This document describes the M40Z300W product from STMicroelectronics, Inc., which can be used to turn SRAM devices into non-volatile RAM (NVRAM). When the supply voltage falls below the preset threshold level, the device write protects the RAM, switches to its internal battery supply, and asserts the reset line. The document also explains the configuration of using two 128K x 8 SRAMs to form a 128K x 16 arrangement.
The document explains how the M40Z300W supervisor can be used to make standard SRAM devices behave like non-volatile RAM (NVRAM). When the supply voltage falls below a preset threshold, the device write-protects the RAM, switches it to an internal battery supply, and asserts the reset line. This allows two 128K x 8 devices to be arranged as a single 128K x 16 configuration.
The device switches the SRAMs to battery power, write-protects them, and drives the reset line low.
The system checks the battery voltage during every power-up and every 24-hour interval while Vcc is valid.
The BL line drives low if the internal battery voltage becomes less than 2.5 volts.