gps.py

from microbit import i2c, sleep
import gc
import math

class GPS(object):
    """
    GPS using i2c
    """

    def __init__(self):
        pass
        
    def GPSChecksumOK(self, Line):
        Count = len(Line)

        XOR = 0

        for i in range(1, Count-4):
            c = ord(Line[i])
            XOR ^= c

        return (Line[Count-4] == '*') and (Line[Count-3:Count-1] == hex(XOR)[2:4].upper())

    def FixPosition(self, Position):
        Position = Position / 100

        MinutesSeconds = math.modf(Position)

        return MinutesSeconds[1] + MinutesSeconds[0] * 5 / 3

	
    def SendGPS(self, Bytes):
        i2c.write(0x42,Bytes)

    def ProcessLine(self, Line):
        Position = None
        if self.GPSChecksumOK(Line):
            if Line[3:6] == "GGA":
                Position = {'fix': 0}
                # $GNGGA,213511.00,5157.01416,N,00232.65975,W,1,12,0.64,149.8,M,48.6,M,,*55
                Fields = Line.split(',')
                if Fields[1] != '':
                    Position['time'] = Fields[1][0:2] + ':' + Fields[1][2:4] + ':' + Fields[1][4:6]
                    if Fields[2] != '':
                        Position['lat'] = self.FixPosition(float(Fields[2]))
                        if Fields[3] == 'S':
                            Position['lat'] = -Position['lat']
                        Position['lon'] = self.FixPosition(float(Fields[4]))
                        if Fields[5] == 'W':
                            Position['lon'] = -Position['lon']
                        Position['alt'] = round(float(Fields[9]))
                Position['fix'] = int(Fields[6])
                Position['sats'] = int(Fields[7])
            elif Line[3:6] == "RMC":
                print("Disabling RMC")
                setRMC = bytearray([0xB5, 0x62, 0x06, 0x01, 0x08, 0x00, 0xF0, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x04, 0x40])
                self.SendGPS(setRMC)
            elif Line[3:6] == "GSV":
                print("Disabling GSV")
                setGSV = bytearray([0xB5, 0x62, 0x06, 0x01, 0x08, 0x00, 0xF0, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x03, 0x39])
                self.SendGPS(setGSV)
            elif Line[3:6] == "GLL":
                print("Disabling GLL")
                setGLL = bytearray([0xB5, 0x62, 0x06, 0x01, 0x08, 0x00, 0xF0, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01, 0x2B])
                self.SendGPS(setGLL)
            elif Line[3:6] == "GSA":
                print("Disabling GSA")
                setGSA = bytearray([0xB5, 0x62, 0x06, 0x01, 0x08, 0x00, 0xF0, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02, 0x32])
                self.SendGPS(setGSA)
            elif Line[3:6] == "VTG":
                print("Disabling VTG")
                setVTG = bytearray([0xB5, 0x62, 0x06, 0x01, 0x08, 0x00, 0xF0, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x05, 0x47])
                self.SendGPS(setVTG)
        return Position
    
    def GetGPSPosition(self):
        Position = None
        Line = ''
        while not Position:
            Byte = i2c.read(0x42,1)[0]
            if Byte != 255:
                Character = chr(Byte)
                if Character == '$':
                    Line = Character
                elif len(Line) > 90:
                    Line = ''
                elif (Line != '') and (Character != '\r'):
                    Line = Line + Character
                    if Character == '\n':
                        Position = self.ProcessLine(Line)
                        Line = ''
        return Position

sleep(1000)
print("GPS Test Program")
sleep(1000)

print("Read from UBlox GPS on i2c address 0x42")
print()
MaxFree = 0

while True:
    MaxFree = max(MaxFree, gc.mem_free())
    print(gc.mem_free(), MaxFree)
    
    gps = GPS()
    Position = gps.GetGPSPosition()
    gps = None
    
    if Position:
        print(Position)
        #if Position['sats'] >= 10:
        #    display.show('*')
        #else:
        #    display.show(str(Position['sats']))
    else:
        print("No position")
        #display.show('N')