Am 26.02.2014 um 18:27 schrieb Grazvydas Ignotas:
On Wed, Feb 26, 2014 at 2:30 PM, Dr. H. Nikolaus Schaller hns@goldelico.com wrote:
Ideas so far: a) DIP switch (i.e. a permanent switch) b) reuse some shoulder/other button to be pressed during boot c) GPIO (but the BootROM is not able to operate the GPIO and we can't modify the BootROM!)
Unfortunately we have no card-detect on such uSD readers. So we can't detect that a uSD is inserted to activate the switch. Even if we had, I would not recommend to use it - because we then never have access to the eMMC and can't even reprogram it.
But what we can do is to mix the GPIO + button solution by using some series resistor on the GPIO and work with pull-up/pull down.
I don't really understand how you are going to make it work at all, ignoring the software switch issues. The data lines would be shared between eMMC and uSD, wouldn't they be? Even if you don't power the one you don't need, wouldn't they both interfere?
They are not wired in parallel but there is an electronic switch chip for the data and clock:
http://www.ti.com/product/txs02612
I don't think SD cards are designed for the case of somebody else driving data lines while they are powered off. Wouldn't the voltages differ too, like the SD card uses ~3V and I'd expect eMMC 1.8V, although I haven't checked.
It also handles the voltage level translation.
I think some old Nokia tablet (N800?) used multiple cards on the same controller somehow, at least I remember seeing some kernel code for that..
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