Setting the UCTXSTP bit will generate a STOP condition. After setting
UCTXSTP, a NACK followed by a STOP condition is generated after reception
of the data from the slave, or immediately if the USCI module is currently
waiting for UCBxRXBUF to be read.
If a master wants to receive a single byte only, the UCTXSTP bit must be set
while the byte is being received. For this case, the UCTXSTT may be polled
to determine when it is cleared:
这个是datasheet手册上的一段，请问我何时发送stop信号，