MSP430 DCO产生8MHz时钟

MSP430F4152

  SCFI0 |= FN_2;                            // x2 DCO, 4MHz nominal DCO
  SCFQCTL = 31;                             // (121+1) x 32768 = 4Mhz
  FLL_CTL0 |= DCOPLUS + XCAP11PF;
 

这段程序产生8MHz时钟，原理是什么？