[time-nuts] gpsd 1pps pulse?

Chris Albertson albertson.chris at gmail.com
Wed May 8 13:19:51 EDT 2013

I was going to write the same thing.  That the DCD line has nothing at
all to do with baud rate.  But that applies to a "real" serial port.
If this is being done on a Serial->USB dongle that all bets are off
and you'll never get really good result as USB adds to many software

You can buy a new motherboard with a real rs232 port for under $100
and used ones for even less.  I'd not recommend uSB for the PPS
signal.  It is OK for serial data but not for PPS. (that is if you
care about best accuracy.)

On the USB converter the fact that the dCD signal changed has to go
over the USB bus in a serial data packet and gt queued and then read
and acted on.  It is orders of magnitude worse.

But all that said, if you get NTP sync'd to within 20 uSec you are
more than good enough.  It takes some effort to get to the 2 uSec
level and heroic effort to break 1 uS.  For practical purposes you
have to decide what is good enough.  Some people are happily with 10
milli Sec.

> Sara, I'd be interested to know if you ran into the problem and changing baud rate fixed it. I ask that as my understanding of the most common UART indicates that the DCD signal line is not managed with the baud rate clock. At least for the most common 16550x variety, the DCD level changes are signaled in the MSR and then if interrupts are enabled an interrupt is queued. However t there is an anti-metastability filter of 2 CLK cycles before pin level changes are reflected in the MSR. So.. if your UART is on a slow clock,  it could be that a 10us transition will be  too fast to pass the filter. However, I do agree that trying is the easiest test.


Chris Albertson
Redondo Beach, California

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