C2510Fx / CC2511Fx
13.14.2.3 Slave Select pin (SSN)
end of the byte sent / received. If this is not the
case, the next received byte will be corrupted.
If there is a rising edge on SSN in the middle
of a byte, this should be followed by a USART
flush to avoid corruption of the following byte.
When the USART is operating in SPI slave
mode, a 4-wire interface is used with the Slave
Select (SSN) pin as an input to the SPI (edge
controlled). The SPI slave becomes active
after a falling edge on SSN and will receive
data on the MOSI input and send data on the
MISO output. After a rising edge on SSN, the
SPI slave is inactive and will not receive data.
Note that the MISO output is not tri-stated
when the SPI slave is inactive. Also note that
the rising edge on SSN must be aligned to the
In SPI master mode, the SSN pin is not used.
When the USART operates as an SPI master
and a slave select signal is needed by an
external SPI slave device, a general purpose
I/O pin should be used to implement the slave
select signal function in software.
Figure 40: SPI Dataflow
13.14.3
Baud Rate Generation
where F is the system clock frequency set by
the selected system clock source.
An internal baud rate generator set up the
UART baud rate when operating in UART
mode and the SPI master clock frequency
when operating in SPI mode.
The register values required for standard baud
rates are shown in Table 55 (F = 26 MHz) and
Table 56 (24 MHz). The tables also give the
difference in actual baud rate to standard baud
rate value as a percentage error.
The
UxBAUD.BAUD_M[7:0]
and
UxGCR.BAUD_E[4:0] registers define the
baud rate used for UART transfers and the
rate of the serial clock (SCK) for SPI transfers.
The baud rate is given by the following
equation:
The maximum baud rate for UART mode is
F/16
(UxGCR.BAUD_E[4:0]=16
and
UxBAUD.BAUD_M[7:0]=0).
The maximum baud rate for SPI master mode
(256 + BAUD _ M )∗2BAUD _ E
and
thus
SCK
frequency
is
F/8
and
Baudrate =
∗ F
(UxGCR.BAUD_E[4:0]=17
228
SWRS055D
Page 154 of 243
TAOS [ TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS ]
