SD Voltages

I am reading a CPU eval board data sheet that says it supports 1.8
volt interface for MMC cards.  They don't seem to support SD cards.
Looking at wikipedia they say, "All SD card families initially use a
3.3-volt electrical interface. On command, SDHC and SDXC cards switch
to 1.8-volt operation."  How is that intended to work?  I don't see
the utility of requiring a voltage converter chip so you can first run
at 3.3 volts then switch to 1.8 volts.  Is this the only way to make
this work if you don't want to have two types of cards?  I guess I
would have expected the interface to work from what ever voltage is
present on the power pin, no?

So how is this used in a design typically? The CPU I am considering
can only work with 1.8 volt I/Os. Do I really have to use a level
shifter chip to complete this interface?

Rick

> I am reading a CPU eval board data sheet that says it supports 1.8
> volt interface for MMC cards.  They don't seem to support SD cards.
> Looking at wikipedia they say, "All SD card families initially use a
> 3.3-volt electrical interface. On command, SDHC and SDXC cards switch
> to 1.8-volt operation."  How is that intended to work?

Ask the manufacturer of the CPU eval board.

For HW design, just dump in a SN74AVC4T774 and be done with it.

Leo Havm�ller.