23.1 Overview

The Frequency Meter (FREQM) can be used to accurately measure the frequency of a clock source, GCLK_FREQM_MSR, by comparing it to a known reference clock. It 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/fGCLK_FREQM_REF and stored in the Value register (VALUE.VALUE). REFNUM is the number of Reference clock cycles selected in the Configuration A register (CFGA.REFNUM) over which the duration of a measurement is to be done.

The frequency of the measured clock, fCLK_MSR, is calculated by:

f G C L K _ F R E Q M _ M S R = ( V A L U E ⁢   +   1 ⁢   +   E r r o r R E F N U M ⁢   +   1 )   f G C L K _ F R E Q M _ R E F

Where Error represents the error introduced by the synchronization mechanism. The error can be a maximum of two measured clock cycles.