On Mon, Apr 18, 2011 at 9:28 AM, Rob <[email protected]> wrote: > You assume that your measurement scale is very coarse. Your reference > is seconds and you can only measure plus or minus one second. > But that is not true in NTP with a local reference clock. It can measure > to microseconds or even nanoseconds.
The measurement scale is always very coarse compared to desired precision. That same applies if the scale is seconds or nanoseconds. If you can measure to one part in N and desire the rate be set to one part in M and N<M then you most wait for time equal to or greater than to M/N. There is another layer over this, your reference clock might have som uncertainty or jitter on it. In the case of NTP the 1PPS signal causes an interrupt and the local clock is used to time stamp the pps event. Neither the local clock nor the a pps is perfect so one must average to reduce the noise. I think NTP does this by using a long time constant on the loop, in other words by not adjusting the rate quickly The OP asked for a simple answer about why it takes so long to do something so seemingly simple as setting a clock. The answer was that you should expect it to because you always have to wait to see it a clock is running at the desired rate. The more precision desired the longer you need to wait. -- ===== Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
