6.10.1 Oscillator and System Clock

The Unified Clock System (UCS) module includes support for a 32768-Hz watch crystal oscillator, an internal very-low-power low-frequency oscillator (VLO), an internal trimmed low-frequency oscillator (REFO), an integrated internal digitally controlled oscillator (DCO), and a high-frequency crystal oscillator. The UCS module is designed to meet the requirements of both low system cost and low-power consumption. The UCS module features digital frequency locked loop (FLL) hardware that, in conjunction with a digital modulator, stabilizes the DCO frequency to a programmable multiple of the watch crystal frequency. The internal DCO provides a fast turnon clock source and stabilizes in less than 5 µs. The UCS module provides the following clock signals:

• Auxiliary clock (ACLK), sourced from a 32768-Hz watch crystal, a high-frequency crystal, the internal low-frequency oscillator (VLO), or the trimmed low-frequency oscillator (REFO).
• Main clock (MCLK), the system clock used by the CPU. MCLK can be sourced by same sources made available to ACLK.
• Sub-Main clock (SMCLK), the subsystem clock used by the peripheral modules. SMCLK can be sourced by same sources made available to ACLK.
• ACLK/n, the buffered output of ACLK, ACLK/2, ACLK/4, ACLK/8, ACLK/16, ACLK/32.