U637256

The U637256 has two separate modes of operation: SRAM mode and nonvolatile mode. In SRAM mode, the memory operates as an ordinary static RAM. In nonvolatile operation, data is transferred in parallel from SRAM to EEPROM or from EEPROM to SRAM. In this mode SRAM functions are disabled. The U637256 is a static RAM with a nonvolatile electrically erasable PROM (EEPROM) element incorporated in each static memory cell.

The SRAM U637256 can be read and written an unlimited number of times, while independent nonvolatile data resides in EEPROM. Data transfers from the SRAM to the EEPROM (the STORE operation) take place automatically upon power down using charge stored in an integraed capacitor. Transfers from the EEPROM to the SRAM (the RECALL operation) take place automatically on power up.

The U637256 combines the ease of use of an SRAM with nonvolatile data integrity. STORE cycles also may be initiated under user control via a software sequence.

Once a STORE cycle U637256 is initiated, further input or output are disabled until the cycle is completed. Because a sequence of addresses is used for STORE initiation, it is important that no other read or write accesses intervene in the sequence or the sequence will be aborted. RECALL cycles may also be initiated by a software sequence. Internally, RECALL U637256 is a two step procedure. First, the SRAM data is cleared and second, the nonvolatile information is transferred into the SRAM cells.

The RECALL operation in no way alters the data of U637256 in the EEPROM cells. The nonvolatile data can be recalled an unlimited number of times. The U637256 is pin compatible with standard SRAMs and standard battery backed SRAMs.