NAND flash retention using USB charger?

Question

NAND-based digital flash data storage devices such as SD,SSD,USBstick,eMMC,UFS,etc. do have transistors, where the small amount of electrical charge does slowly leak over time, resulting into data loss when inactive for excessive time spans. Heat does accelerate that process.

In order to recharge the transistors, do I need to actually read the data like on HDDs, or does the storage just need electrical power supply?

And if the latter one is true, how many seconds or minutes does the NAND storage need power supply? Does 5.3V instead of 4.9V make any difference?

Answer 1

Flash does lose charge over time.

In order to recharge the transistors, do I need to actually read the data like on HDDs, or does the storage just need electrical power supply?

Nope, you would need to read all the data and rewrite it to refresh the charge inside the gates of the flash transistors. Bits are stored as electrons (ie, charge) injected inside a microscopic insulated bit of metal. The bits stay there because the electrons are trapped inside a tiny piece of metal which is insulated from everything, so the electrons can't escape. You suggest twiddling with the power supply voltage, but this has no influence on the bits, by design.

Flash storage data retention time is determined by stored electrons leaking out, which depends on heat and radiation. It also depends on how many electrons were in there to begin with (ie, storage density) and flash architecture (multi level cell vs single level, NOR vs NAND, etc).

Where data retention is crucial (ie, in a microcontroller for example, if it forgets its code it is dead) you get low density NOR flash. TI provides some data retention info on page 5 of this appnote which is informative. NOR flash allows random access, so it can't use nifty cheats like block error correction codes.

As for USB keys, SD cards, etc, there is an arms race towards more density at cheaper price, and the fact these are accessed per block and not in a random fashion enables useful things like error correction codes, bad block remapping, etc. This is handled by a microcontroller inside the SD card or USB key.

I've found some data retention info online, it should be between 1 and 10 years for a SD card depending on the number of writes. Writing a flash cell is a bit of a traumatic process for the oxide layer, so a high number of writes makes the cell more leaky.

So, there is actually no reason (besides power saving) why the microcontroller wouldn't be able to periodically read the data and rewrite the sectors that begin to rot. There does seem to be a SD card which refreshes itself and rewrites the data to increase data retention when needed. A bit expensive though.

You can do it manually, but I'm not sure it would be a good idea. Perhaps copy the data from one storage media onto another... like... you know... a backup!

The most probable cause of data loss for a USB key isn't flash death anyway, rather:

• misplaced or lost somewhere
• FAT32 filesystem issues
• ripped connectors and the like

So... RAID array + backups!

Answer 2

Flash data is stored on floating gate transistors like this one:

Initially, the transistor has no charge on its floating gate, so when we apply a positive voltage to the control gate, the transistor conducts, and we call this state 1. When we want to write a 0 to the transistor, control gate will apply a strongly negative voltage so electrons will tunnel across the thin insulator and stay on the floating gate. Now when we apply a positive voltage to the control gate, the transistor can't see it behind the electric field from the negative charge on the floating gate and doesn't conduct.

Because the floating gate is surrounded by insulators, the electrons on it can't leak out and are stuck there until they are forced off by applying a strong positive voltage to the control gate. They will remain there, even if the power is off, for 10 years or more. Additionally, their presence is not changed by reading from the transistor, as none of the voltages used for reading are high enough to cause the electrons to tunnel through the insulator.