Information

maximum of 128 words. For both read and write, the burst length, can be from 1 word to
the maximum of 31 words. The host driver may configure the value according to the
system situation and requirement.
During a multi-block data transfer, the block length may be set to any value between 1
and 4096 bytes inclusive which satisfies the requirements of the external card. The only
restriction is from the external card. It might not support that large of a block or it doesn't
support a partial block access (which is not the integer times of 512 bytes).
For block size not times of 4, i.e., not word aligned, SDHC requires stuff bytes at the end
of each block, because SDHC treats each block individually. For example, the block size
is 7 bytes, there are 12 blocks to write, the system side must write 2 times for each block,
and for each block, the ending byte would be abandoned by SDHC since it only sends 7
bytes to the card and picks data from the following system write, so there would be 24
beats of write access in total.
49.5.1.4 Dividing large data transfer
This SDIO command CMD53 definition, limits the maximum data size of data transfers
according to the following formula:
Max data size = Block size x Block count
The length of a multiple block transfer needs to be in block size units. If the total data
length can't be divided evenly into a multiple of the block size, then there are two ways to
transfer the data which depend on the function and the card design. Option 1 is for the
host driver to split the transaction. The remainder of the block size data is then transferred
by using a single block command at the end. Option 2 is to add dummy data in the last
block to fill the block size. For option 2, the card must manage the removal of the dummy
data.
The following diagram illustrates the dividing of large data transfers. Assuming a kind of
WLAN SDIO card only supports block size up to 64 bytes. Although the SDHC supports
a block size of up to 4096 bytes, the SDIO can only accept a block size less than64 bytes,
so the data must be divided (see example below).
Functional description
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
1434 Freescale Semiconductor, Inc.