10.3.2 Main Clock Selection and Prescaler

All internal oscillators can be used as the main clock source for CLK_MAIN. The main clock source is selectable from software and can be safely changed during normal operation.

Built-in hardware protection prevents unsafe clock switching:

Upon selection of an external clock source, a switch to the chosen clock source will only occur if edges are detected. Until a sufficient number of clock edges are detected the switch will not occur, and it will not be possible to change to another clock source again without executing a Reset.

An ongoing clock source switch is indicated by the System Oscillator Changing (SOSC) flag in the Main Clock Status (CLKCTRL.MCLKSTATUS) register. The stability of the external clock sources is indicated by the respective status (EXTS and XOSC32KS in CLKCTRL.MCLKSTATUS) flags.

CAUTION:
  1. If an external clock source fails while used as the CLK_MAIN source, only the WDT can provide a mechanism to switch back via System Reset.
  2. Do not alter the properties of the external clock source while used as the CLK_MAIN source. This can be interpreted as a clock failure.

CLK_MAIN is fed into a prescaler before it is used by the peripherals (CLK_PER) in the device. The prescaler divide CLK_MAIN by a factor from 1 to 64.

Figure 10-2. Main Clock and Prescaler

The Main Clock and Prescaler configuration (CLKCTRL.MCLKCTRLA, CLKCTRL.MCLKCTRLB) registers are protected by the Configuration Change Protection Mechanism, employing a timed write procedure for changing these registers.