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

[Kent Headley]

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?

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
/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)?
How does linuxPPS detect changes in the PPS source (GPIO, serial pin,
parallel pin, etc.)?

Cheers,
Kent



On 10/28/10 12:08 PM, "Hal V. Engel" <hvengel at gmail.com> wrote:

> On Thursday, October 28, 2010 09:13:10 am Kent Headley wrote:
>> Thanks for your reply Venu,
>> 
>> The GPS has only RS232 output, which is connected to a Keyspan 19-HS
>> USB-serial converter.
> 
> I am not sure that this is supported.  Why not just use a serial port?
> 
>> The Keyspan nominally behaves as a full 9-pin serial port.
>> For example, statserial can detect changes in the handshaking lines on
>> /dev/ttyUSB0.
>> 
>> The PPS signal is connected to Keyspan serial pin 1 (DCD).
>> So both the NMEA and PPS signals are passed to the PC via the Keyspan
>> (/dev/ttyUSB0).
>> 
>> Is it OK for the NMEA and PPS signals to arrive on the same serial
>> connector? I have seen this at various sites on the web (PPS wired to DCD),
>> so figured it¹s OK.
> 
> PPS and the GPS data stream can either be connected to the same device or to
> separate devices.   This is why ntp needs two device sym links since these are
> logically separated even if they are on the same port.
> 
>> With your uBlox GPS, what pin was the PPS signal attached to?
>> 
>> The comments about sym links for gps0 and gpspps0 are interesting; is that
>> required by NTP, or does linuxPPS need these links also?
> 
> NTP only and the logical device created by the sym link is specific to the
> clock driver and how it is configured.
> 
>> 
>> I just tried creating the links you suggested, exactly as you recommend,
>> but also with
>> 
>>     ln -s /dev/ttyUSB0 /dev/gps0
>>     ln ­s /dev/ttyUSB0 /dev/gpspps0
>> instead of
>>     ln -s /dev/ttyUSB0 /dev/gps0
>>     ln ­s /dev/ttyS0 /dev/gpspps0
>> 
>> since my PPS is arriving via /dev/ttyUSB0 (on my system, ttyS0 doesn¹t seem
>> to have an actual UART associated with it).
>> However, neither of these seems to have the desired effect.
>> 
>> Do you know if any special permissions for /dev/ttyUSB0 (currently
>> /dev/ttyUSB0 system has ³rw-rw---- root dialout² ).
>> 
>> Thanks again!
> 
> Since I don't use a serial to USB device I have no idea how this should be
> configured or even if it can be made to work.  But you can separate the PPS
> output and route it to either a serial port or parallel port.  Do you have a
> parallel port on your machine?
> 
> Also have you checked to see what kind of stuff is happening on /dev/ttyUSB0?
> Can you see the NMEA messages?
> 
>> 
>> Cheers,
>> Kent
>> 
>> On 10/28/10 1:11 AM, "Venu Gopal" <neo.venu at gmail.com> wrote:
>>> Hello Kent,
>>> 
>>> NTP requires NMEA messages and the PPS signal.
>>> If I am not wrong, the GPS receiver is connected to the PC via USB
>>> (/dev/ttyUSB0),
>>> though which the NMEA messages are read. And how is PPS connected to the
>>> PC if it doesn't have serial ports ?
>>> 
>>> I have used uBlox GPS receiver with both USB and serial connectivity.
>>> You need to create soft links for NMEA and PPS as shown below:
>>> 
>>> 1. For NMEA data:
>>> ln -s /dev/ttyUSB0 /dev/gps0
>>> 
>>> 2. For PPS:
>>> ln -s /dev/ttyS0 /dev/gpspps0
>>> 
>>> You need to have PPS signal reaching to PC either over a serial port or a
>>> parallel port.
>>> 
>>> Regards,
>>> Venu
> 
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