Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 19

These are advantages to using SPI:
• Many microcontrollers include hardware support for SPI. The hardware
support simplifies programming.
• All SPI signals are unidirectional so the host doesn't need to have bidirec-
tional port pins.
• A variety of chips and modules in addition to MultiMediaCards have SPI
interfaces. The options include EEPROMs, analog-to-digital converters,
and other I/O functions. A microcontroller can thus use one bus to
access multiple components.
• For interfaces that don't require error checking, an SPI host can instruct a
card to ignore error checking. Error checking is mandatory with the Mul-
tiMediaCard bus.
These are advantages to using the MultiMediaCard bus:
• The host doesn't require a chip-select line for each card. Instead,
addresses are assigned via firmware.
• The host can broadcast commands to multiple cards.
• The host can perform stream reads and writes, where the data isn't in
defined blocks and the card or host transmits continuously until the host
issues a STOP_TRANSMISSION command. SPI hosts can perform
block reads and writes only.
• MMCplus, MMCmobile, and MMCmicro cards can use a parallel data
bus for faster transfers.
A host selects the MultiMediaCard bus or SPI by controlling the CS pin on
the card when sending the GO_IDLE_STATE command to the card. To
use SPI, the host brings CS low while sending the command. To use the
MultiMediaCard bus, CS remains high. All communications that follow use
the selected bus.
The MultiMediaCard specification doesn't mandate power-consumption
limits. A typical MultiMediaCard consumes 50 mA during read operations
and 60 mA during write operations. Cards can support a low-power sleep
mode when the card isn't being accessed.
Protocols
The MultiMediaCard specification defines a set of MultiMediaCard com-
mands. The host uses the commands to retrieve information about a card