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I've seen news articles talking about how SanDisk released Write Once Read Many (WORM) SD cards, but I can't find anywhere to buy them!
I'm thinking of building a project where I'll want to write reasonably small amounts of data (amounting to maybe 64 MB at most) to permanent storage, for retrieval via USB — can anyone think of a suitable IC for this?
Using a microSD card seems like the best idea to me (using a socket or soldering directly to the contacts). You could add an AT90USB162 or LPC1342 for the USB mass-storage interface.
DataFlash chips may be an alternative but their cost per megabyte is much higher.
The SDCard spec allows for WORM via its various write-protect techniques. Most regular SDCards implement the mechanical write-protect switch, making it the host's responsibility to abide by the switch position. But the SDCard spec also specifies two techniques done only by the card: one is an optional "Card Internal Write Protection" mechanism which I bet hardly any SDCard supports. The other is the ability to password-protect an SDCard (the password is actually stored on the card) After a card is locked with a password, if you don't have the password it can only be erased.
If you need the write once implemented on the chip itself you may have a hard time finding the part you need.
If you are ok with writing some code you could use a separate chip to enforce the write once read many policy. I'd suggest pairing a Microchip part (PIC18F13K50 ) with built in USB support with a data flash chip (SST25VF032B). That will give you 8MB of storage. If you need more memory just keep adding SST25VF032B's to the SPI bus.
Total system cost < $5.
Sandisk normally doesn't sell directly to individuals. They sell directly to distributors, who in turn sell directly to individuals such as you and me. If you want to buy this or any other Sandisk product, your best bet is to go to the list of Sandisk distributors and call up each one until you find one willing to take your money.
The press release for the Sandisk WORM cards specifically lists contact information for obtaining these cards. If you really want these specific cards mentioned in the press release, perhaps using that contact information might be a possibility?
Atmel application note "Migrating from the B to the New D DataFlash Family" says: "The “D” family offers sector lockdown for secure code and data storage. The Lockdown mechanism allows each individual sector to be permanently locked so that it becomes read only. Once the sector is locked down, it can never be erased or programmed, and it can never be unlocked."
It uses a SPI interface very similar to the SPI interface of a SD/MMC card. Alas, I think the details of its protocol are different enough that it can't be directly connected to a standard SD/MMC slot as if it were a standard SD/MMC card. And so I don't think you can pop it into a standard off-the-shelf SD/MMC to USB adapter. And so it looks like you'll need a custom-programmed microcontroller, as mjh2007 suggests. In which case, you might as well program that microcontroller to enforce "write-once" requirement, and use any random easily-available non-volatile memory.
Still, practically all software has bugs, and it might be nice to use this relatively easily-available Dataflash (Newark and Digikey have them in stock now) so you don't have to worry about some bug in the microcontroller accidentally overwriting your data.
They are such a niche market you will have real trouble finding any such parts or cards unless you need millions.
