How's the weather down Houston way ? My sister used to live in suburbs north of Houston, Wood something. Been there once, visiting, back in 82. Now she's in Tulsa.
We're still buying the 5V SRAM parallel interface chips. They were supposed to go EOL but are still available. I didn't know about Cypress. And I think Freescale or TI have some other magneto semiconductor technology for NV memory applications, but I don't have any experience with them.
I'm on Digi-Key's website on another computer and Ramtron isn't even listed anymore. only Cypress and Fujitsu. But if you want the 40MHz SPI part, only Cypress.
We use 100-400 KHz clock depending on other peripherals. Only change major parameters via user interface or computer comm. link when using I2C non-vol backup. The major use in our products use the bytewide parallel SRAM interface to an intel 80188 style uP. These write through in nS bus cycles. We use CRC or checksums and redundancy to protect against loss of power during updates. Batteries not required. Being the software guy, the FRAM is the best, parallel or I2C. Haven't tried the SPI yet.
Only problem is sole-source by Ramtron in Colorado. Supposed to be licensed by Japanese company too, I think Fujitsu. But have had supply problems, and obsolescence issues too. We use 5v tech in our legacy items, now everything is 3v . Also, we used parallel chips so could use for fast bus access RAM just like normal data memory. Now mostly use I2C serial style for non-vol database for embedded products. The quick write, virtually infinite write longevity can't be beat.
@alaskaman66: EPROMS, EEPROMS, Flash all erase in high radiation environments unless they are Rad hardened. FRAM does not, is byte programmable and has no write delay
Atlant: Agreed, and datasheets for SD cards and thumb drives are a rare thing, so you don't really know what kind of random write performance you can achieve.
eric: One "trick" is that manufacturers are somewhat "cagey" (non-descriptive) about which sort of Flash you're dealing with, especially when it's a packaged system like a USB stick or an SD (etc.) card.
Atlant: and that is worse with MLC and TLC devices where there are 4 and 8 possible states per cell, depending on the amount of charge on the floating-gate.
eric: (Re wear leveling) Yes, but we're going to take explicit steps to force our non-scrubbing file system (UBIfs) to scrub by reading through the entire medium from time-to-time.
Alaskaman: The difference between a "1" and a "0" in a Flash (SLC!) cell is now down below 100 electrons, so it doesn't take too many electrons leaking off of the floating gate to flip a bit.
Eric: Right. We've seen the phenomenon where seldom/never-read pages on our device silently sit there accumulating disturb errors and whe the day comes that we finally read the page, blamm! It's got too many errors to be corrected by the ECC.
It's therefore important that one's Flash File System knows that it has to "scrub" its way through all of the Flash every so often, correcting all the accumulated simgle-bit errors.
Atlant: If it's being read often however, you might be getting some recoverable ECC errors, which might trigger a refresh of the block. It it's sitting there idle, however, this won't help.
Eric: More coverage of write-disturb and especially read-disturb would also be good. Many people don't realize that the data stored in a NAND Flash degrades with *EVERY READ* (albeit at a much slower rate than when neighboring cells are written) so there is can be no such thing as a reliable "Read only" Flash memory; bad block recovery still has to be possible.
Atlant: Yes, wouldn't have been a bad idea. For Flash memory usually the typical data retention time is 10 years. This does decrease with read/program disturbances, however.
Eric: It would be good to add "retention time" to your comparison table. While magnetic disks physically wear out (or develop head stiction, etc.), the data they store lasts until the mechanical device fails. But Flash memories (of all sorts) have their bits decay over time so data stored for (say) 10 years on a device may become unreadable even though the device itself hasn't really "failed".
I work on unmanaged flash devices. I write device drivers for Micron's Flash Data Interface (FDI), which is a file system for flash. This free product is especially useful for Mobile Phones (feature phones, not smart phones).
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