Thread: Is there such a thing as a micro SD to RS-MMC adapter?
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fanoush is offline fanoush
2007-08-04 , 21:41
Posts: 2,152 | Thanked: 1,490 times | Joined on Jan 2006 @ Czech Republic
#22
Originally Posted by new299 View Post
MMC doesn't have chip select so any mod would only be compatible with SD cards.
I think there is a little confusion here. There are three modes of communication over SD/MMC slot.
- SPI - supported in theory by both mmc and sd cards, pins are explained for this mode on picture already mentioned, useful for some ancient hardware that can do only SPI
- MMC mode - only mmc cards can do this, preferred mode for MMC cards
- SD mode - only sd cards can do this, preferred mode for SD cards

Both SD and MMC modes are supported by N770 linux kernel, SPI not (for OMAP driver). Not sure about SD but at least MMC can have multiple cards connected to same bus (i.e. same pins). Cards have unique IDs and all listen to commands and only the one with correct ID responds to the command. SD maybe too (or maybe not) but anyway this mode is not regulary used and is now deprecated by MMC association. MMC high speed modes defined in MMC 4.x require only one card to be present. Also I think support for more cards is to be removed from linux kernel since it is really not used and only complicates things.

So to sum it up you may now connect more MMC cards to same pins but they will run in legacy MMC mode (max 20Mhz) and there may be bugs since this was never widely used. I'm also not sure about power requirements of such solution (i.e. if the slot in n770 can cope with powering two cards at once).
Originally Posted by new299 View Post
We'd also need to find an appropriate chip select line. Can this just be a GPIO line? I dunno. We also would need to get an SD card driver in to the kernel (sounds like this has been done).
chip select is only for SPI mode, we probably don't want to use this and the pin is missing in N770 slot anyway. As for SD driver it is already present in the kernel.
Originally Posted by new299 View Post
A nicer option would be to use the second MMC/SD card port on the CPU, but finding this might be problematic. Is support already present in the OMAP1710 kernel for the second MMC port?
No the support is not completely present due to missing hardware. If I understand it correctly having support for two slots in OMAP1710 CPU is not enough. You need additional hardware for powering up and driving the slot. Such second chip is missing for the second slot and if present in must be controlled from SD/MMC driver in linux kernel (i.e. powered up and down at right time) for the slot to work. Search this forum for both n770 and n800 schematics and see how SD/MMC slots are wired.

I think there are only two easy options to have cards over 2GB (current mmcmobile available maximum)
- wait for bigger mmcmobile cards (no problem with support for them)
- wire up >=4GB SD (regular,mini or micro) or MMCplus card to the slot.
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