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Device operation M87C257
10/22
2.9 Electronic signature
The Electronic Signature (ES) mode allows the reading out of a binary code from an
EPROM that will identify its manufacturer and type. This mode is intended for use by
programming equipment to automatically match the device to be programmed with its
corresponding programming algorithm. The ES mode is functional in the 25°C ± 5°C
ambient temperature range that is required when programming the M87C257.
To activate the ES mode, the programming equipment must force 11.5V to 12.5V on
address line A9 of the M87C257, with V
CC
= V
PP
= 5V. Two identifier bytes may then be
sequenced from the device outputs by toggling address line A0 from V
IL
to V
IH
. All other
address lines must be held at V
IL
during Electronic Signature mode. Byte 0 (A0 = V
IL
)
represents the manufacturer code and byte 1 (A0 = V
IH
) the device identifier code. When A9
= V
ID
, AS need not be toggled to latch each identifier address. For the STMicroelectronics
M87C257, these two identifier bytes are given in Tabl e 4 and can be read-out on outputs Q7
to Q0.
2.10 Erasure operation (applies for UV EPROM)
The erasure characteristics of the M87C257 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 M87C257 in about 3 years, while it would take approximately 1 week to
cause erasure when exposed to direct sunlight. If the M87C257 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 M87C257 window to prevent unintentional erasure. The recommended
erasure procedure for the M87C257 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 M87C257
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.
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