The Supply Controller embeds a slow clock generator that is supplied with the VDDIO power supply. As soon as VDDIO is supplied, both the 32.768 kHz crystal oscillator and the Slow RC oscillator are powered, but only the Slow RC oscillator is enabled. This allows the Slow clock (SLCK) to be valid in a short time (about 100 μs).
SLCK is generated either by the 32.768 kHz crystal oscillator or by the Slow RC oscillator.
To select the clock source, the selection is made via the XTALSEL bit in the Supply Controller Control Register (SUPC_CR).