Re: [avr-gcc-list] General programming question/GPS parsing

[Curtis Maloney]

wbounce wrote:
> I am think of trying to parse a GPS feed which is transmitted at 4800
> baud. It can send 7 "sentences" of a max of 80 bytes to a potential of
> 560 bytes every second. Then I need to parse the data. I am concerned
> about how much time and ram it will take to do all of this. With only 4K
> of ram space how would you handle this. I was think of using a round
> robin buffer approach but then that complicates the parsing with having
> to worry about wrapping. Or should the program be fast enough to process
> the 1st 80 bytes by the time the 2nd sentences is done coming in? I am
> running a ATMega 128  16MHZ. Using WinAvr C and was think of using the
> AVRlib  standard strcmp,strsep,strtok_r etc.

Well, let's look at the real world timings, here.  At 4800bps, you have just 
over 2ms to parse each BYTE of incoming data.  In that time, a 16MHz AVR will 
process something approaching 3,333 instructions.

That's PER BYTE.  Now, I'm not sure how much work you need to do on these 
"sentences", but I'm betting you could get a lot of it done well before the 
next byte is due.

I agree that double-buffers of 80bytes each is a good idea.  And in the time 
it takes you to receive 80bytes, you have well over a quarter of a million 
instructions to get things done with.

> Has anyone done this and kept control of a robot at the same time?

Maybe somebody has.  How much trouble you may run into depends on a lot of 
things, such as complexity of parsing the GPS data, complexity of 
"controlling" your robot, and how you schedule tasks.

> Are there timing values for the library routines?

That is a very excellent question.  I'm willing to bet a good deal of effort 
has been made to make these routines size and speed efficient.  Perhaps you'd 
like to try some elemental timings?

--
Curtis Maloney
address@hidden