Texas Instruments MSP430x1xx - Manual (Page 251)

Synchronous Operation
13-5
USART Peripheral Interface, SPI Mode
Figure 13–3. MSP430 USART as Master, External Device With SPI as Slave
Receive Buffer URXBUF
Receive Shift Register
MSB LSB
Transmit Buffer UTXBUF
Transmit Shift Register
MSB LSB
SPI Receive Buffer
Data Shift Register (DSR)
MSB LSB
SOMI SOMI
SIMO SIMO
MASTER SLAVE
Px.x STE
STE
SS
Port.x
UCLK SCLK
MSP430 USART COMMON SPI
The master initiates the transfer by sending the UCLK signal. For the master,
data is shifted out of the transmit shift register on one clock edge, and shifted
into the receive shift register on the opposite edge. For the slave, the data
shifting operation is the same and uses one common register for transmitting
and receiving data. Master and slave send and receive data at the same time.
Whether the data is meaningful or dummy data depends on the application
software:
- Master sends data and slave sends dummy data
- Master sends data and slave sends data
- Master sends dummy data and slave sends data
Figures 13–4 and 13–5 show an example of a serial synchronous data transfer
for a character length of seven bits. The initial content of the receive shift
register is 00. The following events occur in order:
A) Slave writes 98h to the data shift register (DSR) and waits for the master
to shift data out.
B) Master writes B0h to UTXBUF, which is immediately transferred to the
transmit shift register, and starts the transmission.
C) First character is finished and sets the interrupt flags.
D) Slave reads 58h from the receive buffer (right justified).
E) Slave writes 54h to the DSR and waits for the master to shift out data.
F) Master reads 4Ch from the receive buffer URXBUF (right justified).
G) Master writes E8h to the transmit buffer UTXBUF and starts the
transmission.
Note: If USART is in slave mode, no UCLK is needed after D), until G).
However, in master mode, two clocks are used internally (not on UCLK
signal) to end transmit/receive of first character and prepare the
transmit/receive of the next character.