[LinuxPPS] LinuxPPS through USB interface

[clemens at dwf.com]

> 
> I read about a year's worth of questions on the forum and I could not find an exact match to my question so I apologise if my question has been answered. 
> 
> I have a GPS Eval chip from sparkfun and I wired a connection between the PPS port to the DCD port of the USB connection. Basically this card has two ports, serial and USB. The USB port is a serial converter port. When I try and run ppstest with a pps instance of the usb drive I get the following error: 
> 
> time_pps_fetch() error -1 (Connection timed out) 
> 
> If I connect the device through a normal serial port I get no issues with PPS. I have tested the connection with a scope and the data is being properly sent along the line. I'm wondering if LinuxPPS does not support USB serial emulation or if there is some setting I need. 
> 
> Please let me know if anyone has any tweaks or alternative methods to retrieve when the PPS signal arrives. I don't think LinuxPPS supports other pins so if I send the signal to a different port i'd have to modify the code to read from that port. 
> 
> Thank you in advance for any help you provide me. It is greatly appreciated. 
> 
Its probably a waste of time (and hardware) to try to run any PPS application
thru a USB serial converter.

USB serial data is POLLED so there is an unknown delay before it is 
delivered to the system, so the fact that you have PPS is worthless.

Either connect it to your one serial port, or buy a (multi)port serial
card for your machine and run it off of one of those ports, - then you
can have sub-microsecond timing from the PPS line.

-- 
                                        Reg.Clemens
                                        reg at dwf.com