memory products - Al Kossow's Bitsavers

M27256DEVICE OPERATION (Continued)PROGRAM INHIBITProgramming of multiple M27256s in parallel withdifferent data is also easily accoJJJ.Plished. Exceptfor CE, all like inputs (including OE)of the parallelM27256 may be common. A TTL low pulse appliedto a M27256's CE input, with Vp.aEt 12.5V, willprogram that M27256; A high level CE input inhibitsthe other M27256s from being programmed.PROGRAM VERIFYA verify should be performed on the programmedbits to determine that they were correctlyprogrammed. The verify is accomplished with OE atVIL, CE at VIH and Vpp at 12.5V.OPTIONAL VERIFYThe optional verify may be performed instead~the verify mode. It is performed with OE at VIL, CEat VIL (as opposed to the standard verify whic~ hasCE at VIH), and Vpp at 12.5V. The outputs willthree-state according to the signal presented to OE.Th'erefore, all devices with Vpp = 12.5V andOE = \ill.will present data on the bus independentof the CE state. When parallel programming severaldevices which share the common bus, Vpp shouldbe lowered to Vcc (= 6V) and the normal readmode used to execute a program verify.ELECTRONIC SIGNATUREThe Electronic Signature mode allows the readingout of a binary code from an EPROM that will identifyits manufacturer and type. This mode is intendedfor use by programming equipment for the purposeof automatically matching the device to beprogrammed with its corresponding programmingalgorithm. This mode is functional in the 25°C ± 5°Cambient temperature range that is required whenprogramming the M27256. To activate this mode,the programming equipment must force 11.5V to12.5V on address line A9 (pin 24)of the M27256.Two identifier bytes may then be sequenced fromthe device outputs by toggling address line AO(pin 10) from VIL to VIH. All other address linesmust be held at VIL during Electronic Signaturemode. Byte 0 (AO = VIJ represents the manufacturercode and byte 1 (AO = VI H) the device identi.­fier code. For the SGS-THOMSON f\il27256, thesetwo identifier bytes are given below. All identifiersfor manufacturer and device codes will possess oddparity, with the MSB (07) defined as the parity.bit.ERASURE OPERATIONThe erasure characteristic of the M27256 is suchthat erasure begins when the cells are exposed tolight with wavelengths shorter than approximately4000 Angstrom A. It shduld be noted that sunlightand some type of fluorescent lamps havewavelengths in the 3000-4000 A range. Data showsthat constant exposure to room level fluorescentlighting could erase a typical M27256 in about 3years, while it would take approximately 1 week tocause erasure when expose to direct sunlight. Ifthe M27256 is to be exposed to these types of lightingconditions for extended periods of time, it issuggested that opaque labels be put over theM27256 window to prevent unintentional erasure.The recommended erasure procedure for theM27256 is exposure to short wave ultraviolet lightwhich has wavelength 2537·A. The integrated dose(Le. UV intensity x exposure time) for erasureshould be a minimum of 15 W-sec/cm2. Theerasure. time with this dosage is approximately 15to 20 minutes using an ultraviolet lamp with 12000pW/cm2 power rating. The M27256 should beplaced within 2.5 cm (1 inch) of the lamp tubes duringthe erasure. Some lamps have a filter on theirtubes which should be removed before erasure.ELECTRONIC SIGNATURE MODEIDENTIFIER~AO 07 06(10) (19) (18)MANUFACTURER CODE VIL 0 0DEVICE CODE VIH 0 005 04 03 02 01 00 Hex(17) (16) (15) (13) (12) (11) Data1 0 0 0 0 0 200 0 0 1 0 0 046/1068