After the FREQM is enabled, writing a '1' to the START bit in the Control B register (CTRLB.START) starts the measurement. The BUSY bit in Status register (STATUS.BUSY) is set when the measurement starts, and cleared when the measurement is complete.
There is also an interrupt request for Measurement Done: When the Measurement Done bit in Interrupt Enable Set register (INTENSET.DONE) is '1' and a measurement is finished, the Measurement Done bit in the Interrupt Flag Status and Clear register (INTFLAG.DONE) will be set and an interrupt request is generated.
The result of the measurement can be read from the Value register (VALUE.VALUE). The frequency of the measured clock GCLK_FREQM_MSR is then:
In case an overflow condition occurred, indicated by the Overflow bit in the STATUS register (STATUS.OVF), either the number of reference clock cycles must be reduced (CFGA.REFNUM), or a faster reference clock must be configured. Once the configuration is adjusted, clear the overflow status by writing a '1' to STATUS.OVF. Then another measurement can be started by writing a '1' to CTRLB.START.