[aprssig] USB GPS question

Stephen H. Smith wa8lmf2 at aol.com
Sat Feb 25 00:14:06 CST 2012

On 2/24/2012 10:43 PM, Andrew Rich wrote:
> If you take a USB GPS to a PC you can use it as a serial device
> If you take the same GPS and plug it into a micro processor prototype board with ftdi chip, can you still access the NMEA ?

For the most part, NO.      The FTDI (or any other serial<-->USB chip) is 
nothing until it is enabled by a rather complex multi-layer stack of driver 
software.      In systems with "real" operating systems such as Windows, MacOS 
or Linux, a large part of the USB support is already embedded in the OS, with 
the additional device-specific part provided by the driver you install from a 
CD or download.     Implementing this on a micro controller, without an 
existing actual OS, would be a major undertaking.   Note that the USB root hub 
on the computer or microcontroller device is NOT the same chip used in 
serial<-->USB conversions on external devices being run backwards.   It's a 
completely different beast.

It would be far far simpler to just crack open the GPS and get at the NMEA data 
in serial form BEFORE it gets converted to USB.     (All so-called USB GPS 
devices are actually serial internally - they then get converted to USB 
internally using the same kind of chip used in serial<-->.USB dongles.)

It seems like an insane amount of effort to convert a serial data stream to USB 
inside the GPS, run it through two meters of cable, so you can then convert it 
back to serial with a dedicated microcontroller running a rather large amount 
of code that would have to be developed from scratch.

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