9/15

M27W256

On-Board Programming

The M27W256 can be directly programmed in the

application circuit. See the relevant Application

Note AN620.

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 M27W256. To activate the ES

mode, the programming equipment must force

11.5V to 12.5V on address line A9 of the

bytes may then be sequenced from the device out-

resents the manufacturer code and byte 1 (A0 =

the

device

identifier

code.

For

the

STMicroelectronics M27W256, these two identifier

bytes are given in Table 4 and can be read-out on

outputs Q7 to Q0. Note that the M27W256 and

M27C256B have the same identifier bytes.

ERASURE OPERATION (applies for UV EPROM)

The erasure characteristics of the M27W256 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 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

sure time with this dosage is approximately 15 to

20 minutes using an ultraviolet lamp with 12000

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 era-

sure.