[ntp:questions] How bad is USB? (was: Re: Can the line audio out of HF radio be used to sync ntp. Trying to get a cheap ($) radio method.)

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Sun May 3 09:21:40 UTC 2009

>You've both commented that USB has drawbacks, but in reality what
>performance might be obtained?  Not everyone needs microsecond precision,
>and USB might allow millisecond precision - i.e. possibly better than what
>might be obtained by Internet access alone, or by using a radio source.
>Has anyone made any actual tests or measurements?  Are there any results
>available from an actual installation of USB on either Windows or UNIX?

USB is polled by the host.  That gives it a bad reputation.

But that polling is done by hardware.  The basic time scale is 1 ms.
If you don't get your low-latency stuff sorted out on a serial port
you can easily introduce delays longer than 1 ms.

If you want microseconds, USB won't work.

I've been experimenting with low cost GPS gadgets.  I haven't found
anything great.  A main problem is that the software on most of
them is setup for navigation rather than timekeeping.  Some/many
of them send the serial text from a 100 ms timer so the USB
delays are lost in the noise.

Note that the Garmin 18x has this problem where the 18 didn't.

I haven't tried grabbing PPS over USB.

These are my opinions, not necessarily my employer's.  I hate spam.

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