On 7/12/20 7:15 AM, Hal Murray wrote:
[email protected] 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.
[email protected] 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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