GPS module + PIC

4 Attachment(s)

GPS module + PIC

Hello,

I'm trying to communicate with a GPS module http://www.futurlec.com/GPS.shtml.

Before I connect it to a PIC+LCD, I try to communicate with the GPS module via a PC terminal.

Unfortunately, I can't get any "understandable" message from the module; I might receive SIRF information instead of NMEA messages, I don't know.

This is what I see on my terminal:
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4175&stc=1&d=127083345 4">

If my guess is right, I would need to switch from SIRF to NMEA protocol. There are to examples hereunder in the doc (full doc attached).
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4174&stc=1&d=127083336 4">

I don't understand how to calculate the checksum.

I found a cool checksum calculator, HashCalc from SlavaSoft Inc. But I can't find a similar checksum corresponding to both examples.

Can someone tell me in "simple words" how the checksum in both samples above have been calculated and the exact string I would have to send the data to the module?

Is it "$PSRF100,1,4800,8,1,0*0D0A" or "$PSRF100,1,4800,8,1,0*0D,0A" or "$PSRF100,1,4800,8,1,0*<0D><0A>" or anything else?

I've spent quite a few hours on this one and no success so far so any help or info is very welcome ;)

You need an inverter chip or circuit . (MAX232)

1 Attachment(s)

Thanks Dave,

The MAX232 in my drawer did the trick ;)

Any idea about the checksum calculation?

This is what I did up to now in EXCEL but trying to convert HEX to BIN will rise an error.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4178&stc=1&d=127089284 5">

10001000000
is the number excel failed on but other than that I do not know. I will have to look into it.

Check Sum

Here is one check sum solution for gps:

Code:

gpsdata var byte[82] ' Max length of a GPS sentence is 82 bytes. DoCS: i = 0 ' Initialize the position counter WHILE gpsdata[i] != "*" ' Count everthing until the * cs = cs ^gpsdata[i] ' XOR for checksum i = i + 1 ' Increment position counter WEND ' A quick and easy atoh function ' Make it a seperate label if we add more atoh conversions in the future LOOKDOWN gpsdata[i+1],["0123456789ABCDEF"],tempbyte gpscs = tempbyte * 16 LOOKDOWN gpsdata[i+2],["0123456789ABCDEF"],tempbyte gpscs = gpscs + tempbyte ' Flag bad data "V" if checksum doesn't add up. ' Or if the sentence is less then 20 chars long ' Which means there is no active waypoint to go to. ' Should be "A" otherwise. IF (cs != gpscs) OR (i < 20 )THEN gpsvalid = 0 ELSE gpsvalid = 1 ENDIF RETURN

2 Attachment(s)

Can't "talk" towards my GPS module...

- Reading & parsing NMEA sentences is ok;
- Calculating checksum is ok;
- Sending data to GPS module is not ok.

I have connected the GPS module via the MAX232 chip to my serial PC port both ways so I should be able to "talk" to my module.

Unfortunately, only reading out (receiving) the GPS module is working.

I checked the MAX chip with my oscillo and the signals are okay (TX & RX) on both levels (RS232 and TTL).

I'm sending an example from the module's datasheet to enable a VTG sentence but it won't work.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4205&stc=1&d=127141338 1">

I just never see this VTG data appear.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4207&stc=1&d=127141381 2">

Any idea?

It works now!

Strange....

I just restarted the terminal and... it works now - I can sent messages to the module.

Must be a kind of magic (?)

Is it Hyperterminal?

Ioannis

No, it's MicroCode Studio's terminal.

But I checked with HT too and did the same (?).

Nevermind, it works now :)

Timeout routine not working

Hello there,

I just finished this piece of code and it works fine. It will simply display the GPS course & speed.

I finally connected the GPS module's TX directly to a TTL port of my PIC (no level-shifter such as MAX232 used) with the correspondant mode settings.

Everything is working fine appart from the NOGPS routine.

When I disconnect the GPS's TX from the PIC's port, the NOGPS routine will never appear.

Any idea what's wrong?

Code:

