GPS decoding problems

Hello everybody. Does anyone have experience with GPS data string decoding? I surfed Internet for a while and found that most successful it can be do by Basic Stamp BS2p. This microcontroller employs scratchpad RAM for collect serial input. Once the string is in the scratchpad, it can be parsed and converted as required. My question- is it possible to avoid using scratchpad memory for handling GPS data? If it isn’t possible which PICs have scratchpad memory? So, anyone who has idea how to handle this problem please let me know.
Thank you in advance
Vladimir

I use a PIC 16F877 to decode. I use an array of 85 characters to collect the input data string then set about unpacking the string based on counting the commas.


GetData:
ProcessRMC:
serin2 rx232, 188, 2000, nogps,[WAIT("$GPRMC")] ' find start
for i = 0 to 84 ' plenty of length - can be cut back
serin2 rx232, 188, [a[i]]
next i

FindTrackField:
commacount = 0
' lcdout $FE, $01
for i = 0 to 84
e = a[i]
if e = "," then commacount = commacount + 1
if commacount = 8 then gettrack 'Track follows 8th comma
'10 gives MagVar - 8 gives Track
next i

HTH
Brian

Thank you Brian!
I will try your code today.
Thanks again.
Vladimir

Brian Please Help!
Please post all code for “Find Field Track Routine”

I already tested your code and it successfully finding “GPRMC” string and storing data in 65 elements array (memory of PIC16F872 doesn’t allow to create bigger array).
But still cannot figure out how to find right field and extract numeric data from the string.

This is a sample of string I have:

$G,092713.867,A,4913.0722,N,12256.9039,W,1.16,73.6 5,291206,,*20
$G,092714.867,A,4913.0712,N,12256.9078,W,0.81,78.5 6,291206,,*25 and so on

Maybe I made mistake in my code?

'---------------- Definitions---------------------------------------------
define OSC 20
'------------- Define LCD registers and bits------------------------------
Define LCD_DREG PORTB 'Set LCD data port
Define LCD_DBIT 0 'Set starting data bit
Define LCD_RSREG PORTB 'Set LCD register select port
Define LCD_RSBIT 4 'Set LCD register select bit
Define LCD_EREG PORTB 'Set LCD enable port
Define LCD_EBIT 5 'Set LCD enable bit
'------------- Define DEBUG registers and bits----------------------------
define DEBUG_REG PORTC 'Set DEBUD port
DEFINE DEBUG_BIT 3 'Set DEBUG pin
DEFINE DEBUG_BAUD 4800 'Set DEBUG baud rate
DEFINE DEBUG_MODE 0 'Set DEBUG mode

'------------Allocate Variables for GPS-----------------------------------
TimeOut CON 3000 'Time out delay
baudGPS CON 16572 '16572 - 4800 baud inverted
ParseCycle var byte ' Parse counter
commacount var byte 'Commas counter
e var byte
GPSdata var byte[65] ' Data array
TEMPdata var byte ' Temp debug data

'--------------------- Pins definitions-----------------------------------
GPSin VAR PORTc.7 'pin
'-----------------------Program routine-----------------------------------


GetData:

serin2 gpsin,baudgps, timeout, nogps,[WAIT("$GPRMC")] ' find start of string
for ParseCycle = 0 to 64
serin2 gpsin, baudgps, [GPSdata[ParseCycle]]
next ParseCycle

'for parsecycle = 0 to 64 'test routine
'TEMPdata = gpsdata[parsecycle] 'test routine
'Debug TEMPdata 'test routine


FindTrackField:
commacount = 0
for ParseCycle = 0 to 64
e = GPSdata[ParseCycle]
if e = "," then commacount = commacount + 1
if commacount = 8 then gosub gettrack 'Track follows 8th comma

debug e
'debug " Gps data value ", gpsdata '10 gives MagVar - 8 gives Track

next ParseCycle


goto getdata

Nogps:
LCDOut $FE, 1 'Clear Screen
LCDOut "No GPS"
low portc.0
Pause 2300
high portc.0 'error LED flashes
pause 2300
low portc.0
goto getdata


gettrack:
lcdout $FE, $01
lcdout $fe, 1, "Time" 'not finished line
lcdout $fe, $c0, "Speed " 'not finished line
return

Brian, please push me forward one more time.
Vladimir

Here is a routine that will extract all fields of the RMB sentence. You can modify it for other strings.

