[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?
> 
> 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.


it will support pretty high baud rates, though: 1,228,800 bps



https://github.com/torvalds/linux/blob/master/drivers/usb/serial/pl2303.c

I suspect it might have something to do with how interrupts are handled 
in Linux.  Typically when an interrupt comes in, the interrupt handler 
sets a flag in the driver saying "interrupt came in", perhaps latching 
some status, and then returns from the interrupt immediately (so that 
the time spent in interrupt state is small).  Then, at the next 
scheduling interval, a thread that is blocked waiting for the interrupt 
becomes alive, and actually does the rest of the processing.



> 
> 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.
> 
> 
>