Hardware Reference
In-Depth Information
Figure 8-2.
SD card circuit
Looking at the wiring in Figure
8-2
, it might be assumed that all data transfers are 4
bits wide (GPIO 50 through GPIO 53). However, as the following sections will describe,
this depends on the SD card media used.
SD Card Basics
The SD card includes an internal controller, also known as a Flash Storage Processor
(FSP). In this configuration, the Linux host merely provides a command and waits for the
response. The FSP takes care of all erase, programming, and read operations necessary
to complete the command. In this way, Flash card designs are permitted to increase in
complexity as new performance and storage densities are implemented.
The SD card manages data with a sector size of 512 bytes. This was intentionally
made the same as the IDE magnetic disk drive for compatibility with existing operating
systems. Commands issued by the host include a sector address to allow read/writes of
one or more sectors.
■
Note
operating systems may use a multiple of the 512-byte sector.
Commands and data are protected by CRC codes in the FSP. The FSP also
automatically performs a read after write to verify that the data is written correctly.
21
If
the data write is found defective, the FSP automatically corrects it, replacing the physical
sector with another if necessary.
