[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-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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