The precision frequency detector measures the frequency offset or error (in ppm) of all input clocks relative to the frequency accuracy of the XO input, which is considered as the "0-ppm reference clock" for frequency comparison. The valid and invalid ppm frequency thresholds are configurable through the registers. The monitor will clear the LOR_FREQ flag when the relative input frequency error is less than the valid ppm threshold. Otherwise, the monitor will set the LOR_FREQ flag when the relative input frequency error is greater than the invalid ppm threshold. The ppm delta between the valid and invalid thresholds provides hysteresis to prevent the LOR_FREQ flag from toggling when the input frequency offset is crossing these thresholds.

A measurement accuracy (ppm) and averaging factor are used in computing the frequency detector register settings. A higher measurement accuracy (smaller ppm) or higher averaging factor will increase the measurement delay to set or clear the flag, which allow more time for the input frequency to settle, and can also provide better measurement resolution for an input with high drift or wander. Note that higher averaging reduces the maximum frequency ppm thresholds that can be configured.