Cheers
Brian

GetData:
' Add code here for GPRMC, etc

GetRMBData: ' capture 84 character frame in real time
serin2 rx232, 188, 2000, nogps,[WAIT("$GPRMB")] ' find header
for i = 0 to 83 'grab data stream
serin2 rx232, 188, [a[i]]
next i
' now we have an 84 character array starting with the first comma after
' $GPRMB. $GPRMB is of variable length depending on field values.
' Processing is now done while the next GPS frames pass by. Need a bigger
' RAM to handle RMB & RMC on every GPS frame.

UnpackRMB: ' unpack frame & show on SEROUT
CommaCnt = 0
for i = 0 to 83
if a[i] = "," then
CommaCnt = CommaCnt + 1 ' found a new field
serout bootout, 2, [$0D, $0A, #CommaCnt , ","]
if CommaCnt = 1 then getvalidity
if CommaCnt = 2 then getxte
if CommaCnt = 3 then getsteer
if CommaCnt = 4 then getorigin
if CommaCnt = 5 then getdestination
' if CommaCnt = 6 then getdestnlatt
' if CommaCnt = 7 then gethemisphere
' if CommaCnt = 8 then getdestlong
' if CommaCnt = 9 then geteorw
' if CommaCnt = 10 then getrangetogo
if CommaCnt = 11 then getbearing
' if CommaCnt = 12 then getclosingvelocity
if CommaCnt = 13 then getarrival
if CommaCnt = 14 then getdata 'all done so get next RMC
GetFieldData:
for j = i+1 to (i+15) 'get the next character
if a[j] = "," then
goto getnextfield
endif
serout bootout, 2, [a[j]]
next j
endif
GetNextField:
next i

goto getdata

GetValidity:
' This is needed since both Track and Bearing collapses to 0.0 when no
' GPS data is being received. This could be falsely interpreted as
' traveling due North with the destination dead ahead.
' A FailSafe check must be done here to prevent this. Currently omitted.
if a[i+1] = "V" then
serout bootout, 2, ["V = bad frame"]
goto GetNextField
endif
if a[i+1] = "A" then
serout bootout, 2, [ "A = good frame"]
goto GetNextField
endif
serout bootout, 2, [ "NO sync"]
goto GetNextField

GetXTE:
xte = 0
for j = (i+1) to (i+8) ' get (up to) the next 8 characters
if a[j]="," then xtedone ' end of field or blank field found
if a[j]<>"." then ' ignore any decimal point
e = a[j] - 48
'ASCII representation - subtract 48 to get numeric value
xte = 10* xte + e
endif
next j
XTEDone:
' at this point we have XTE * 10
serout bootout, 2, ["XTE = ", #xte/10, ".", #xte//10]
goto GetNextField

GetSteer:
if a[i+1] = "L" then
serout bootout, 2, ["Turn LEFT"]
goto GetNextField
endif
if a[i+1] = "R" then
serout bootout, 2, [ "Turn RIGHT"]
goto GetNextField
endif
serout bootout, 2, [ "NO turn info"]
goto GetNextField

GetOrigin:
'This is an alphanumeric field - needs unpacking
serout bootout, 2, ["Origin = "]
for j = (i+1) to (i+15) ' get (up to) the next 15 characters
if a[j]="," then origindone ' end of field or blank field found
serout bootout, 2, [ a[j]]
next j
OriginDone:
goto GetNextField

GetDestination:
'This is an alphanumeric field - needs unpacking
serout bootout, 2, ["Destination = "]
for j = (i+1) to (i+15) ' get (up to) the next 15 characters
if a[j]="," then destinationdone ' end of field or blank field found
serout bootout, 2, [ a[j]]
next j
DestinationDone:
goto GetNextField

