the simulator via the USB interface. The EPROM simulated in this way is then integrated into the actual circuit structure via a plug-in adapter.
The circuit to be tested in each case recognizes the simulated EPROM as a real EPROM and behaves accordingly. Another method is the use of battery-backed memory modules that are equipped with a write protection switch. These modules can be programmed directly via an EPROM burner and then used and checked with the activated write protection on the respective test circuit.
Use of ROM in the PC
The so-called read-only memory of a PC, the ROM, is also an EPROM. With the help of its small utility program, the boot loader, it helps the computer to boot up and, after testing the hardware environment, to load the operating system into the main memory. For example, the first PC-ATs only had 5 ¼ inch drives. When the 3.5 inch floppy disk drives appeared on the market, many computers could only be converted if their ROMs were also adapted to the new hardware. This made it necessary to replace the ROM module. In some cases the EPROM could be reprogrammed without problems. Only then were older computers able to work with 3.5 inch floppy drives.
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