The frequency of the 32.768 kHz crystal oscillator can be monitored by means of logic driven by the Main RC oscillator known as a reliable clock source. This function is enabled by configuring the XT32KFME bit of CKGR_MOR. Prior to enabling this frequency monitor, the 32.768 kHz crystal oscillator must be started and its startup time be elapsed. Refer to details on the Slow clock generator in the section “Supply Controller (SUPC)”.
An 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 frequency monitor is disabled.
When the Main RC oscillator frequency is set to 4 MHz, the accuracy of the measurement is ±40% as this frequency is not trimmed during production. Therefore, ±10% accuracy is obtained only if the Main RC oscillator frequency is configured for 8 or 12 MHz.
The monitored clock frequency is declared invalid if at least 4 consecutive clock period measurement results are over the nominal period ±10%. Note that modifying the trimming values of the Main RC oscillator (PMC_OCR) may impact the monitor accuracy and lead to inappropriate failure detection.
Due to the possible frequency variation of the Main RC oscillator acting as reference clock for the monitor logic, any 32.768 kHz 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 32.768 kHz frequency ensures that the error flag will not be asserted. The permitted drift of the crystal is 10000 ppm (1%), which allows any standard crystal to be used.
If the Main RC oscillator frequency range needs to be changed while the frequency monitor is operating, the monitoring must be stopped prior to change the Main RC oscillator frequency. Then it can be re-enabled as soon as PMC_SR.MOSCRCS is set.
The error flag can be defined as an interrupt source of the PMC by setting PMC_IER.XT32KERR. This flag is also routed to the RSTC and may generate a reset of the device.