<html><head></head><body><pre><code>' PIC 16F690 Fuses @ DEVICE FCMEN_OFF @ DEVICE IESO_OFF @ DEVICE BOD_OFF @ DEVICE CPD_OFF @ DEVICE PROTECT_OFF @ DEVICE MCLR_OFF @ DEVICE PWRT_OFF @ DEVICE WDT_OFF @ DEVICE XT_OSC ' Registers 76543210 OPTION_REG = %10000000 'PORT A&B Pull-Ups disabled (look WPUA & WPUB) ANSEL = %00000000 'Disable analog inputs Channels 0 to 7 ANSELH = %00000000 'Disable analog inputs Channels 8 to 11 ADCON0 = %00000000 'A/D Module is OFF CM1CON0 = %00000000 'Comparator1 Module is OFF CM2CON0 = %00000000 'Comparator2 Module is OFF INTCON = %00000000 'INTerrupts CONtrol TRISA = %00000001 'Set Input/Output (0 to 5) PORTA = %00000000 'Ports High/Low (0 to 5) TRISB = %00000000 'Set Input/Output (4 to 7) PORTB = %00000000 'Ports High/Low (4 to 7) TRISC = %00000000 'Set Input/Output (0 to 7) PORTC = %00000000 'Ports High/Low (0 to 7) '------------------------------------------------------------------------------- ' LCD cabling '- ( 1) Vss - GND '- ( 2) Vdd / Vcc +5V '- ( 3) Vee / Vo / Contrast adjust / Pot 4k7(1-2-W3) or R2k2(GND-3) 'PORTC.1 ( 4) RS '- ( 5) R/W -> to Vss 'PORTC.2 ( 6) E 'PORTB.4 (11) D4 'PORTB.5 (12) D5 'PORTB.6 (13) D6 'PORTB.7 (14) D7 DEFINE LCD_DREG PORTB 'LCD data port DEFINE LCD_DBIT 4 'LCD data starting PORT.bit (0 or 4) DEFINE LCD_RSREG PORTC 'LCD register select port DEFINE LCD_RSBIT 1 'LCD register select bit DEFINE LCD_EREG PORTC 'LCD enable port DEFINE LCD_EBIT 2 'LCD enable bit DEFINE LCD_BITS 4 'LCD bus size 4 or 8 DEFINE LCD_LINES 2 'Number lines on LCD '------------------------------------------------------------------------------- INIT: Ctr_A VAR BYTE 'Counter GPS_D VAR BYTE(80)'Incomming data bit array Spd_U VAR WORD 'Speed Units GPSfrom VAR PORTA.0 'use a TTL I/O port on PIC! GPSbps VAR WORD GPSbps = 32956 'Open,True,N,4800bps (TTL level) Timeout CON 3000 CommaCnt VAR BYTE 'comma count PAUSE 1000 LCDOUT $FE, 1 ' Custom character definition LCDOUT $FE,$40,$0C,$12,$12,$0C,$00,$00,$00,$00 'character 0 = "°" LCDOUT $FE,$48,$14,$18,$14,$00,$1E,$15,$15,$00 'character 1 = "km" LCDOUT $FE,2,"GPS v1.1" PAUSE 3000 '------------------------------------------------------------------------------- GPVTG: ' Get VTG sentence and store it into GPS_D var until "*" is found SERIN2 GPSfrom,GPSbps,Timeout,NOGPS,[WAIT("$GPVTG,"),STR GPS_D\"*"] ' extract COURSE data Ctr_A = 0 cou_u = 0 WHILE GPS_D(Ctr_A) != "." Cou_u = 10 * cou_u + (GPS_D(Ctr_A)-48) Ctr_A = Ctr_A + 1 WEND ' locate SPEED data after 6th comma Ctr_A = 0 CommaCnt = 0 WHILE CommaCnt < 6 IF GPS_D(Ctr_A) = "," THEN CommaCnt = CommaCnt + 1 Ctr_A = Ctr_A + 1 WEND ' extract SPEED data - leave tenths Spd_U = 0 WHILE GPS_D(Ctr_A) != "." Spd_U = 10 * Spd_U + (GPS_D(Ctr_A)-48) Ctr_A = Ctr_A + 1 WEND LCDOUT $FE,1 LCDOUT $FE,2,DEC cou_u,0," ",DEC Spd_U,1 GOTO GPVTG: '------------------------------------------------------------------------------- NOGPS: LCDOUT $FE,2," No GPS " PAUSE 1000 GOTO GPVTG: END </code></pre></body> </html>

At what level is the Serial input of the PIC when you disconnect the GPS module? I guess the timeout is blocked by the incorrect level.

You should have a pull up resistor there.

Ioannis

It's working... yes and no

I have tried both a 10k pull-down and pull-up resistor.

I was thinking a pull-down would be correct since the PIC gets a "true" signal, but as it didn't work, I tried the pull-up way; no success :(

Meanwhile, I have seen that the GPS module will always send the string my SERIN2 command is waiting for ("$GPVTG").

If the module has no signal (no satellite coverage), it will still send the NMEA sentence with empty data, just like this "$GPVTG,,T,0,M,,N,,K*xx".

So, if I disconnect the GPS from the PIC, it works but... it is not the way I would like to have it working.

Product specs and supplier

Just for the record, I found the supplier's website for the GPS module I use; it is http://www.globalsat.com.tw/ and the specific module is here: http://www.globalsat.com.tw/products...t_cnt_folder=2.