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M27W256 Device description
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these two identifier bytes are given in Tabl e 3 and can be read-out on outputs Q7 to Q0.
Note that the M27W256 and M27C256B have the same identifier bytes.
2.10 Erasure operation (applies for UV EPROM)
The erasure characteristics of the M27W256 is such that erasure begins when the cells are
exposed to light with wavelengths shorter than approximately 4000 Å. It should be noted
that sunlight and some type of fluorescent lamps have wavelengths in the 3000-4000 Å
range. Research shows that constant exposure to room level fluorescent lighting could
erase a typical M27W256 in about 3 years, while it would take approximately 1 week to
cause erasure when exposed to direct sunlight. If the M27W256 is to be exposed to these
types of lighting conditions for extended periods of time, it is suggested that opaque labels
be put over the M27W256 window to prevent unintentional erasure. The recommended
erasure procedure for the M27W256 is exposure to short wave ultraviolet light which has
wavelength 2537Å. The integrated dose (i.e. UV intensity x exposure time) for erasure
should be a minimum of 15 W-sec/cm
2
. The erasure time with this dosage is approximately
15 to 20 minutes using an ultraviolet lamp with 12000 µW/cm
2
power rating. The M27W256
should be placed within 2.5 cm (1 inch) of the lamp tubes during the erasure. Some lamps
have a filter on their tubes which should be removed before erasure.
Table 3. Electronic Signature
Identifier A0 Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 Hex Data
Manufacturer’s
Code
V
IL
00100000 20h
Device Code V
IH
10001101 8Dh
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