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6.0 SPI Protocol Definition
6.1
SPI Bus Protocol
While the MultiMediaCard channel is based on
command and data
initiated by a start bit and terminated by a stop
bit, the SPI channel is byte oriented. Every
command or data block is built of 8 bit bytes and is
byte aligned (multiples of 8 clocks) to the CS
signal.
Similar to the MultiMediaCard protocol, the SPI
messages are built from command, response and
data-block tokens. All communication between
host and cards is controlled by the host (master).
The host starts every bus transaction by asserting
the CS signal low.
The response behavior in SPI mode differs from
the MultiMediaCard mode in the following three
aspects:
• The selected card always responds to t h e
command.
• An 8 or 16 bit response structure is used.
• When the card encounters a data retrieval
problem, it will respond with an error
response (which replaces the expected
data block) rather than time-out as in t h e
MultiMediaCard mode.
Only single block read write operations are
supported in SPI mode. In addition
command response, every data block sent to t h e
card during write operations will be responded
with a special data response token. A data block
may be as big as one card sector and as small as a
1
single byte.
6.1.1
Mode Selection
The
MultiMediaCard
MultiMediaCard mode. It will enter SPI mode i f
the CS signal is asserted (negative) during t h e
reception of the reset command (CMD0). If t h e
card recognizes that the MultiMediaCard mode is
1)
The default block length is as specified in the CSD
(512 bytes). A set block length of less than 512
bytes will cause a write error. The only valid write
set block length is 512 bytes. CMD16 is not
mandatory if the default is accepted.
SanDisk MultiMediaCard Product Manual Rev. 2 © 2000 SANDISK CORPORATION
bit-streams
which
are
to t h e
wakes
up
in
t h e
MultiMediaCard Product Manual
required it will not respond to the command and
remain in the MultiMediaCard mode. If SPI mode
is required, the card will switch to SPI mode and
respond with the SPI mode R1 response.
The only way to return to the MultiMediaCard
mode is by power cycling the card. In SPI mode,
the MultiMediaCard protocol state machine is not
observed. All the MultiMediaCard commands
supported in SPI mode are always available.
The CMD (pin 2) and DAT[0] (pin 7) lines start up
2
in "Open Drain"
mode and must be sourced
externally. Once the card is in SPI mode, the lines
are in "push-pull" mode until power is cycled.
Since the card defaults to MultiMediaCard mode
after a power cycle, Pin 1 (CS) must be pulled low
and CMD0 (40h) must be sent on the CMD (DataIn,
pin 2) line in order for the card to enter SPI mode.
The
default
command structure/protocol
MultiMediaCard mode is to have CRC checking
enabled.
The default command structure/protocol for SPI
mode is that CRC checking is disabled. Since t h e
card powers up in MultiMediaCard mode, CMD0
must be followed by a valid CRC byte (even
though the command is sent using the
structure). Once in SPI mode, CRCs are disabled by
default.
CMD0 is a static command and always generates
the same 7 bit CRC of 4Ah. Adding the "1," end
bit (bit 0) to the CRC creates a CRC byte of 95h.
The following hexidecimal sequence can be used to
send CMD0 in all situations for SPI mode, since
the CRC byte (although required) is ignored once
in SPI mode. The entire CMD0 sequence appears as
40 00 00 00 00 95 (hexidecimal).
2)
See section 4.2.
for
SPI
61
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