ZHCSHN9A February 2018 – April 2018 LMK05028
PRODUCTION DATA.
The precision frequency detector measures the frequency offset (in ppm) for all input clocks relative to a 0-ppm reference clock, which can be selected from either the XO or TCXO input. The valid and invalid ppm frequency thresholds are configurable through the registers. The monitor will clear its LOR_FREQ flag when the relative input frequency error is less than the valid ppm threshold. Otherwise, it 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 frequency measurement averaging factor is also used in computing the frequency detector register settings. A higher averaging factor increases 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.