Conversion (Char to Int) for a Field in an NMEA sentence?

Projects Programming
1

This question is about the simplest way to convert the data (char) from a field (field 7) in an $GPRMC NMEA sentence?

I would like to take field 7 from the NMEA sentence, convert that 'Knots' reading into an int form, divide it by .54 and output it to a PWM pin that I can RC filter and output via an LM358 to give me an analog output...

I'm having problems finding an easy way to convert the char field into an int variable? I think I have missed something!

I'm happy with this sketch otherwise and have not used TinyGPS.

I also want to output to an LED the status of field 2 (LED on if field is "A") so need to test for that as well at some stage?

Here is the current raw sketch that needs the additions....

Any help appreciated! Thanks.....

#include <SoftwareSerial.h>

SoftwareSerial gpsSerial(10, 11); // RX, TX (TX not used)
const int sentenceSize = 80;

char sentence[sentenceSize];

void setup()
{
  Serial.begin(9600);
  gpsSerial.begin(9600);
}

void loop()
{
  static int i = 0;
  if (gpsSerial.available())
  {
    char ch = gpsSerial.read();
    if (ch != '\n' && i < sentenceSize)
    {
      sentence[i] = ch;
      i++;
    }
    else
    {
     sentence[i] = '\0';
     i = 0;
     displayGPS();
    }
  }
}

void displayGPS()
{
  char field[20];
  getField(field, 0);
  if (strcmp(field, "$GPRMC") == 0)
  {
  Serial.println("Speed over the ground in knots is:"); 
    getField(field, 7);  // Knots value from GPS
  Serial.println(field);
  
[color=red][font=Verdana]  // I would like to take field 7 from the 
  // NMEA sentence, convert that 'Knots' reading into an int form,
  // div it by .54 and output it to a PWM pin that I can
  // RC filter and output via an LM358 to give me an analog output....[/font][/color]
  }
}

void getField(char* buffer, int index)
{
  int sentencePos = 0;
  int fieldPos = 0;
  int commaCount = 0;
  while (sentencePos < sentenceSize)
  {
    if (sentence[sentencePos] == ',')
    {
      commaCount ++;
      sentencePos ++;
    }
    if (commaCount == index)
    {
      buffer[fieldPos] = sentence[sentencePos];
      fieldPos ++;
    }
    sentencePos ++;
  }
  buffer[fieldPos] = '\0';
}

Edit: add code tag! sorry....

2

You probably would have received an answer sooner had you placed your code into code tags.

getField(field,7);
int knots = atoi(field);   // ascii to integer
3

Thanks....
Now added Code Tags!

4

atoi works perfectly! never heard about it before....

Just read - "its part of the C++ standard libraries (stdlib.h), which are included automatically in all sketches"

Thanks again.

5

mightyboyev:
atoi works perfectly! never heard about it before....
Just read - "its part of the C++ standard libraries (stdlib.h), which are included automatically in all sketches"

I don't know how easy it is to find things by description, but http://www.learncpp.com/ is a great resouce for learning C++.

6

After that amazingly simple successful outcome, is there any simpler way to do a character compare (“A” or “V”) on field 2 of these NMEA sentences? Just want to turn an LED on if field 2 = A…

Can do it the standard method but just interested in how that could/might be cut down as well!

Looking at that learncpp link now….. Thanks for that tip.

7

I don't know what you consider the "standard" way, but you can't get much simpler or faster than

getField(field, 2);
if (field[0] == 'A') {
    //turn LED on
}