Wanted: Write-Once MicroSD Card !
Allegedly these exist! --- though I have only been able to find them offered for sale by the railroad car.
For certain applications, nothing else will really suffice.
If any of my readers know of (or wish to become) a vendor offering, in (for starters) mid-three-digit quantities:
- a) One Time Programmable MicroSD card
- b) MicroSD card with a true hardware write-protect feature (specifically not the idiot floppy-style software-read tab
...please write in!
Via email, from a reader:
Hi Stanislas
via mail because comments on loper-os.org are b0rked, reCAPTCHA is no longer working.
You’re probably aware of this, but the SD spec defines flag PERM_WRITE_PROTECT in the CSD of a compliant SD card. If you set this (one time programmable) bit, the card’s firmware must reject all write or erase requests. Now, the pages in the card aren’t *physically* OTP, so one could, for example, replace the controller to get around it, but if you’re at that point, one could also replace the *pages* in a card that is physically OTP. Signed data + PERM_WRITE_PROTECT should do it for you, I’d think.
Samsung doc - http://www.mt-system.ru/sites/default/files/klmxgxge4a-x001mmc4_41_2ynm_based_emmc1_1.pdf
SanDisk doc : https://elinux.org/images/d/d3/Mmc_spec.pdf
Some flash chips have *actual* OTP pages, but they tend to be small, ~64k per chip or so, not sure if they are avalable packaged in SD, and if the SD protocol would even allow access.
Oh, and more FFA, please.
cheers
Simon