CCISx Bit
13-12
Capture/compare input select. These bits select the TACCRx input signal.
See the device-specific datasheet for specific signal connections.
00 CCIxA
01 CCIxB
10 GND
11 VCC
看看msp430x1xx系列用户指南中的表述:
The capture inputs CCIxA and CCIxB are connected to external pins or internal signals and are selected with the CCISx bits.
是用来选择捕捉的输入信号的。