Texas Instruments MSP430x4xx - Manual (Page 293)

USART Operation: SPI Mode
15-7 USART Peripheral Interface, SPI Mode
15.2.4 SPI Enable
The SPI transmit/receive enable bit USPIEx enables or disables the USART
in SPI mode. When USPIEx = 0, the USART stops operation after the current
transfer completes, or immediately if no operation is active. A PUC or set
SWRST bit disables the USART immediately and any active transfer is
terminated.
Transmit Enable
When USPIEx = 0, any further write to UxTXBUF does not transmit. Data
written to UxTXBUF will begin to transmit when USPIEx = 1 and the BRCLK
source is active. Figure 15−4 and Figure 15−5 show the transmit enable state
diagrams.
Figure 15−4. Master Mode Transmit Enable
Idle State
(Transmitter
Enabled)
Transmit
Disable
Transmission
Active
USPIEx = 0
No Data Written
to Transfer Buffer
Not Completed
USPIEx = 1
USPIEx = 0
USPIEx = 1,
Data Written to
Transmit Buffer
Handle Interrupt
Conditions
Character
Transmitted
USPIEx = 1
USPIEx = 0 And Last Buffer
Entry Is Transmitted
SWRST
PUC
Figure 15−5. Slave Transmit Enable State Diagram
Idle State
(Transmitter
Enabled)
Transmit
Disable
Transmission
Active
USPIEx = 0 No Clock at UCLK
Not Completed
USPIEx = 1
USPIEx = 0
USPIEx = 1
Handle Interrupt
Conditions
Character
Transmitted
USPIEx = 1
USPIEx = 0
SWRST
PUC
External Clock
Present