MEMO

SD spec:
--------
physical_layer_simplified_specification_ver6.00.pdf

SD commands:
------------
CMD    -standard command
ACMD   -application specific command (CMD55 before)

SD init for UHS-I(3.9.4):
-------------------------
CMD0
CMD8     -verify host and card are able to talk
ACMD41   -check if card can use requested voltage
CMD11    -issue voltage change
CMD2     -identification
CMD3
CMD7
CMD42    -unlock the card
ACMD6    -set to 4 bits mode
CMD6     -set driver strength
CMD19    -send tuning block to the host (mandatory for SDR104,  -optional for SDR50)

SD speeds (3.9.6):
------------------
SDR104: 104MB/s, 208MHz clk, 1.8v
SDR50:   50MB/s, 100MHz clk, 1.8v
DDR50:   50MB/s,  50MHz clk, 1.8v
SDR25:   25MB/s,  50MHz clk, 1.8v
SDR12: 12.5MB/s,  25MHz clk, 1.8v
HS:      25MB/s,  50MHz clk, 3.3v
DS:    12.5MB/s,  25MHz clk, 3.3v