[time-nuts] Raspberry Pi NTP server

[jimlux]

On 7/12/20 7:15 AM, Hal Murray wrote:
> stevesommarsntp at gmail.com said:
>> I'd like to reduce the USB polling contribution by polling at 125
>> microseconds as the Linux PPS folks suggest (http://linuxpps.org/doku.php/
>> technical_information)    Would an FTDI-based USB convertor do the trick?
> 
> It depends on which FTDI chip you use.  Both Prolific and FTDI make many
> chips.  The common ones are slow, polling at 1 ms.
> 
> The FT232R does poll at 125 microseconds.  I got mine on a breakout board from
> AdaFruit.  I also got a GPS chip on a breakout board to connect up to it.
> 
> Don't forget hanging bridges.  I don't know of any NTP software that knows
> about them.
> 
> 
> jimlux at earthlink.net said:
>> I would not expect another kind of USB to serial converter to do better.  The
>> problem is higher up in the way that Linux handles USB devices.
> 
> I can't quite figure out what you are saying.  The USB rules say that the
> chips commonly used for serial ports get polled at 1 ms.  The FT232R says it
> goes faster and it works as expected at 125 microseconds.
> 
> Linux may have troubles with USB code, but I'm not sure how that impacts PPS
> processing.  The PPS timestamp is captured at interrupt time.  What else
> matters after that?
> 

Is the PPS via USB CTS stamped at interrupt time? or is it stamped 
higher up in the stack?

I started tracing this out, but then decided I'm not going to be writing 
Linux USB drivers any time soon, so gave it up.

I could easily imagine that the interrupt comes in, marks a thread as 
"ready to run" and the "oh CTS has changed state" is detected at a 
higher level.






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