memory products - Al Kossow's Bitsavers

ST27C256FN·ST27C256PDEVICE OPERATIONThe seven modes of operation of the ST27C256are listed in the Operating Modes table. A single5V power supply is required in the read mode. Allinputs are TTL levels except for Vpp.READ MODEThe ST27C256 has two control functions, both ofwich must be logically active in order to obtain dataat the outputs. Chip Enable (CE) is the powercontrol and should be used for device selection.Output Enable (OE) is the output control and shouldbe used to gate data to the output pins, independentof device selection. Assuming that addressesare stable, address access time (tACe) is equal tothe delay from CE to Output (tCE). Data is availableat the ol!!Q..uts after a delay of tOE from the fallingedge of OE, assuming that CE has been low andaddresses have been stable for at least tACC-tOE.STANDBY MODEThe ST27C256 has a standby mode which reducesthe maximum power dissipation to 5.25 mW.The ST27C256 is placed in the standby mode byapplying a TTL high signal to the CE input. When instandby mode, the outputs are.ln.a high impedancestate, independent of the OE input.OUTPUT OR-TYINGBecause OTPs are usually used in larger memoryarrays, we have provided two control lines whichaccomodate this multiple memory connection. Thetwo control lines allow for:a) the lowest possible memory power dissipation,andb) complete assurance that output bus contentionwill not occur.To use these control lines most efficiently, CEshould be decoded and used as the primary deviceselecting function, while OE should be made acommon connection to all devices in the array andconnected to the READ line from the system controlbus. This assures that all deselected memorydevices are in their low power standby modes andthat the output pins are active only when data isdesired from a particular memory device.PROGRAMMING MODESCaution: Exceeding 14Von Vpp pin will damagethe ST27C256.Initially, all bits olthe ST27C256 are in the" 1" state.Data is introduced by selectively programming"Os" into the desired bit locations. Although only"Os" will be programmed, both "1s" and "Os" canbe presented in the data word.The ST27C256 is in the programming mode whenthe Vpp input is at 12.5 V and CE and PGM areboth at TTL Low. It is required that a 0.1 /"F capacitorbe placed across Vpp, VCC and ground tosuppress spurious voltage transients which may damagethe device. The data to be programmed isapplied 8 bits in parallel to the data output pins.The levels required for the address and data inputsare TTL.Programming of multiple ST27C256s in parallelwith the same data can be easily accomplished dueto the simplicity of the programming requirements.Like inputs of the paralleled ST27C256s may beconnected together when they are programmedwith the same data. A low level TTL pulse appliedto the CE input programs the paralleledST27C256s.HIGH SPEED PROGRAMMINGThe high speed programming algorithm describedin the flow chart rapidly programs ST27C256 usingan efficient and reliable method particularly suitedto the production programming environment. Typicalprogramming times for individual devices areon the order of 5 minute.PROGRAM INHIBITProgramming of multiple ST27C256s in parallelwith different data is also easily accomplished.Qyusing the program inhibit mode. A high level on CEinputs inhibits the other ...§J27C256s from beingprogrammed. Except for CE, all like inputs (includingOE) of the parallel ST27C256s may becommon. A......JTL low-level pulse applied to aST27C256 CE input with Vpp at 12.5V will programthat ST27C256.PROGRAM VERIFYA verify may be performed on the programmed bitsto determine that they were correctly pro.9@mmed.The verify is performed with OE at VIL, CE at V'H,and Vpp at 12.5 V.ELECTRONIC SIGNATURE MODEElectronic signature mode allows the reading outof a binary code that will indentify the EPROMs manufacturerand type. This mode is intended for useby programming equipment for the purpose of automaticallymatching the device to be programmedwith its corresponding programming algorithm. Thismode is functional in the 25°C ± 5°C ambient temperaturerange that is required when programmingthe ST27C256. To activate this mode the programmingequipment must force 11.5V to 12.5V on addressline A9 of the ST27C256. Two bytes may thenbe sequenced from the device outputs by togglingaddress line AO from V,L to V,H. All other addresslines must be held at V,L during electonic signaturemode.5/10191