说明：   SPI master talks to SPI slave using 3-wire mode. Incrementing data is sent by the master starting at 0x01. Received data is expected to be same as the previous transmission TXData = RXData-1. USCI RX ISR is used to handle communication with the CPU, normally in LPM0. ACLK = ~32.768kHz, MCLK = SMCLK = DCO ~ 1MHz. BRCLK = ACLK/2.