Frequency Meter (FREQM)
Overview
FREQM can be used to accurately measure the frequency of a clock by comparing it to a known reference clock.
Features
- Ratio can be measured with 24-bit accuracy
- Accurately measures the frequency of an input clock with respect to a reference clock
- Reference clock can be selected from the available GCLK_FREQM_REF sources
- Measured clock can be selected from the available GCLK_FREQM_MSR sources
Block Diagram
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/fCLK_REF and is 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, fCLK_MSR, is calculated by:
$f_{CLK\_MSR} = \frac{VALUE + 1 + Error}{REFNUM + 1} f_{CLK\_REF}$ , where "Error" represents the error introduced by the synchronization mechanism. The error can be a maximum of two measured clock cycles.
The reference clock frequency, fCLK_REF, must be slower than the measurement clock frequency, fCLK_MSR.
Refer to the "FREQM – Frequency Meter" chapter from the product data sheet for more details.