[time-nuts] Raspberry Pi NTP server

[jimlux]

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.
> 
> 
>