32.18 32.768 kHz Crystal Oscillator Frequency Monitor

The frequency of the 32.768 kHz crystal oscillator can be monitored by means of logic driven by the 12 MHz RC oscillator known as a reliable clock source. This function is enabled by configuring CKGR_MOR.XT32KFME.

The error flag XT32KERR in PMC_SR is asserted when the 32.768 kHz crystal oscillator frequency is out of the ±10% nominal frequency value (i.e., 32.768 kHz). The error flag can be cleared only if the Slow clock frequency monitoring is disabled.

The monitored clock frequency is declared invalid if at least four consecutive clock period measurement results are over the nominal period ±10%.

Due to the possible frequency variation of the embedded 12 MHz RC oscillator acting as reference clock for the monitor logic, any Slow clock crystal frequency deviation over ±10% of the nominal frequency is systematically reported as an error by means of PMC_SR.XT32KERR. Between -1% and -10% and +1% and +10%, the error is not systematically reported.

Thus, only a crystal running at a 32.768 kHz frequency ensures that the error flag is not asserted. The permitted drift of the crystal is 10000 ppm (1%), which allows any standard crystal to be used.

The error flag can be defined as an interrupt source of the PMC by setting PMC_IER.XT32KERR.