The but count should be set to 7 when entering RXDATA. previously it was
not reset or left at 0, which caused the first byte to only have the lsb
set and all other bits to be read incorrectly.
After a data read (e.g. CMD17) the data received from the SD card is
stored into a buffer which can be read back one byte at a time by the
CPU through address 5.
There is also a flag which is set when data is received. This can be
checked by reading the CMD register, which doubles as the status
register.
Read flag is set when the sd controller reads response data in from the
sd card. When the cpu reads from the controller, the flag is reset.
This flag does not trigger an interrupt, it mmust be polled.
The SD card expects data to transition on falling edges and be stable on
rising edges.
Additionally, writes from the CPU were not handled with correct timing.
Now, there is an extra state when writing to the command register so
that the command is properly latched before the CRC is calculated.
Adds the start of the SD card controller which is capable of sending
commands using the SD protocol.
It is accessed by writing the arguments first and triggered by writing
the command number.
