memory products - Al Kossow's Bitsavers

M27512DEVICE OPERATIONThe six modes of operations of the M27512 are listedin the Operating Modes. A single 5V power supplyis required in the read mode. All inputs are TTLlevels except for OE/vpp and 12Von A9 for ElectronicSignature.READ MODEThe M27512 has two control functions, both of whichmust be logically active in order to obtain data atthe outputs. Chip Enable (CE) is the power controland should be used for device selection. OutputEnable (OE/Vpp) is the output control andshould be used to gate data to the output pins, independentof device selection. Assuming that addressesare stable, the address access time (tAcclis equal to the delay from (;E to output (tccl. Datais available at the outputs after dela~ tOE fromthe falling edge of OE, assuming that CE has beenlow and addresses have been stable for at leasttACC-tOE'STANDBY MODEThe M27512 has a standby mode which reducesthe maximum active power current from 125 mAto 40 mAo The M27512 is placed in the standbymode by applying a TTL high signal to the CE input.When in the standby mode, the outputs arein a high impedance state, independent of theDE/Vpp input.TWO LINE OUTPUT CONTROLBecause EPROMs are usually used in largermemory arrays; the product features a 2 line controlfunction which accommodates the use of multiplememory connection. The two line control functionallows:a) the lowest possible memory power dissipationb) complete assurance that output bus contentionwill not occur.For the most efficient use of these two control lines,CE should be decoded and used as the primarydevice selecting function, while OE/vpp should bemade a common connection to all devices in thearray and connected to the READ line from the systemcontrol bus. This assures that all deselectedmemory devices are in their low power standbymode and that the output pins are only active whendata is desired from a particular memory device.SYSTEM CONSIDERATIONSThe power switching characteristics of NMOS-E3EPROMs require careful decoupling of the devices.The supply current, Icc, has three segments thatare of interest to the system designer: the standbycurrent level, the active current level, and transientcurrent peaks that are produced by the failingand rising edges of (;E. The magnitude of thistransient current peaks is dependent on the outputcapacitive and inductive loading of the device.The associated transient voltage peaks can be suppressedby complying with the two line output con-trol and by properly selected decoupling capacitors.It is recommended that a 1 /LF ceramic capacitorbe used on every device between Vee andGND. This should be a high frequency capacitorof low inherent inductance and should be placedas close to the device as possible. In addition, a4.7 /LF bulk electrolytic capacitor should be usedbetween Vee and GND for every eight devices.The bulk capacitor should be located near wherethe power supply is connected to the array. Thepurpose of the bulk capacitor is to overcome thevoltage drop caused by the inductive effects of PCBtraces.PROGRAMMINGCaution: exceeding 14Von pin 22 (OENpp) willpermanently damage the M27512.When delivered, and after each erasure, all bits ofthe M27512 are in the "1" state. Data is introducedby selectively programming "Os" into the desiredbit locations. Although only "Os" will beprogrammed, both "1s" and "Os" can be presentin tile data word. The only way to change a "0"to Ii "1" is by ultraviolet light erasureJ.he M27512is in the programming mode when OENpp inputis at 12.5V and (;E is at TTL-low. The data to beprogrammed is applied 8 bits in parallel to the dataoutput pins. The levels required for the address anddata inputs are TTL. The M27512 can usePRESTO· Programming Algorithm that drasticalyreduces the programming time (typically less than50 seconds) nevertheless to achieve compatibilitywith all programming equipments, standard FASTProgramming Algorithm can be used as well.FAST PROGRAMMING ALGORITHMFast Programming Algorithm rapidly programsM27512 EPROMs using an efficient and reliablemethod suited to the production programming environment.Programming reliability is also ensuredas the incremental program margin of each byteis continually monitored to determine when·it hasbeen successfully programmed. A flowchart of theM27512 Fast Programming Algorithm is shown inthe next page. The Fast Programming Algorithmutilizes two different pulse types: initial and overprogram.The duration of the initial CE pulse (s) isone millisecond, which will then be followed by alonger overprogram pulse of length 3X msec. (Xis an iteration counter and is equal to the numberof the initial one millisecond pulses applied to a particularM27512 location), before a correct verify occurs.Up to 25 one-millisecond pulses per byte areprovided for before the over program pulse is applied.The entire sequence .2!.Jlrogram pulses isperformed at Vee =6Vand OENpp =12.5V (byteverifications at Vee = 6V and OENpp = VII). Whenthe Fast Programming cycle has been completed,all bytes should be compared to the original datawith Vee = 5V.5/1577