6.10.2 Oscillator and Clock System (CS)

The clock system includes support for a 32-kHz watch crystal oscillator XT1 (LF mode), an internal very-low-power low-frequency oscillator (VLO), an integrated internal digitally controlled oscillator (DCO), and a high-frequency crystal oscillator XT1 (HF mode). The clock system module is designed to meet the requirements of both low system cost and low power consumption. A fail-safe mechanism exists for all crystal sources. The clock system module provides the following clock signals:

• Auxiliary clock (ACLK), sourced from a 32-kHz watch crystal (XT1 LF mode), a high-frequency crystal (XT1 HF mode), the internal VLO, or the internal DCO.
• Main clock (MCLK), the system clock used by the CPU. MCLK can be sourced by the same sources made available to ACLK.
• Sub-Main clock (SMCLK), the subsystem clock used by the peripheral modules. SMCLK can be sourced by the same sources made available to ACLK.