The final clock system discussed in this module will be the 4xx clock system which is also called the FLL (Frequency-Locked Loop). The primary difference between the FLL and the basic clock system is the calibration of the DCO occurs automatically. There is still a 32768 LFXT1 crystal oscillator (shown in the upper left), a DCO (shown in the middle), the same clock signals and distribution tree (shown on the right), but included additionally for the FLL is the frequency integrator. This provides automatic DCO calibration and stabilization to the preprogrammed frequency.