23.4.2 Slow Clock Generator

The SUPC embeds a slow clock generator that is supplied with the VDDIO power supply. As soon as the VDDIO is supplied, both the 32.768 kHz crystal oscillator and the slow RC oscillator are powered up, but only the slow RC oscillator is enabled. When the slow RC oscillator is selected as the slow clock source, the slow clock stabilizes more quickly than when the 32.768 kHz crystal oscillator is selected.

The user can select the 32.768 kHz crystal oscillator to be the source of the slow clock, as it provides a more accurate frequency than the slow RC oscillator. The 32.768 kHz crystal oscillator is selected by setting the XTALSEL bit in the SUPC Control register (SUPC_CR). The following sequence must be used to switch from the slow RC oscillator to the 32.768 kHz crystal oscillator:

  1. The PIO lines multiplexed with XIN32 and XOUT32 are configured to be driven by the oscillator.
  2. The 32.768 kHz crystal oscillator is enabled.
  3. A number of slow RC oscillator clock periods is counted to cover the startup time of the 32.768 kHz crystal oscillator. Refer to the section “Electrical Characteristics” for information on the 32.768 kHz crystal oscillator startup time.
  4. The slow clock is switched to the output of the 32.768 kHz crystal oscillator.
  5. The slow RC oscillator is disabled to save power.

The switching time may vary depending on the slow RC oscillator clock frequency range. The switch of the slow clock source is glitch-free. The OSCSEL bit of the SUPC Status register (SUPC_SR) indicates when the switch sequence is finished.

Reverting to the slow RC oscillator as a slow clock source 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 should be left unconnected.

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