Accessing Flash Memory Cards - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 80

Signals and Power
SPI has no official specification document other than the data sheets for the
components that support the interface. As a result, different implementa-
tions use different names for the signals (Table 4-1). Note that the MultiMe-
diaCard signal names are from the perspective of the MultiMediaCard card:
DataIn is an input on the card, while DataOut is an output on the card.
Data and Clock Lines
Table 4-2 shows the pin functions for a MultiMediaCard in SPI mode.
SCLK (also called SCK) is the card's clock input. The clock provides transi-
tions that determine when to write and read data on the data lines. The SPI
host generates the clock pulses.
DataIn (also called MOSI, or Master Out Slave In, or SDI) is the card's data
input. The host uses the DataIn line to send commands and to send data to
the storage media.
DataOut (also called MISO, or Master In, Slave Out, or SDO) is the card's
data output. The card uses the DataOut line to send responses to com-
mands, other status information, and data requested from the storage
media.
CS (also called /SS, or Slave Select) is the device's chip-select input. The host
must control a unique chip-select output for each card on the bus. The host
selects a card by bringing the card's CS input low.
Power
Each card also has a power-supply pin (VDD) and two ground pins (VSS
and VSS2). A high-voltage MultiMediaCard requires a supply voltage of
3.3V. A low-voltage MultiMediaCard can be powered at either 3.3V or
1.8V.
Example Circuit
An SPI host can be a microcontroller with hardware SPI support or any
generic microcontroller or other intelligent hardware with available port
pins and the ability to implement communications entirely in firmware.