U637H256

The U637H256 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 U637H256 is a fast static RAM 25 ns) with a nonvolatile electrically erasable PROM (EEPROM) element incorporated in each static memory cell. The SRAM can be read and written an unlimited number of times, while independent nonvolatile data resides in EEPROM. Data of U637H256 transfers from the SRAM to the EEPROM (the STORE operation) take place automatically upon power down using charge stored in an integrated capacitor. Transfers from the EEPROM to the SRAM (the RECALL operation) take place automatically on power up.

The U637H256 combines the high performance and ease of use of a fast SRAM with nonvolatile data integrity. STORE cycles also may be initiated under user control via a software sequence. Once a STORE cycle 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 U637H256 cycles may also be initiated by a software sequence. Internally, RECALL U637H256 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 in the EEPROM cells. The nonvolatile data of U637H256 can be recalled an unlimited number of times. The U637H256 is pin compatible with standard SRAMs and standard battery backed SRAMs.