PPS_AVG
v_usec = (v_usec << PPS_AVG) - pps_jitter;
pps_jitter += v_usec / (1 << PPS_AVG);
pps_stabil -= -v_usec >> PPS_AVG;
pps_stabil += v_usec >> PPS_AVG;
pps_freq -= -u_usec >> PPS_AVG;
pps_freq += u_usec >> PPS_AVG;
#define MAXTIME (200 << PPS_AVG) /* max PPS error (jitter) (200 us) */
ntv.jitter = pps_jitter >> PPS_AVG;