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M27W801 Device operation
11/24
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 M27W801. To activate
the ES mode, the programming equipment must force 11.5V to 12.5V on address line A9 of
the M27W801. 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. For the STMicroelectronics M27W801, these two identifier bytes are given in
Table 3 and can be read-out on outputs Q7 to Q0.
Note that the M27W801 and M27C801 have the same identifier byte.
2.10 Erasure operation (applies to UV EPROM)
The erasure characteristics of the M27W801 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 M27W801 in about 3 years, while it would take approximately 1 week to cause
erasure when exposed to direct sunlight. If the M27W801 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 M27W801 window to prevent unintentional erasure. The recommended erasure
procedure for the M27W801 is exposure to short wave ultraviolet light which has wavelength
2537 Å. The integrated dose (i.e. UV intensity x exposure time) for erasure should be a
minimum of 30 W-sec/cm
2
. The erasure time with this dosage is approximately 30 to 40
minutes using an ultraviolet lamp with 12000 µW/cm
2
power rating. The M27W801 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 2. Operating modes
(1)
1. X = V
IH
or V
IL
, V
ID
= 12V ± 0.5V.
Mode E GV
PP
A9 Q7-Q0
Read V
IL
V
IL
X Data Out
Output Disable V
IL
V
IH
X Hi-Z
Program V
IL
Pulse V
PP
X Data In
Program Inhibit V
IH
V
PP
X Hi-Z
Standby V
IH
X X Hi-Z
Electronic Signature V
IL
V
IL
V
ID
Codes
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