[LinuxPPS] FW: Linux PPS with Garmin 18xLVC using 2.6.35.7 linux Kernel

[Kent Headley]

Thank you very much Rodolfo - This definitively answers my question!
I will keep this in mind as I choose different hardware to work from.

In another note, you said that using USB for PPS is wrong...I'm not familiar
enough with USB to understand why it can't work.

Is it due to latencies in the signal chain or implementation differences
that make it non-deterministic?

Thanks again for taking time to answer, and for your great work on linuxPPS.

Cheers,
Kent

On 10/29/10 12:05 AM, "Rodolfo Giometti" <giometti at enneenne.com> wrote:

> On Thu, Oct 28, 2010 at 12:33:14PM -0700, Kent Headley wrote:
>> Hi Hal, thanks for your note.
>> 
>> The PC I'm using (Asus Eee PC) doesn't have any external serial or parallel
>> ports. It is a convenient platform of opportunity to try out linuxPPS, so if
>> it PPS support can't be done using USB-serial adapters, I'll have to move to
>> another platform with real serial ports. Ultimately, I want to use a small
>> single board computer, so this was the closest computer I have on hand.
>> 
>> Does linuxPPS require a 8250/16550 UART?
> 
> LinuxPPS just require a real serial port with proper support for DCD
> change IRQ. Currently few real serial ports support this.
> 
>> So it is NTP that needs the links names gps0 and gpspps0 - thanks for
>> clarifying that.
>> 
>> The /dev/ttyUSB0 device works fine - I can see NMEA messages (e.g. cat
> 
> They works fine for serial data but __not__ for PPS!
> 
>> /dev/ttyUSB0). Also, utilities like statserial will show that the DCD pin
>> (where PPS is connected) is changing reliably. I'm not sure how statserial
>> detects DCD changes (interrupts? polling?), but perhaps it's not the same
>> mechanism used by linuxPPS.
>> 
>> It seems like the USB serial device uses a usb serial driver rather than the
>> core serial drivers. lsmod shows
>> 
>>     serial_core 12121   1 8250
>>     usbserial   21796   3 keyspan
>>     usbcore     96374   7 keyspan, usbserial (and others)
>>     pps_ldisc   1358    1
>>     pps_core    5257    1 pps_ldisc
>> 
>> I don't see any dependencies between the pps modules and any serial modules
>> though. 
>> 
>> Does linuxPPS use a particular serial driver (module)?
> 
> No, it uses a ldisc.
> 
>> How does linuxPPS detect changes in the PPS source (GPIO, serial pin,
>> parallel pin, etc.)?
> 
> It depends where you connect your PPS source. :)
> 
> Ciao,
> 
> Rodolfo