velocity meter with GPS module code help

[sonealx]

I have GPS module PIC 16F877A and 2x16 lcd. I am trying to do a velocity meter. I use $GPVTG data strings from GPS signals in my code. The hardware is complete. When I give the voltage to my hardware, lcd responds lately. I think this is normal. But the lcd shows the velocity info 3-4 times, then it stops (blank screen). I thought that the problem is in the code. Here is my code:

[andrewg]

I can see several problems:

1. You need to add "errors" to your "#use rs232".
2. You don't provide a size for your c[] array.
3. You shouldn't sit in a loop in your interrupt handler. All your handler should be doing is saving the received character into a buffer. Your mainline should be reading that buffer and doing the processing.
_________________
Andrew

[ckielstra]

A few other small issues:

[Douglas Kennedy]

Well inside your isr you call getc more than once. The way the interrupt works is that it is called whenever and only whenever a char has been received. The hardware is capable of having the last and current character in its buffer but this is not guaranteed. All that is guaranteed is that at least one char is available when the interrupt triggers. If by chance two chars are available then the reading of exactly one char in your isr and then returning will cause automatically an immediate new interrupt to process the second char.
To test for a header label
like $GPVTG try something like this in your isr....
With a global flag set the flag to 1 if $ is the result of your getc
set the flag to 2 if it was 1 and G is the result of getc
set the flag to 3 if it was 2 and P is the result of getc
set the flag to 4 if it was 3 and V is the result of getc
set the flag to 5 if it was 4 and T is the result of getc
set the flag to 6 if it was 5 and G is the result of getc
If the flag is 6 then use getc to place the sentence into your buffer array.
set a global ptr to the position of your sentence in your circular buffer
When the sentence is complete set a global sentence available flag and set a global ptr to the end of your sentence and reset the global flag from 6 to 0. The next sentence will begin at this end address plus one modulo the size of your buffer.
In main you will parse the buffer knowing that your isr will interrupt and process exactly one char at a time as the char is received. The isr will be receiving the next $GPVTG

I highly highly suggest with RS232 that you always use an interrupt and a circular buffer. It is not that it can't be done another way its just that the interrupt and the circular buffer is so robust an approach that you will find it extremely reliable.

[sonealx]

Thank you andrewg and ckielstra for your advice. I changed the code in your advices. but it didnt work again :( here is the changed code:

[andrewg]

The post from Douglas Kennedy is good except I'd change it a little so that when the flag gets to 6, it starts counting commas, i.e.

set the flag to 7 if it was 6 and , is the result of getc
set the flag to 8 if it was 7 and , is the result of getc
etc

then when you get to the 7th comma:

if the flag is 14 then add the getc characters to your c[] array
if the flag is 15 then set "c array is ready flag"

In your mainline, check the flag, parse and display it, then clear the flag.

Also, make sure your code never accesses past the end of the c[] array, eg c[55], c[56], etc. Add checks to the index variable to ensure that.
_________________
Andrew

[andrewg]

In general, using a circular buffer and simply having the serial ISR put stuff into that buffer is the way to go. CCS include many examples with their compiler and the relevant one in this case is "EX_SISR.C".

The idea is that the serial interrupt handler should be as simple as possible. However, my suggestion was that you can actually perform some simple processing in the interrupt handler using a flag to keep track of where you're up to and only save the velocity field into your buffer for display.

Also, while it isn't important for such a simple task as what you're doing, generally floating point is avoided as much as possible. Floating point is slow and takes up lots of valuable code space.

In fact, for your project the simplest solution is not to parse the velocity data at all but simply copy the velocity field text to a buffer, then copy that text to the display.
_________________
Andrew

[sonealx]

thank you for helping. But i still dont know how to put the info in a circular buffer after gateflag.i tried some code and CCS didn't happen an error. Do you think that are they true for isr part and in main program part?? if not, can you help me to correct it?? here is the code :

[Douglas Kennedy]

With respect you may lack the basic expertise to complete a project like this.
I PM'd you with some of the code you are now cutting and pasting.

Many are disappointed when they cut and paste code and with hope alone expect it to work. Many feel they are just one element of advice or one line of code away from success. Reality is that for all but the most basic things a real understanding is required. Rarely is the loudness of a cry for help or the frequency of the cry sufficient to create success. Success often requires starting small such that a foundation is built that enables greater things to be done. To write a book you must first learn the language in which it is to be written.

[bkamen]