30.4.2 32.768 kHz Crystal Oscillator

By default, the 32.768 kHz oscillator is disabled. To use this oscillator, the XIN32 and XOUT32 pins must be connected to a 32.768 kHz crystal or to a ceramic resonator. Refer to the section “Electrical Characteristics” for appropriate loading capacitors selection on XIN32 and XOUT32.

Note that the user is not obliged to use the 32.768 kHz crystal oscillator and can use the Slow RC oscillator instead. Using the 32.768 kHz crystal oscillator provides a more accurate frequency than the Slow RC oscillator.

To select the 32.768 kHz crystal oscillator as the source of SLCK, the bit SUPC_CR.XTALSEL must be set. This results in a sequence which first configures the PIO lines multiplexed with XIN32 and XOUT32 to be driven by the crystal oscillator, then enables the 32.768 kHz crystal oscillator and then disables the Slow RC oscillator to save power. The switch of SLCK source is glitch-free.

Reverting to the Slow RC oscillator is only possible by shutting down the VDDIO power supply. If the user does not need the 32.768 kHz crystal oscillator, the XIN32 and XOUT32 pins can be left unconnected since by default the XIN32 and XOUT32 system I/O pins are in PIO input mode with pullup after reset.

The user can also set the 32.768 kHz crystal oscillator in Bypass mode instead of connecting a crystal. In this case, the user must provide the external clock signal on XIN32. For input characteristics of the XIN32 pin, refer to the section “Electrical Characteristics”. To enter Bypass mode, the OSCBYPASS bit of the Supply Controller Mode register (SUPC_MR) must be set prior to setting SUPC_CR.XTALSEL.