[LinuxPPS] LinuxPPS for Raspberry Pi?

[tlhackque]

On 27-Feb-13 06:01, folkert wrote:
> Maybe you can draw schematics for connecting the garmin to the RPI?
> I can solder but I cannot figure out how to alter the voltage levels or
> inverting.
> thanks
>
>
Wow, the mailing list is slow - I posted a month ago!  Thanks to the 
several folks who replied to my query.

My status:  I have my garmin 18x-lvc running on my RPi - it seems pretty 
reasonable; I have about a month of uptime.

# ntpq -pn
      remote           refid      st t when poll reach   delay offset  
jitter
==============================================================================
o127.127.20.0    .GPPS.           0 l   46   64  377    0.000 -0.015   0.013

The offset and jitter fluctuate (a few 10s of usec) over time.

It was a long saga - not quite finished.

Look at http://www.raspberrypi.org/phpBB3/viewtopic.php?f=41&t=1970 for 
the history.  Somewhere in there (currently 2nd to last page) you'll 
find photos of what I ended up building.  I had a small board with an 
RS232 level shifter that I modified.  But it's not the best choice; I 
used what I had on-hand. 
http://www.compsys1.com/workbench/On_top_of_the_Bench/Max233_Adapter/max233_adapter.html 
has the website.  I used their A232DBH3v, with some modifications. 
Starting from scratch, consider *A232DBCT3V 
<../../../workbench/BenchOrders/bench_orders.html#dbct3v> *as it already 
has enough channels for the PPS data.  You don't need the DB9 connector, 
but it's handy for debugging.  It's about $20 assembled - saves a lot of 
effort.  But you can build your own level converter using the maxim 
parts for less $ but more time. These days, you need to be able to 
handle SMT parts...

I used a female header to connect to the RPi GPIO connector - you can 
connect directly to the pins, but a header is a lot more flexible.  easy 
to find at any electronics parts supplier.  It's the standard 0.100 in 
dual-row header.

The connections are pretty simple:
    garmin data comes in as rs232, goes thru the MAX232 inverter and 
comes out as 3.3V signal.  Wire that to the RPi UART receive pin. data 
from the RPI UART transmit pin goes to the MAX232 inverter and comes out 
as 12V - goes to the garmin.

The PPS signal comes from garmin, send it thru the MAX232 and connect 
the output to a GPIO input pin.

Be careful with voltages.  The garmin runs on +5V; the RPi on +3.3V.

Also, the inversion means that the PPS signal is seen by the RPi as 
FALLING edge.  So configure PPS for that.  Don't try to do this with 
fudge - only one edge (rising as it leaves the GPS, falling after 
inversion) is controlled.  So you'll see bad jitter if you tell NTP to 
look at the wrong edge.

If your antenna is outside, you might want to worry about surge 
suppression - I didn't because it costs more than the RPi  Garmin to do 
right.

I posted my config files.  NTP looks like this (just the GPS lines):

server 127.127.20.0 mode 1  prefer
fudge  127.127.20.0 time1 0.000 time2 0.643 refid GPPS flag1 1 flag2 0 
flag4 1

The open item is that after rebuilding the RPi kernel, the kernel isn't 
talking to the RPi firmware.  See 
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=31680&p=273629#p273629 
for that.

Someone else in the same thread used the adafruit ultimate GPS, which 
looks interesting.  Less expensive than the garmin, and no level 
conversion needed.  One caution: the adafruit board uses 3.3V, but has a 
5V -> 3.3V LDO regulator.  Their article recommends powering from 5V.  I 
don't - report is that the regulator introduces enough noise that the 
jitter is greater.  Haven't personally verified, but if I were starting 
from scratch, I think I'd try one one of these.  Maybe if I decide to 
invest in another couple of reference clocks for redundancy.  (1 = you 
know the time; 2 = you've now clue; 3 = you can vote...)

Geoff - the precompiled kernel is fine for the adafruit, but due to the 
PPS inversion, not for the garmin.  I ended up getting the development 
tools and cross-compiling a kernel on an x86 linux server  - which 
almost worked.  Exception is the firmware issue.

The garmin has battery backup.  The adafruit is supposed to accept a 
lithium battery - with that you shouldn't need an RTC module.  Why 
didn't you go that way?  Is the LCD display really helpful?

Thanks again.  Hope this info is useful.

-- 
This communication may not represent my employer's views,
if any, on the matters discussed.

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