U635H256

a: Stresses greater than those listed under „Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress rating only, and functional operation of the device at condition above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect reliability.

The U635H256 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 of U635H256 are disabled.

The U635H256 is a fast static RAM (25, 35, 45 ns), with a nonvolatile electrically erasable PROM (EEPROM) element incorporated in each static memory cell. The SRAM U635H256 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 system capacitance. Transfers from the EEPROM to the SRAM (the RECALL operation) take place automatically on power up. The U635H256 combines the high performance and ease of use of a fast SRAM with nonvolatile data integrity.

STORE cycles U635H256 also may be initiated under user control via a software sequence.

Once a STORE cycle is initiated,further input or output of U635H256 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 U635H256 cycles may also be initiated by a software sequence.

Internally, RECALL U635H256 is a two step procedure. First, the SRAM data is cleared and second, the nonvolatile information is transferred into the SRAM cells.

The RECALL U635H256 operation in no way alters the data in the EEPROM cells. The nonvolatile data can be recalled an unlimited number of times.

The U635H256 is pin compatible with standard SRAMs.