Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 16

Figure 1-5: Each MultiMediaCard contains memory, registers, and an intelligent
controller.
For many embedded systems, a MultiMediaCard host is a good choice
because the cards are small, many microcontrollers can interface to them,
and MultiMediaCard hosts have no licensing fees. A MultiMediaCard host
can also communicate with SD Cards if the connector accepts the slightly
thicker SD cards. Other options are an SD-Card host or CompactFlash
host.
MultiMediaCard
A MultiMediaCard contains these elements (Figure 1-5):
• Memory for data storage. The memory is typically flash memory but
ROM-based MultiMediaCards are also available.
• Five registers that can store configuration and status information such as
valid power-supply voltages and whether the card has completed its
power-up procedure.
• An interface that supports communicating via the MultiMediaCard bus
and SPI.
• A controller that executes MultiMediaCard commands.
There are three classes of MultiMediaCards. The Read/Write (RW) class
encompasses cards that can read and write to storage media, typically flash
memory. Read-only Memory (ROM) cards support reading but not writing
to the storage media. I/O cards perform additional functions beyond data
storage.