Power failure adviser for MCU

[wraper]

Please elaborate since the datasheet you yourself quote in some post above clearly states:

Probably not all IC's have this feature, have not used those IC's, links just as example. Certainly have read FRAM datasheets which explicitly stated this. http://www.ti.com/lit/an/slaa526a/slaa526a.pdf:

The write-back mechanism is also protected from power loss and is ensured to
complete safely under all power conditions.

That in combination with a serial I2C protocol can be desastrous on an unexpected power outage if you do not monitor it and have some extra time to shut down properly, thus coming back to the original issue. I do not see a big difference. I rather have an externa spi flash than an I2C external fram for that matter, as long as the nr of write cycles is not the issue that is.

What prevents from using SPI FRAM if particularly I2C is your concern? http://eu.mouser.com/ProductDetail/Cypress-Semiconductor/FM25L04B-GTR/?qs=sGAEpiMZZMthnG%252bV7QrURGCbJW7ee0dy5BEjZwdm1fs%3d

I call the I2C hardly instantly, you first have to adress the chip then write the datablocks, go count on 100kHz bus and a coming powerfailure, you also have to buffer the chips.

You can use up to 1MHz I2C speed. Once data is sent, it is written instantly, no programming delay like in EEPROM or FLASH. Or SPI is you need the speed so much.

that is too late, you need a pre-warning. I still would isolate the mcu in this case and use some buffer capacitors, if the costs is an issue then so is the price of the fram.
One of our requirements for external storage was to hold two firmwares of 256kB, good luck with fram on that requirement .

What too late? That was told about destructive read. Even if the IC have no protection feature just don't read it while below certain voltage level for which BOD in MCU is perfectly sufficient. OP don't need 256kB as considered using internal EEPROM of xmega. 256-512 kB of FRAM is not something exceptional either. http://eu.mouser.com/Semiconductors/Integrated-Circuits-ICs/Memory/F-RAM/_/N-98x7s?P=1z0w12pZ1z0w12o
FRAM FAQ from TI  http://www.ti.com/lit/ml/slat151/slat151.pdf1.pdf

[wraper]

True, but broken records should be taken into account. If you update the administration (fat) after verification of the data written it would be ok

No way, it means rewrite for no reason.

Speed. FRAM has fast write times. Beyond all the other
operations, the actual write time to an FRAM memory
cell is less than 50ns. That is up to 1000x faster than
Flash/EEPROM. Additionally, unlike EEPROM where you
must have two steps to write data: a write command,
followed by a read/verify command; FRAM’s write
memory function happens in the same process as read
memory. There is only one memory access command,
one step for either reading or writing. So in effect, all the
time associated with an EEPROM write transaction is
eliminated in an FRAM-based smart IC.

FRAM is very reliable BTW. And because it is RAD resistant and don't experience bit flips is used in space and medical equipment. Goes into automotive as well.

EDIT: Fujitsu recommends it particularly for this purpose. http://www.fujitsu.com/global/products/devices/semiconductor/memory/fram/overview/features/index.html

1. Performance Improvement
FRAM's high-speed writing can take backup data at an instantaneous power supply interruption. Not only that, FRAM can record data more frequent than EEPROM and Flash memories. When writing the data, EEPROM and Flash memories need high voltage and thus, consume more power than FRAM. The battery of battery-powered device lives longer if FRAM embedded. In summary, FRAM is;
Able to take backup data at an instantaneous power supply interruption
Able to take frequent data records
Able to keep a battery life longer

[Kjelt]

It sure does have its usefull purposes, except the price which is rather high for the larger devices.
But then I can think of it as a buffer or temporary storage for larger storage, so for instance logfiles etc. till it is almost full then write the entire contents to the much cheaper flash for final storage.