It is totally possible to save pre-power down state to an EEPROM. You need to look at how much data you need to save and how fast it can be saved to the EEPROM. The typical low cost EEPROM has a serial interface (SPI or I²C) and there is a time consideration to perform the serial shifting of the commands, address and data bytes to the EEPROM. Once a write command is givem there is also a delay time while the write actually occur. The write time for the various types of available EEPROM will vary from a few hundred microseconds to several milliseconds.

Considering the time to store the data it is necessary to ensure that the early warning shutdown signal comes with early enough time ahead of the loss of the Vcc to ensure that the writes to the EEPROM can be completed.

Sometimes there is a better design approach where by the data can get stored into the EEPROM at the time the parameter gets changed instead of waiting until the power down warning occurs. This of course also must take into account the life time expectation of the EEPROM because they are only specified for a certain number of guarenteed write cycles per location.

Michael Karas