[time-nuts] gpsd 1pps pulse?

[mike cook]

Le 7 mai 2013 à 20:00, Sarah White a écrit :

> 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.
> 
> Chuck,
> 
> 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.
> 
> 
snipped

> 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.
> 

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.