Merged crc7.py into sdcard.py, fixed many little things per suggestions.
Removed hardwired crcs from cmd, always recompute.
Signed-off-by: Marcus Mendenhall <mendenmh@gmail.com>
Originally by @peterhinch.
See https://github.com/micropython/micropython/pull/6007 for discussion.
The summary is that on some cards (especially older Kingston ones) if the
bus is shared with other SPI devices, then it seems to require that MISO
is high for a few cycles before the transaction is initiated.
Because CS is high, this change should otherwise be a no-op.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
Marcus Mendenhall