Texas Instruments MSP430x4xx - Manual (Page 295)

USART Operation: SPI Mode
15-9 USART Peripheral Interface, SPI Mode
15.2.5 Serial Clock Control
UCLK is provided by the master on the SPI bus. When MM = 1, BITCLK is
provided by the USART baud rate generator on the UCLK pin as shown in
Figure 15−8. When MM = 0, the USART clock is provided on the UCLK pin by
the master and, the baud rate generator is not used and the SSELx bits are
don’t care. The SPI receiver and transmitter operate in parallel and use the
same clock source for data transfer.
Figure 15−8. SPI Baud Rate Generator
Bit Start
mX
BRCLK
8 8
UCLKI
ACLK
SMCLK
SMCLK 11
BITCLK
10
01
00
20 27 28 215
Compare (0 or 1)
Modulation Data Shift Register
(LSB first)
16−Bit Counter
Q0
............
Q15
m0 m7
... ...
8
UxBR1 UxBR0
Toggle
FF
N =
R
R
R
UxMCTL
SSEL1 SSEL0
The 16-bit value of UxBR0+UxBR1 is the division factor of the USART clock
source, BRCLK. The maximum baud rate that can be generated in master
mode is BRCLK/2. The maximum baud rate that can be generated in slave
mode is BRCLK The modulator in the USART baud rate generator is not used
for SPI mode and is recommended to be set to 000h. The UCLK frequency is
given by:
Baud rate =
BRCLK
UxBR
with UxBR= [UxBR1, UxBR0]