Datasheet

350
32133D–11/2011
UC3D
20.6.4 SPI Mode
The USART features a Serial Peripheral Interface (SPI) link compliant mode, supporting syn-
chronous, full-duplex communication, in both master and slave mode. Writing 0xE (master) or
0xF (slave) to MR.MODE will enable this mode. A SPI in master mode controls the data flow to
and from the other SPI devices, who are in slave mode. It is possible to let devices take turns
being masters (aka multi-master protocol), and one master may shift data simultaneously into
several slaves, but only one slave may respond at a time. A slave is selected when its slave
select (NSS) signal has been raised by the master. The USART can only generate one NSS sig-
nal, and it is possible to use standard I/O lines to address more than one slave.
20.6.4.1 Modes of Operation
The SPI system consists of two data lines and two control lines:
• Master Out Slave In (MOSI): This line supplies the data shifted from master to slave. In
master mode this is connected to TXD, and in slave mode to RXD.
• Master In Slave Out (MISO): This line supplies the data shifted from slave to master. In
master mode this is connected to RXD, and in slave mode to TXD.
• Serial Clock (CLK): This is controlled by the master. One period per bit transmission. In both
modes this is connected to CLK.
• Slave Select (NSS): This control line allows the master to select or deselect a slave. In
master mode this is connected to RTS, and in slave mode to CTS.
Changing SPI mode after initial configuration has to be followed by a transceiver software reset
in order to avoid unpredictable behavior.
20.6.4.2 Baud Rate
The baud rate generator operates as described in ”Baud Rate in Synchronous and SPI Mode”
on page 341, with the following requirements:
In SPI Master Mode:
• The Clock Selection field (MR.USCLKS) must not equal 0x3 (external clock, CLK).
• The Clock Output Select bit (MR.CLKO) must be one.
• The BRGR.CD field must be at least 0x4.
• If USCLKS is one (internal divided clock, CLK_USART/DIV), the value in CD has to be even,
ensuring a 50:50 duty cycle. CD can be odd if USCLKS is zero (internal clock, CLK_USART).
In SPI Slave Mode:
• CLK frequency must be at least four times lower than the system clock.
20.6.4.3 Data Transfer
• Up to nine data bits are successively shifted out on the TXD pin at each edge. There are no
start, parity, or stop bits, and MSB is always sent first. The SPI Clock Polarity (MR.CPOL),
and SPI Clock Phase (MR.CPHA) bits configure CLK by selecting the edges upon which bits
are shifted and sampled, resulting in four non-interoperable protocol modes see Table 20-7.
A master/slave pair must use the same configuration, and the master must be reconfigured if
it is to communicate with slaves using different configurations. See Figures 20-18 and 20-19.