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M27C256B Datasheet(PDF) 9 Page - STMicroelectronics |
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M27C256B Datasheet(HTML) 9 Page - STMicroelectronics |
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9 / 16 page  9/16 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 VCC = VPP = 5V. Two identifier bytes may then be sequenced from the device out- puts by toggling address line A0 from VIL to VIH. All other address lines must be held at VIL during Electronic Signature mode. Byte 0 (A0 = VIL) rep- resents the manufacturer code and byte 1 (A0 = VIH) 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/cm2. The erasure time with this dosage is approximately 15 to 20 minutes using an ultravi- olet lamp with 12000 µW/cm2 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. |
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