578
SAM9G45 [DATASHEET]
6438K–ATARM–12-Feb-13
33.7.7
SPI Mode
The Serial Peripheral Interface (SPI) Mode is a synchronous serial data link that provides communication with
external devices in Master or Slave Mode. It also enables communication between processors if an external pro-
cessor is connected to the system.
The Serial Peripheral Interface is essentially a shift register that serially transmits data bits to other SPIs. During a
data transfer, one SPI system acts as the “master” which controls the data flow, while the other devices act as
“slaves'' which have data shifted into and out by the master. Different CPUs can take turns being masters and one
master may simultaneously shift data into multiple slaves. (Multiple Master Protocol is the opposite of Single Mas-
ter Protocol, where one CPU is always the master while all of the others are always slaves.) However, only one
slave may drive its output to write data back to the master at any given time.
A slave device is selected when its NSS signal is asserted by the master. The USART in SPI Master mode can
address only one SPI Slave because it can generate only one NSS signal.
The SPI system consists of two data lines and two control lines:
Master Out Slave In (MOSI): This data line supplies the output data from the master shifted into the input of the
slave.
Master In Slave Out (MISO): This data line supplies the output data from a slave to the input of the master.
Serial Clock (SCK): This control line is driven by the master and regulates the flow of the data bits. The master
may transmit data at a variety of baud rates. The SCK line cycles once for each bit that is transmitted.
Slave Select (NSS): This control line allows the master to select or deselect the slave.
33.7.7.1
Modes of Operation
The USART can operate in SPI Master Mode or in SPI Slave Mode.
Operation in SPI Master Mode is programmed by writing at 0xE the USART_MODE field in the Mode Register. In
this case the SPI lines must be connected as described below:
The MOSI line is driven by the output pin TXD
The MISO line drives the input pin RXD
The SCK line is driven by the output pin SCK
The NSS line is driven by the output pin RTS
Operation in SPI Slave Mode is programmed by writing at 0xF the USART_MODE field in the Mode Register. In
this case the SPI lines must be connected as described below:
The MOSI line drives the input pin RXD
The MISO line is driven by the output pin TXD
The SCK line drives the input pin SCK
The NSS line drives the input pin CTS
In order to avoid unpredicted behavior, any change of the SPI Mode must be followed by a software reset of the
transmitter and of the receiver (except the initial configuration after a hardware reset). (See
Section 33.7.8.2).
33.7.7.2
Baud Rate
In SPI Mode, the baudrate generator operates in the same way as in USART synchronous mode:
See “Baud RateIn SPI Master Mode:
The external clock SCK must not be selected (USCLKS
≠ 0x3), and the bit CLKO must be set to “1” in the Mode
Register (US_MR), in order to generate correctly the serial clock on the SCK pin.