[time-nuts] gpsd 1pps pulse?

[Sarah White]

On 5/7/2013 2:00 PM, Sarah White wrote:

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

((snip))

On 5/8/2013 6:53 AM, mike cook wrote:

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.

--- begin reply ---

[CITATION] --> http://www.ntp.org/ntpfaq/NTP-s-config-adv.htm

(section) 6.2.4.6.1. What is the shortest Width for a Pulse
connected to the DCD Pin of an RS-232 Interface?

That depends. The higher the quality your serial port is, the longer
pulses will be needed. This is because of the ESD protection of the chip.

26 usec should be about the bit time for a 38400 serial line, so you
could connect it to the RxD line instead and see if you receive
characters that way when the port is set for 38400 or faster.

Another thing to try is to configure the serial port for 115200, some of
the chips base their deglitching on the baud rate, often requiring a
full symbol before they react.

[END CITATION]

Mike, I think that's basically what you were saying:

(quote of mike cook)

... 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.

(end of last quote)

Or did I misunderstand what you said?

Yes. On principle, for some but not 100% of serial ports, the baud rate
clock won't matter at all, and the hardware interrupts will still happen
with low latency, regardless of the baud rate setting. What will change
however, is filtering which MIGHT be related to the baud rate setting.
(as per the citation I included in this post)

--Sarah

P.S.  I had trouble understanding the quote & reply style / layout...
... refactored the layout by band to look cleaner (my own tastes)
... basically made it easier for me to proofread everything, etc.

Also, seems the person who started this topic is currently silent...
... I think it was: "Chuck Forsberg WA7KGX N2469R" <caf at omen.com>