' demonstration of SiRF GPS configuration and
' serial port input with circular line buffer
' (c) Robert Rozee 2016


' configure circular line buffer
' ******************************

Dim CB$(10)
head = 1
tail = 1
running = 0

A$ = ""
B$ = ""

Const clreol$ = Chr$(27)+"[K"

Open "COM1:9600,1024,isr,S2" As #1


' configure SiRF chipset
' **********************

SS1$ = "$PSRF100,0,9600,8,1,0*0C"+Chr$(13)+Chr$(10)
For I = 1 To 10: Read J: SS2$ = SS2$+Chr$(J): Next I ' disable static nav
For I = 1 To 10: Read J: SS3$ = SS3$+Chr$(J): Next I ' enable statis nav
For I = 1 To 32
Read J
If (I>=5) And (I<=28) Then K = K+J ' checksum accumulated in K
If I=29 Then J = K \ 256
If I=30 Then J = K Mod 256
SS4$ = SS4$+Chr$(J)
Next I

Print #1, SS1$; ' enter SiRF binary mode
Pause 100
Print #1, SS2$; ' disable static navigation
Pause 100
Print #1, SS4$; ' return to NMEA mode
Pause 100
running = 1
clrscr


' main loop
' *********

Do
If head<>tail Then ' collect NMEA sentence from buffer
B$ = CB$(tail)
tail = (tail Mod 10)+1
validate(B$) ' verify checksum
EndIf


If Instr(B$,"$GPRMC")=1 Then ' check if sentence is the one we want

Print at$(1,4); B$; clreol$
Print clreol$

For I = 1 To 16 ' extract parameters one at a time
J = Instr(B$,",")
If J=0 Then Exit For
B$ = Mid$(B$,J+1) '$GPxxx is first discarded (param 0)

If I=1 Then GPS.time = Val(B$)
If I=2 Then GPS.OK = (Left$(B$,1)="A")
If I=3 Then
K = Instr(B$,".")
IP% = Val(Left$(B$,K-1))
FP% = Val(Mid$(B$,K+1))
GPS.lat% = IP%*10000+FP% ' 1/10000th of a degree per count
EndIf
If (I=4) And (Left$(B$,1)="S") Then GPS.lat% = -GPS.lat%

If I=5 Then
K = Instr(B$,".")
IP% = Val(Left$(B$,K-1))
FP% = Val(Mid$(B$,K+1))
GPS.long% = IP%*10000+FP% ' 1/10000th of a degree per count
EndIf
If (I=6) And (Left$(B$,1)="W") Then GPS.long% = -GPS.long%

If I=7 Then GPS.sog = Val(B$)*1.852 ' convert knots to km/h
If I=8 Then GPS.cog = Val(B$)
If I=9 Then GPS.date = Val(B$)

Next I

time.H = GPS.time \ 10000
time.M = (GPS.time \ 100) Mod 100
time.S = GPS.time Mod 100

date.D = GPS.date \ 10000
date.M = (GPS.date \ 100) Mod 100
date.Y = GPS.date Mod 100

Print at$(1,8);
Print "time = ",
Print Str$(time.H,2,0," "); "h",
Print Str$(time.M,2,0,"0"); "m",
Print Str$(time.S,2,0,"0"); "s"

Print "date = ",
Print Str$(date.Y,2,0,"0"); "y",
Print Str$(date.M,0,0,"0"); "m",
Print Str$(date.D,0,0,"0"); "d"
Print
Print "lat = ", GPS.lat% \ 10000, ". "; Abs(GPS.lat% Mod 10000)
Print "long = ", GPS.long% \ 10000, ". "; Abs(GPS.long% Mod 10000)
Print
Print "sog = ", GPS.sog; " km/h"; clreol$
Print "cog = ", GPS.cog; " degrees"; clreol$
Print
If GPS.OK Then Print "FIX OK";
Print clreol$;

EndIf

B$ = ""

Loop
End ' end of program



Sub isr ' serial port handler
Local I, S$ length 80
S$ = Input$(72,#1) ' grab data into temporary buffer
If running Then
A$ = Right$(A$,255-Len(S$))+S$ ' add to end of linear buffer
I = Instr(A$,Chr$(13)+Chr$(10)) ' check for eol marker (CR+LF)
Do While I<>0
CB$(head) = Left$(A$,I-1) ' transfer into circular buffer
head = (head Mod 10)+1
If I<254 Then A$ = Mid$(A$,I+2) Else A$=""
I = Instr(A$,Chr$(13)+Chr$(10)) ' check for any more eol markers
Loop
EndIf
End Sub



Sub validate(B$) ' verify B$ is undamaged
Local header, footer, checksum, I
header = Instr(B$,"$") ' header marker, $
footer = Instr(B$,"*") ' footer marker, *
checksum = 0
If (header<>1) Or (footer<>(Len(B$)-2)) Then
B$ = ""
Else
For I = header+1 To footer-1 ' xor together everything in between
checksum = checksum Xor Asc(Mid$(B$,I,1))
Next I
If Hex$(checksum,2)<>Right$(B$,2) Then B$ = ""
EndIf
End Sub ' B$ is cleared if anything is wrong



Function at$(x,y) ' VT100 position cursor
at$ = Chr$(27)+"["+Str$(Y)+";"+Str$(X)+"H"
End Function



Sub clrscr ' VT100 clear screen and home cursor
Print Chr$(27)+"[2J"+Chr$(27)+"[H";
End Sub



' disable static navigation
Data &hA0, &hA2 ' count + header
Data &h00, &h02 ' payload length (2 bytes)
Data &h8F, &h00 ' command + SN flag
Data &h00, &h8F ' payload checksum
Data &hB0, &hB3 ' footer

' enable static navigation
Data &hA0, &hA2 ' count + header
Data &h00, &h02 ' payload length (2 bytes)
Data &h8F, &h01 ' command + SN flag
Data &h00, &h90 ' payload checksum
Data &hB0, &hB3 ' footer

' return to NMEA mode
Data &hA0, &hA2 ' count + header
Data &h00, &h18 ' payload length (24 bytes)
Data &h81, &h02 ' command + mode
Data &h00, &h01 ' GGA (1) (interval + CS flag)
Data &h00, &h01 ' GLL (interval + CS flag)
Data &h00, &h01 ' GSA (1) (interval + CS flag)
Data &h00, &h01 ' GSV (5) (interval + CS flag)
Data &h01, &h01 ' RMC (1) (interval + CS flag) ## 1 sec ##
Data &h00, &h01 ' VTG (1) (interval + CS flag)
Data &h00, &h01 ' MSS (interval + CS flag)
Data &h00, &h00 ' <unused>
Data &h00, &h01 ' ZDA (interval + CS flag)
Data &h00, &h00 ' <unused>
Data &h25, &h80 ' baud rate (9600)
Data &hFF, &hFF ' payload checksum (calculated at runtime)
Data &hB0, &hB3 ' footer