20.6.1 Principle of Operation

FREQM counts the number of periods of the measured clock (GCLK_FREQM_MSR) with respect to the reference clock (GCLK_FREQM_REF). The measurement is done for a period of REFNUM/f_{CLK_REF_MUX} and stored in the Value register (VALUE.VALUE). REFNUM is the number of Reference clock cycles selected in the Configuration A register (CFGA.REFNUM).

The frequency of the measured clock, $f_{CLK_MSR}$ , is calculated by the formula:

f_{CLK_MSR} = (\frac{VALUE + 1 + Error}{REFNUM + 1}) f_{CLK_REF_MUX}

, where Error represents the error introduced by the synchronization mechanism. The error can be maximum two measured clock cycles.