[time-nuts] Msg to N.Z. time nuts

[Philip Pemberton]

Hal Murray wrote:
> I started collecting low cost GPS receivers a year or two ago.  I thought I 
> had some with the SiRF-II chips.  Either I can't find them or I didn't 
> actually get any.

I've been "collecting" the OEM modules for about the same amount of time. That 
said, I haven't got that many -- a pair of Trimble SVeeSixes with 4.12 
firmware and an Axiom Sandpiper (SiRF2 based). I've just got a pair of Fastrax 
"iTrax" IT321 receivers, which are surface-mount (i.e. solder-down to a PCB) 
receiver modules based on the SiRF3 chipset.

>   GSW3.2.4_3.1.00.12-SDK003P1.00a  GlobalSat BU-353
>   GSW3.2.2_3.1.00.12-SDK003P1.01a  AmbiCom GPS-USB Rev 2.1
>   GSW3.2.2_3.1.00.12-SDK003P1.01a  Navibe GM-720
>   GSW3.2.4Pat2_3.1.00.12-SDK001P1.00  Navisys GR-300

All of which are running the SiRF 3.2 firmware, so if there is a firmware bug 
in play, they're all going to be doing much the same thing...

> They all use the Prolific PL2303 serial-to-USB chip.

That might be what's causing the timing jitter, especially if there are other 
devices on the USB bus. USB-to-serial chips aren't known for accurate timing 
-- a few of them buffer incoming data and then send it over the USB bus in one 
burst.

That's not to say the SiRF chipset isn't the cause of the error, but it might 
not be the only variable. I'd be tempted to tie a TTL=>RS232 level translator 
to the RXD_IN pin on the PL2303, then connect the output of the translator to 
a spare 'proper' RS232 port on a PC. The timing should be far more accurate 
(though probably not fantastic).

-- 
Phil.
lists at philpem.me.uk
http://www.philpem.me.uk/