GetBearing:
bearing = 0
for j = (i+1) to (i+8) ' get (up to) the next 8 characters
if a[j]="," then bearingdone ' end of field or blank field found
if a[j]<>"." then ' ignore any decimal point
e = a[j] - 48
'ASCII representation - subtract 48 to get numeric value
bearing = 10* bearing + e
endif
next j
BearingDone:
' at this point we have BEARING * 10
serout bootout, 2, ["Bearing = ", #bearing/10, ".", #bearing//10]
lcdout $FE, $01, "Track ", #track/10, ".", #track//10
lcdout $FE, $C0, "Bearing ",#bearing/10, ".", #bearing//10
goto GetNextField

GetArrival:
if a[i+1] = "A" then
serout bootout, 2, ["Arrived"]
goto GetNextField
endif
if a[i+1] = "V" then
serout bootout, 2, [ "not there yet"]
goto GetNextField
endif
serout bootout, 2, [ "NO arrival info"]
goto GetNextField

NoGPS:
lcdout $FE, $01, "No GPS data " , $FE, $C0, "Check GPS "
pause 1000
goto startup

anybody can provide full description for this project ???

Here is how I decode the $gprmc sentence...

Partial code below

Code:

---

GPRMC VAR BYTE[64]

hh      VAR BYTE  'hours
mm      VAR BYTE  'minutes
ss      VAR BYTE  'seconds
sss    var word  'milliseconds
fix    VAR WORD  'GPS fix
latdeg  VAR BYTE  'degrees latitude
latmin  VAR BYTE  'minutes latitude 
latminn var word  'fractional minutes latitude
NS      VAR BYTE  'north or south
londeg  VAR BYTE  'degrees longitude
lonmin  VAR BYTE  'minutes longitude
lonminn var word  'fractional minutes longitude
EW      VAR BYTE  'east or west
Knots  VAR byte  'speed in knots (units)
Knotss  var byte  'speed in fractional knots
course  var word  'heading
dy      VAR BYTE  'day
mt      VAR BYTE  'month
yr      VAR BYTE  'year


'============= Main Program ==========================================
Main:
high gpwr
pause 3000

GPSin:
    serin2 gps,188,3000,tmout,[WAIT("$GPRMC,"),STR GPRMC\63\"$"] 'Data IN from GPS
   
    arrayread gprmc,[DEC2 hh,DEC2 mm,dec2 ss,_
      wait(","),fix,wait(","),DEC2 latdeg,DEC2 latmin,wait("."),dec4 latminn,_
      wait(","),NS,wait(","),DEC3 londeg,DEC2 lonmin,wait("."),dec4 lonminn,_
      wait(","),EW,wait(","),dec knots,dec Knotss,dec course,_
      wait(","),DEC2 dy,DEC2 mt,DEC2 yr]      'parse the GPS array into parts

if fix = "V" then gpsin  'jump to "waiting for fix" display

low gpwr
high xpwr
    pause 1000
   
    SEROUT2 xbtx,188,[$0D,$0A,"$GPRMC,",STR GPRMC\63]            'Data OUT to XBEE
    pause 500

---

Hope this helps

Thanx so much .. I really appreciate it
Could you please provide me with GPS hardware connections with the pic ?

What are you using for the GPS? A handheld unit or a module?

Quote:

---

Originally Posted by mackrackit

What are you using for the GPS? A handheld unit or a module?

---

I am using a module.

The module data sheet should give the pin out. RX and TX.

Quote:

---

Originally Posted by mackrackit

The module data sheet should give the pin out. RX and TX.

---

Thanks, but to which pins of the pic16f877 should these pins be connected ??

Well.... Back to the data sheet if you are planning to use the USART. Pins RC6 and 7.

If you are going to use SERIN2 then just about any of the pins will work.

Anyway does anyone have experience with GPS data string decoding for pic18LF4620?

Quote:

---

Originally Posted by piceighteen

Anyway does anyone have experience with GPS data string decoding for pic18LF4620?

---

It would be the same as with any other MCU.

can you post schematic of GPS receiver

Quote:

---

Originally Posted by nguyenphuocho

can you post schematic of GPS receiver

---

Do you mean how the receiver is connected to a MCU?