[ntp:questions] Time server question

Chris xxx.syseng.yyy at gfsys.co.uk
Wed Jul 17 13:27:07 UTC 2019


On 07/17/19 12:59, William Unruh wrote:

 > I had some indication that the parallel port was faster.

That would make sense, since the rs232 devices tend to be slew
rate limited for noise rejection. Found some DS8921 driver / receiver
devices, originally designed for hard drive data path use. Delay
around 10-15 nS or so, which should be more than good enough.
Single 5 volt rail as well.

 > I suspect it will be a bad outcome. The rpoblem is that you get
 > interrupt contention, and the two interrups will put in time delays into
 > the second one processed.

Remember hand optimising 6502 asm interrupt handlers years
ago to tune timer accuracy, but that was a 1uS cycle  machine,
with handlers stretching out to 100uS or more.
Don't have data, but modern cpus are much faster and in
any case, there are other interrupt sources within the system
which may contribute to jitter. Don't know enough to say, but
perhaps ntp will average out the two to give a more accurate
result ?. Would be interesting to hook it up and see what
happens anyway...

Chris




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