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[clm42]

I was just thinking of hooking my old garmin etrex up to the internal serial port and trying to find some software for it.

[pgf]

we need to poke around and find the serial port, both in s/w (is there a ttyS0? i don't have an XO yet to see for myself) and in h/w.  there's a page here that describes it, and the adapter that OLPC uses for connecting to external devices:

    http://wiki.laptop.org/go/Serial_adapters

note that if you wanted to use an existing GPS unit like an etrex, you'd still need a TTL/RS232 level converter.

[sloe]

OK folks,  I've got news.  I tore apart the XO tonight and discovered that GPS is possible!  Thanks to the link that Paul posted, I was able to locate both serial ports on the mobo - one (J1) has a connector already attached, and the other (CN24) is just pads on the board that can be soldered to.

I'll have pics up with more explanation tomorrow.

-sloe

[Torokun]

yes please~!

[pgf]

I found a site that sells OEM GPS devices.  The product listing is here, and it seems they may have a solution for us. ..  I did find one that is only 43.3mm x 31.4mm x 5.8mm that seems to be easy to work with at the "hobby" level, ...

I did find one interesting tidbit, though - they will do sample orders of 10 units or more.  ...

i'm having trouble navigating and browsing their site.  which units were you looking at?

have you looked at http://www.sparkfun.com?  that's where i would probably look first, since they're a little more geared towards non-OEM customers, and have info about antennas, etc.

paul

[sloe]

I did look at sparkfun, but I didn't get a warm fuzzy from them.  Can't remember why at the moment, so I'll go back and take another look.  As for the one device I was thinking of, it is this one here - It's small, but has pins for the connections instead of pads.

One thing I noted previously was that we would need a LVTTL>USB adapter in order to use a USB device.  That, in fact, is not true.  The OLPC serial Adapters are themselves LVTTL -> USB/RS232 adapters, which says the XO itself operates on LVTTL (3.3v TTL).  This is a bonus, because the GPS module I linked above outputs LVTTL, as well as requires 3.3V for operation, which can be provided from Pin 1 of J1 or CN24 on the XO.  Basically what it boils down to is that all that is required for this to work is the GPS module, an antenna (which is going to be the hard one), and a cable to hook it to J1.  Oh, and some software to utilize it

Now, for the pikturs I mentioned....  I"ll be posting smallish ones here. If you want the big 'uns in their full 10MP resolution, just click on the pic, it'll take you there.  Each 10MP pic is about 4-5MB in size, though, so it will take a bit to get them if you've got a slow connection.

First, the money shot - what an XO-1 looks like without it's clothes on:


Not much there.  Just to the bottom left of the heatshield, however, you can see an empty jack on the board.  This is J1.



Taking the heatshield off, we also find CN24 hidden near the southbridge.


Showin' off my "m4d ph0t0g sk1llz":


Ok, that laid out where our connections need to be.  Now, on the backside of the mobo, there is about 4mm of height available, but I'm leery of mounting anything there - all that space to the right of the WiFi card is open.  Depending on where else I can fit one, this may be our only option for the antenna, though.  The GPS module will fit under the LCD - there is a 1-2mm lip running around the edge of the LCD, and it sits on pads that raise it another 2mm.  Cutting a section out of the plastic (it's actually metal on the other side for shielding) would yield a mounting height of 5 mm.  There is also some room near the backlight connector, maybe enough to mount a module.  Unfortunately, I had no form of measurements with me while I had it apart, so I was only guessing where things would fit.  Here are some shots of the underside of the LCD:



Detail of backlight and data connectors:



I'll need to build dummy models for size out of paper and/or cardboard to verify my findings, but things are looking good.  They did pack this thing full and made the case as tight as possible, but we can squeeze a little bit out of it.

Oh, and I did notice one branding trick - on the main handle of the XO, where it starts to curve in, the texture of the case becomes little XO guys instead of just dots.  I didn't take a pic because I already had the flash card in the computer, but if you look closely at the handle of your XO you'll see what I'm talking about, that is, if you have one 

[sloe]

Hmmm....I don't remember why I didn't stick around at sparkfun, but now that I took a closer look it appears that the GS405 Helical GPS Receiver could be a decent solution.  It's biggest downfall is price - $90.  For that much, I'd just as soon buy a mouse GPS unit.  I'll have to work out it's spacing requirements, but I may be able to mount this in the handle and stick the antenna out a hole.  Well see.

[pgf]

but if you look closely at the handle of your XO you'll see what I'm talking about, that is, if you have one

gee, thanks for rubbing it in.  ;-)

thanks for the pics, and for looking for space.

regarding power:  remember that wherever it comes from has to go off in suspend mode -- not just when the system if off.  i think it would also be very clever of us to figure out how to power it down when it's not in use.  not sure about that at the moment.  are there other sections of the system that are, or can be, powered down individually?   if so, could we piggy-back on one of those?  at worst we may be able to find a spare gpio somewhere and use it to swich the gps module on and off, but that's getting further from KISS.

paul

[sloe]

Yeah, I've been keeping the power req's in the back of my mind.  That's one of the things I look at when checking these modules, and the GS405 I mentioned actually isn't that good in this regard.  It states a 100mA load, but doesn't give details.  The unit from mightygps way up at the top of this page states 67 - 90 mA, which I've come to discover is the difference between acquisition mode and tracking mode.  Only a few of them list the current required in standby for a "warm" start which can be provided by a battery (and is, on quite a few).

The simplest answer I've come up with for turning it on and off is a new hole drilled in the case with a momentary push button switch.  I'll have to break out the mutlimeter when I have time and start taking voltage readings in the three states: off, on, and standby.  I'm curious about that SW11 pad...I wonder if I could get an answer out of someone at OLPC about it?

[pgf]

new hole:  yuck.  (but i agree it would simple, in some sense.)

what "sw11 pad" are you referring to?

[mattd]

new hole:  yuck.  (but i agree it would simple, in some sense.)

Actually, pfg, with attention to a little detail, putting buttons in plastic with a professional look is easy.

I've done it several times for mounting small momentary switchs in car dashes for a 'stealth' garage door opener.

sloe, how hard is it to get in the handle area? I think in the strap holes or handle would be a perfect spot for the GPS antenna. And if you could find a white one.....

[pgf]

Actually, pfg, with attention to a little detail, putting buttons in plastic with a professional look is easy.

i'm supposed to hear about "attention to detail" from somone that can't get my initials right?  :-)

your point is well taken -- a switch (non-momentary :-) could work, but i'd much rather it be software-driven.

paul

[sloe]

your point is well taken -- a switch (non-momentary :-) could work, but i'd much rather it be software-driven.

paul

I agree - I would much rather prefer a s/w switch, but I'm a hardware geek.  I don't know programming all that well.  How 'bout I build the circuit, and you write the control instructions?

[pgf]

How 'bout I build the circuit, and you write the control instructions?

it's a deal.

i'm happy to report that for once, the OLPC wiki responded beautifully to the first two keywords i searched on:  "power domains".
   http://wiki.laptop.org/go/Hardware_Power_Domains

given that, and this diagram:
   http://wiki.laptop.org/images/4/4c/Tinderbox_C2.pdf

i'm thinking we should look for a spare gpio, and create a new domain.  i think sharing the USB domain would be the next most preferable, depending on how often it already gets turned off/on.

paul

[Wayan Vota]

Sloe & pgf,

I love what y'al are doing - GPS on the XO, and I'd love to have this whole idea highlighted on OLPC News through a Guest Post from y'all.  Please look for PM's from em ont he details.