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' GPS CLOCK
' Geoff Graham, March 2014
'
' Demonstration of the Micromite
' This program will get the time from an EM-408 GPS module, add the timezone
' and daylight saving then display on a two line LCD.
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' Configuration constants
TimeZone = 8.0        ' hours from GMT (+ or -)
UseDST = 0            ' set to 1 to enable daylight saving for AWST

' global arrays
Dim md(12), arg$(20)
Data 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
For i = 1 To 12: Read md(i): Next i

LCD Init 23, 24, 25, 26, 17, 18              ' setup the LCD for display
Open "COM1:4800" As #1                       ' open the GPS serial interface

' this is the main program loop, it never exits
Do
  KeepSearching:
  Do
    GetGPSRecord                             ' get a GPS record
  Loop Until arg$(0) = "GPRMC"               ' we only want the RMC record
  If arg$(2) <> "A" Then                     ' "A" means valid record
    LCD 1, C16, "Searching"
    LCD 2, C16, "For Satellites"
    GoTo KeepSearching                       ' go back and keep looking
  EndIf

  ' the GPS has the valid time
  ' first extract the elements of the date/time from the GPS record
  year = Val(Right$(arg$(9), 2))             ' extract the date
  month = Val(Mid$(arg$(9), 3, 2))
  day = Val(Left$(arg$(9), 2))
  hour = Val(Left$(arg$(1), 2))              ' extract the time
  min = Val(Mid$(arg$(1), 3, 2))
  sec = Val(Mid$(arg$(1), 5, 2))

  ' convert the time to minutes since midnight 1st Jan 2014
  ' then add/subtract the timezone and if required add daylight saving
  ' this calculation takes just 15mS at 40MHz
  mins = GetMins(year, month, day, hour, min)
  mins = mins + TimeZone * 60                ' adjust for the timezone
  If UseDST Then                             ' if we observe daylight saving
    If mins < GetDST(year, 4, 2) Or mins > GetDST(year, 10, 2) Then
      mins = mins + 60                       ' adjust for AWST DST
    EndIf
  EndIf

  ' because we will display the time at the start of the next second
  ' we have to add 1 second to the current time
  sec = sec + 1
  If sec >= 60 Then sec = 0 : mins = mins + 1

  ' finally convert the minutes back into the current date/time
  Line1$ = GetDate$(mins)
  Line2$ = GetTime$(mins)

  ' we now have the date/time for the next second ready for display
  ' we have to wait for the end of the data stream from the GPS
  ' then we wait for the start of the next second and only then update the LCD
  Do : Loop While Timer < 950                ' wait for the data to finish
  Do : Loop While Input$(200, #1) <> ""      ' clear the input buffer
  Do While Input$(1, #1) <> "$" : Loop       ' wait for a new second to start
  Timer = 0

  ' display the date and time for this second
  LCD 1, C16, Line1$
  LCD 2, C16, Line2$
Loop


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' subroutine to get a GPS record into the array arg$()
Sub GetGPSRecord
  Do
    Do While Input$(1, #1) <> "$" : Loop   ' wait for the start
    For i = 0 To 20
      arg$(i) = ""                         ' clear ready for data
      Do                                   ' loops until a specific exit
        x$ = Input$(1, #1)                 ' get the character
        If x$ = "," Then Exit Do           ' new data item, new field
        If x$ = "*" Then Exit Sub          ' end of record, so return with it
        arg$(i) = arg$(i) + x$             ' add to the data
      Loop                                 ' keep going
    Next i                                 ' increment the field
  Loop
End Sub


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' calculate the minutes since midnight 1st Jan 2014
Function GetMins(yr, mth, day, hr, min)
  GetMins = (yr - 14) * (365 * 24 * 60) + ((yr - 13) \ 4) * (24 * 60)
  GetMins = GetMins + (md(mth) * (24 * 60))
  GetMins = GetMins + ((day - 1) * (24 * 60))
  GetMins = GetMins + (hr * 60)
  GetMins = GetMins + min
  If (yr - 16) Mod 4 = 0 And mth > 2 Then GetMins = GetMins + (24 * 60)
End Function


' convert minutes back into the time (as a string)
Function GetTime$(minutes)
  Local hr, min, am$
  am$ = "AM"
  hr = (minutes \ 60) Mod 24
  If hr > 12 Then am$ = "PM" : hr = hr - 12
  If hr = 0 Then hr = 12
  min = minutes Mod 60
  GetTime$ = Str$(hr) + ":"
  GetTime$ = GetTime$ + Right$("0" + Str$(min), 2) + ":"
  GetTime$ = GetTime$ + Right$("0" + Str$(sec), 2) + " " + am$
End Function


' convert minutes back into the date (as a string)
Function GetDate$(minutes)
  Local yr, mth, day, mths$, days$
  mths$ = "  JanFebMarAprMayJunJulAugSepOctNovDec"
  days$ = "SunMonTueWedThuFriSat"
  For yr = 14 To 99
   If minutes < GetMins(yr + 1, 1, 1, 0, 0) Then Exit For
  Next yr
  For mth = 1 To 12
    If minutes < GetMins(yr, mth, 1, 0, 0) Then Exit For
  Next mth
  mth = mth - 1
  day = ((minutes - GetMins(yr, mth, 1, 0, 0)) \ (24 * 60)) +  1
  GetDate$ = Mid$(days$, ((((minutes \ (24 * 60)) + 3) Mod 7) * 3) + 1,3)+"  "
  GetDate$ = GetDate$ + Right$(" " + Str$(day), 2)
  GetDate$ = GetDate$ + "-" + Mid$(mths$,mth*3,3) + "-" + Str$(yr + 2000)
End Function


' get the minutes that DST will start or end in a month
Function GetDST(yr, mth, hr)
  Local d, m
  m = GetMins(yr, mth, 1, hr, 0)
  d = ((m \ (24 * 60)) + 3) Mod 7
  GetDST =  m + (((7 - d) Mod 7) * 24 * 60)
End Function