The G2xx1 chips are capable of using external digital clock sources. The LaunchPad has a 12MHz digital clock signal on board. I made an “unauthorized” modification to utilize that 12KHz signal as the external digital clock for the G2xx1 chip.
This 12MHz clock is driven by MSP430F1612 (U1) P5.5/SMCLK (pin#49). It is designed to drive TUSB3410 (U3) X1 (pin#27) via a 2-to-1 voltage divider R19-R22. This signal goes through a small hole on the PCB near pin#49 of U1. I soldered one end of a 330 Ohm resistor there and “blue wires” the other end of the resistor to the G2xx1 (IC1) P2.6/XIN (pin#13/19). The resistor is to slow down the edges of the 12KHz clock to reduce RF noise. It also helps to protect against contention in case I mistakenly programmed P2.6 as an output pin.
After that, all I need to do is set the LFXT1Sx bits to 11 and I got a 12MHx ACLK for the G2xx1.
BTW, 12MHz divvied by 104 is very close to 115.2k -- a very commonly used BAUD rate.
