[LinuxPPS] PPS via FTDI 232 doesn't work, many questions

[Hal V. Engel]

On Sunday, November 20, 2011 03:59:35 PM Cirilo Bernardo wrote:
> On Fri, Nov 18, 2011 at 6:48 AM, Ben Adler
> 
> <adler at informatik.uni-hamburg.de> wrote:
> > Hello list!

snip
> 
> >  - How long should that pulse be? Is the answer a function of the
> > port's baudrate? Can the pulse be too long?
> 
> The pulse cannot really be too long.  The minimum required pulse
> depends on the hardware you are using; you will need to read the FTDI
> documentation. One of the reasons is that some hardware is polled (in
> which case the minimum duration has some relation to the bitrate
> clock) while other hardware uses a different detection scheme and can
> detect pulses as short as a few ns.  In all hardware I had worked
> with, the duration of the GPS PPS signal is sufficient.

Most timing GPS's use a 200ms pulse that goes high at the epoch (IE. it is low 
for about 800ms before the epoch).  Hardware that is interupt driven (IE. not 
polled hardware) like a real UART will work correctly with much shorter 
pulses.  LinuxPPS is really designed with interupt driven hardware in mind.  
Most of us using it expect to achive microsecond accuracy and this requires 
that the PPS pulse be hooked up to a device that responds to interupts.

> 
> >  - Is the "kernel consumer" the same as "hardpps"? From what I
> > understand, hardpps directly adjusts the clock in kernel-space, whereas
> > using /sys/class/pps/pps0/{assert,clear} means that I'd need a running
> > ntp-server to interpret these files and work the clock. Is that right?
> 
> Using NTP is the best way to discipline your system clock.

You need NTP in both cases.  The kernel comsumer works with NTP and is 
dependant on the NTP supplied clock drivers.  With hardpps (IE. the kernel 
consumer) NTP lets the kernel handle some of the functionality since it is 
closer to the hardware.

> 
> >  - Is the latency_timer value above also relevant for state changes on
> > CD or just for data bytes?
> > 
> >  - Is it possible for hardpps to change the status of another line X on
> > the serial port after it has received a PPS signal on the CD line? I'd
> > like to use this to measure the time from PPS clock on CD line to the
> > status change on line X using an oscilloscope. I hope this would allow
> > me to approximate a latency between the real PPS and the time hardpps
> > thinks it is.
> > 
> > These are my questions after reading about LinuxPPS for a day - I hope
> > that you can enlighten me on at least a few of them. Thanks for reading
> > to the end!
> > 
> > Ben Adler
> > 
> > _______________________________________________
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> > Wiki: http://wiki.enneenne.com/index.php/LinuxPPS_support
> 
> I can't find references to "Hard PPS" (is that the old obsolete Hard
> PPS project?) Anyway, to change a status line you will need to modify
> the PPS driver.
> 
> - Cirilo
> 
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