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Forum Index : Microcontroller and PC projects : Reading from UART too fast ? GPS checksum
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vk4tec Senior Member  Joined: 24/03/2012 Location: AustraliaPosts: 239 |
Hello When I was coding in Assembly and PIC's I know it was a good practice to not blindly rush into the receive section of th USART but observe a few rules. For instance check the presence of a character and also don't let the recieve buffer get over run. I am seeing something like this in MMBASIC If I try and read two characters really quickly, from a GPS the second of the two characters is sometimes not populated. If if put a "pause 10" in there, it works 100% get_nmea ( this return $.........................* ) $GPRMC,122030.2,A,2720.25427,S,15302.59721,E,000.00,167.7,29 1212,011.0,E* Then I use a statment to get two more characters to "tack" on the end. "45" $GPRMC,122030.2,A,2720.25427,S,15302.59721,E,000.00,167.7,29 1212,011.0,E*45 Maybe I can use CR or LF instead as the trigger ??? It as if I am reading from the USART too quickly. What it is is the checksum from a NMEA GPS stream. The parsing code takes in all characters from $.............*XY $GPRMC,122030.2,A,2720.25427,S,15302.59721,E,000.00,167.7,29 1212,011.0,E* The code forms msg$ from "$" to "*" and misses XY the checksum. Some programs want the check sum. So I just read two characters and append on the end of MSG$ 45 I have to slow it down and put a pause 10 in between reading X and Y Otherwise I get "$.............................*X < and no Y > Curious - Andrew - Andrew Rich VK4TEC www.tech-software.net |
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| rhamer Senior Member Joined: 06/06/2011 Location: AustraliaPosts: 174 |
That data seems to be fixed length, so you need to sit in a while loop until you have received all characters. Something like; Do while char count < xx Read char Increment char count Loop Not the right syntax, and it assumed the read char is blocking, but you get the idea. Cheers Rohan Rohan Hamer HAMFIELD Software & Hardware Solutions Makers of the Maximite Expander. http://www.hamfield.com.au |
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| cwilt Senior Member Joined: 20/03/2012 Location: United StatesPosts: 147 |
Andrew, The way I did it was to parse the fields and not the whole sentence. Each field ends with a comma and checksum ends with CRLF. As the fields were parsed I would fill variables. When the variables were full then display, save to file, or both. This allowed the buffer time to fill while parsing each field. Maybe not the best way but it allowed me to write 1 NEMA handling routine for all nema strings that way if I change gps units that may have different outputs it still works. |
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Geoffg Guru  Joined: 06/06/2011 Location: AustraliaPosts: 3165 |
Instead of an arbitrary pause you could use: DO WHILE LOC(#1) < 2 : LOOP Geoff Graham - http://geoffg.net |
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