Merge pull request #4 from dherrada/master

Merging

Author: evaherrada

README.rst

@@ -41,9 +41,9 @@ Feather boards and many other circuitpython boards will round to two decimal pla
     >>> float('1234.5678')
     1234.57
 
-This isn't ideal for gps data as this lowers the accuracy from 0.1m to 11m. 
+This isn't ideal for GPS data as this lowers the accuracy from 0.1m to 11m. 
 
-This can be fixed by using string formatting when the gps data is outputted.
+This can be fixed by using string formatting when the GPS data is output.
 
 An implementation of this can be found in examples/gps_simpletest.py
 
@@ -88,7 +88,71 @@ These two lines are the lines that actually solve the issue:
 
     print('Latitude: {0:.6f} degrees'.format(gps.latitude))
     print('Longitude: {0:.6f} degrees'.format(gps.longitude))
-
+   
+About NMEA Data
+===============
+This GPS module uses the NMEA 0183 protocol.
+
+This data is formatted by the GPS in one of two ways.
+
+The first of these is GGA. GGA has more or less everything you need.
+
+Here's an explanation of GGA:
+::
+
+                                                        11
+           1         2       3 4        5 6 7  8   9  10 |  12 13  14   15
+           |         |       | |        | | |  |   |   | |   | |   |    |
+    $--GGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh
+
+
+1. Time (UTC)
+2. Latitude
+3. N or S (North or South)
+4. Longitude
+5. E or W (East or West)
+6. GPS Quality Indicator,
+
+   * 0 - fix not available,
+   * 1 - GPS fix,
+   * 2 - Differential GPS fix
+    
+7. Number of satellites in view, 00 - 12
+8. Horizontal Dilution of precision
+9. Antenna Altitude above/below mean-sea-level (geoid)
+10. Units of antenna altitude, meters
+11. Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level below ellipsoid
+12. Units of geoidal separation, meters
+13. Age of differential GPS data, time in seconds since last SC104 type 1 or 9 update, null field when DGPS is not used
+14. Differential reference station ID, 0000-1023
+15. Checksum
+
+The second of these is RMC. RMC is Recommended Minimum Navigation Information.
+
+Here's an explanation of RMC:
+::
+
+                                                               12
+           1         2 3       4 5        6 7   8   9   10   11|
+           |         | |       | |        | |   |   |    |   | |
+    $--RMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,xxxx,x.x,a*hh
+
+1. Time (UTC)
+2. Status, V = Navigation receiver warning
+3. Latitude
+4. N or S
+5. Longitude
+6. E or W
+7. Speed over ground, knots
+8. Track made good, degrees true
+9. Date, ddmmyy
+10. Magnetic Variation, degrees
+11. E or W
+12. Checksum
+
+
+`Info about NMEA taken from here
+<https://www.tronico.fi/OH6NT/docs/NMEA0183.pdf>`_.
 
 Contributing
 ============
@@ -100,7 +164,7 @@ before contributing to help this project stay welcoming.
 Building locally
 ================
 
-To build this library locally you'll need to install the
+To build this library locally, you'll need to install the
 `circuitpython-build-tools <https://github.com/adafruit/circuitpython-build-tools>`_ package.
 
 .. code-block:: shell

adafruit_gps.py

@@ -171,6 +171,11 @@ def _parse_sentence(self):
         # Parse any NMEA sentence that is available.
         # pylint: disable=len-as-condition
         # This needs to be refactored when it can be tested.
+
+        # Only continue if we have at least 64 bytes in the input buffer
+        if self._uart.in_waiting < 64:
+            return None
+
         sentence = self._uart.readline()
         if sentence is None or sentence == b'' or len(sentence) < 1:
             return None