Hardware Reference
In-Depth Information
The purpose of the window is to allow ultraviolet light to reach the chip die because the
EPROM is erased by exposure to intense UV light. The window is quartz crystal because
regular glass blocks UV light. You can't get a suntan through a glass window!
Note
The quartz window makes the EPROMs more expensive than the OTP PROMs. This extra
expense is needless if erasability is not important.
The UV light erases the chip by causing a chemical reaction, which essentially melts the
fuses back together. Thus, any binary 0s in the chip become 1s, and the chip is restored
to a new condition with binary 1s in all locations. To work, the UV exposure must be at
a specific wavelength (2,537 angstroms), at a fairly high intensity (12,000 uw/cm
2
), in
close proximity (2cm-3cm, or about 1 inch), and last for between 5 and 15 minutes. An
EPROM eraser is a device that contains a UV light source (usually a sunlamp-type bulb)
above a sealed compartment drawer in which you place the chip or chips (see
Figure 5.5
).
Figure 5.5
A professional EPROM eraser.
This figure shows a professional-type EPROM eraser that can handle up to 50 chips at a
time.Iuseamuchsmallerandlessexpensiveonethaterasesuptofourchipsatatimeand
is both economical and portable. Several low-cost EPROM erasers are sold by DigiKey
(
www.digikey.com
)
and other sources of EPROM programming equipment.
The quartz crystal window on an EPROM typically is covered by tape, which prevents
accidental exposure to UV light. UV light is present in sunlight, of course, and even in
standard room lighting, so that over time a chip exposed to the light can begin to degrade.
For this reason, after a chip is programmed, you should put a sticker over the window to
protect it.
