M27C256B
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 am-
bient temperature range that is required when pro-
gramming the M27C256B. To activate the ES
mode, the programming equipment must force
11.5V to 12.5V on address line A9 of the
M27C256B, with V
CC
= V
PP
= 5V. Two identifier
bytes may then be sequenced from the device out-
puts 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
) rep-
resents the manufacturer code and byte 1
(A0 = V
IH
) the device identifier code. For the ST-
Microelectronics M27C256B, these two identifier
bytes are given in Table 4 and can be read-out on
outputs Q7 to Q0.
ERASURE OPERATION (applies for UV EPROM)
The erasure characteristics of the M27C256B is
such that erasure begins when the cells are ex-
posed to light with wavelengths shorter than ap-
proximately 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 fluo-
rescent lighting could erase a typical M27C256B in
about 3 years, while it would take approximately 1
week to cause erasure when exposed to direct
sunlight. If the M27C256B is to be exposed to
these types of lighting conditions for extended pe-
riods of time, it is suggested that opaque labels be
put over the M27C256B window to prevent unin-
tentional erasure. The recommended erasure pro-
cedure for the M27C256B is exposure to short
wave ultraviolet light which has wavelength
2537Å. The integrated dose (i.e. UV intensity x ex-
posure 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 ultravi-
olet lamp with 12000 µW/cm
2
power rating. The
M27C256B 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.
9/16
STMICROELECTRONICS [ STMICROELECTRONICS ]
