[time-nuts] Raspberry Pi NTP server

[Bill Notfaded]

USB 1.0/Low-Speed: 1.5 Mbps
USB 1.1/Full-Speed: 12 Mbps.
USB 2.0/Hi-Speed: 480 Mbps.
USB 3.0/SuperSpeed: 5 Gbps.
USB 3.1/SuperSpeed: 10 Gbps.
This is the actual bitrate for these serial interfaces.

Bill
On Wed, Jul 8, 2020, 5:16 PM jimlux <jimlux at earthlink.net> wrote:

> On 7/8/20 4:40 PM, Hal Murray wrote:
> >
> > stevesommarsntp at gmail.com said:
> >> My RPi4 (Raspbian Buster) has a GPS+PPS/USB.  Serial->USB uses Prolific
> >> PL2303, which supports USB 2.0
> >
> >> which means 1 msec polling of the PPS signal.   I've been unable to
> poll more
> >> frequently
> >
> > As far as I know, the PL2303, 067b:2303, is an old/slow chip.  (I forget
> the
> > right magic USB terms)  Why do you expect it to go faster than 1 ms?
>
>
> The underlying frame rate on USB (even the slowest USB1.0) is 8kHz, so
> it's not unreasonable to expect that a chip might be able to "poll" a
> status signal at that rate.
>
>
> >
> > It and the FTDI chip(s) are popular and widely known to be well
> supported on
> > Linux.  I'll be very surprised if it goes faster.
>
> There is that.  OTOH, drivers do get updated. One could probably, if you
> were hard core, look at the device driver code in the Linux distro and
> figure it out.
>
>
> >
> > What sort of device are you using?  One way to tell if it is likely to go
> > faster than 1 ms is the thickness of the wire.  Faster speeds need more
> > shielding for EMI reduction (or something like that) which turns into
> fatter
> > cables.  It's pretty easy to tell if you have samples of both in front
> of you.
> >   I think you can only use the thinner cable if t runs at 1 ms and you
> hard
> > wire the chip to the end of the cable as is typical of a GPS mouse.
> >
> >
> >
>
>
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