Battery back-up supervisor for dual SRAM banks

The CMOS bq2205 SRAM non-volatile controller with reset provides all the necessary functions for converting one or two banks of standard CMOS SRAM into non-volatile read/write memory.

A precision comparator monitors the 3.3-V VCC input for an out-of-tolerance condition. When out-of-tolerance is detected, the two conditioned chip-enable outputs are forced inactive to write-protect both banks of SRAM.

Power for the external SRAMs, VOUT, is switched from the VCC supply to the battery-backup supply as VCC decays. On a subsequent power-up, the VOUT supply is automatically switched from the backup supply to the VCC supply. The external SRAMs are write-protected until a power-valid condition exists. The reset output provides power-fail and power-on resets for the system. During power-valid operation, the input decoder, A, selects one of two banks of SRAM.