The Micro (STM23F4xx) I'm using happens to include a SDIO peripheral that is supposed to support the 4-bit parallel bus SD Card protocol. I'm aware that the SD Association holds patents on the protocol and requires a fee for using it. I'm wondering whether this also applies to me when I'm just using the peripheral, or whether ST Micro has payed for it in order to implement it.

Also: supposed I route all signals required for 4-bit mode (for testing, experimenting, ect.), but then happen to only use SPI mode, does the fee still apply?

  • \$\begingroup\$ If someone reverse engineers your product and finds out you used the 4-bit protocol in your software stack, and also somehow finds out you did not pay SD for it, you will get in trouble. \$\endgroup\$ – KyranF Oct 7 '14 at 7:36
  • \$\begingroup\$ I'd ask ST directly whether they've licensed it, and if so whether their license applies to designers that use their MCU's. I suspect they have but it doesn't, but I do wonder from time to time - so if you can find out and report back, that'd be nice... re the SPI/SD interface, an annoyance that I have is that (on the STM32's I've looked at) they haven't routed an SPI module onto the matching pins for the SDIO module. \$\endgroup\$ – markt Oct 7 '14 at 8:21
  • \$\begingroup\$ I've contacted ST - let's see what they come up with. \$\endgroup\$ – Arne Oct 7 '14 at 10:12

First of all, some terms. SDIO refers to an I/O (not memory) card protocol that is based on the SD card protocol. The STM32F40xxx datasheet says the chip supports 1 and 4-bit SDIO, 1, 4, and 8-bit MMC (MultiMediaCard, and 1 and 4-bit SD cards.

ST Micro most certainly licensed the protocol to design and test the interface, but I doubt very much the SD Card Association would let them make the software protocol available at no cost to anyone who buys their chips, even under an NDA agreement.

I don't see any problem with running the extra wires for the 4-bit mode on your board, and having some sort of jumper options to select between 1 and 4-bit mode. In fact, I've done exactly that myself. As far as I know, the proprietary aspect of the 4-bit interface is in the software, not the hardware interface, which is well-0documented.

  • \$\begingroup\$ You can stop assuming the hardware supports 4-bit SD cards. It does. \$\endgroup\$ – markt Oct 7 '14 at 9:50

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