[ntp:questions] help with setting up NTP on windows with a USB GPS

Dan Drown dan-ntp at drown.org
Tue Jul 30 16:39:25 UTC 2019

On 07/30/19 03:48, Jakob Bohm wrote:
> USB-to-RS-232 converters generally completely loose the precision timing
> abilities of traditional serial port circuits (a 16550 or equivalent
> chip connected to the main CPU bus like in the serial adapters for the
> original 1981 IBM PC).
> This is because the standard for USB-to-RS-232 is all about transferring
> the data over an essentially "polled" USB 1.1/2.0 bus that polls once
> per ms, with the conversion chip buffering stuff until the next
> available 1ms USB frame.
> As a transitional solution for such a technique, one could use an
> enhanced USB-to-RS-232 adapter including a timestamp in its reporting of
> "modem-control" inputs, such as "CD line went high 2.3456ms before the
> start of this USB frame" (because it happened 0.3456ms before a frame,
> but 2 frames were busy).

I took pretty much this strategy with my project here:  

It's custom USB-to-serial firmware, that measures the delay between  
the PPS and the USB poll.  The kernel timestamps the CD transition in  
the USB interrupt handler, and the Linux software combines that with  
the firmware's delay message before reporting it to ntp via SHM.

It works, but it's not as good timing quality as just using a platform  
that supports pps-gpio or PPS on the NIC's 1588 timers.

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