[time-nuts] gpsd 1pps pulse?

Sarah White kuzetsa at gmail.com
Tue May 7 14:00:52 EDT 2013

On 5/6/2013 9:38 AM, Chuck Forsberg WA7KGX N2469R wrote:
> I would like to set up a NTP server on a machine running Linux
> (currently Korora 18).
> What are the requirements for the 1 PPS signal fed to the carrier detect
> pin?
> I presume the 10 us 1pps from the Thunderbolt is too narrow.
> I assume the rising edge of CD is the one gpsd triggers on.


10 microseconds? That sounds fine actually... You could simply try it?

Really, I mean it. Just try and it might work with your serial port.

I don't recommend any clever hardware hacks which will somehow piggyback
the 1 PPS output onto the DCD line on the same serial port you're using
for the TSIP protocol.

If you have problems with PPS, just set the serial port to a higher baud
rate... This is the reason I recommend against using the DCD line on the
same serial port you're using for TSIP. Trimble tends to use 9600 baud
on the TSIP port, and the signaling on that is too narrow.

For PPS, you'll be fine with 115200 BAUD (bit/s) ... This speed comes
out to 8680.55 nanoseconds, more than a full microsecond "narrower" than
the 10 microsecond wide PPS.

I had trouble getting the right headers to connect to the onboard serial
port on the motherboard (not installed by default) ... So I tried a
USB-serial, and ultimately switched to a "PCI Express to Serial Port"

This is from the product description on my current setup:

(quote) Based on a native single chip design (no bridge chip), this
2-port serial adapter card allows you to harness the full capability
offered by PCI Express (PCIe), and reducing the load applied to the CPU
by as much as 48% over conventional serial cards.

^ I'm really not sure if this helps the latency for hardware interrupts,
or makes them worse, but it DEFINITELY works better than the USB-serial
adapter I was trying to use before.

At less than $100, the price is right though (And now my PPS hardware
interrupts are via "native PCIe" so it only makes sense that it would
work better than USB)

Hope this helps

P.S. If anyone needs a good serial port and not sure where to look, I
got mine from newegg, they have tons: http://goo.gl/CEzZA ... Mine was
specifically, this model, made by startech: http://goo.gl/S9EHx

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