Venus modules

GPS/GNSS Development Boards
26

You have to preset the baud rate manually or using something else (only tested at 57600 or faster). It enables agps at the end after uploading.

Lots of holes and places to lock if something goes wrong but shows the method in just over 100 lines.

You also need to download Eph.dat

e.g. make sure Eph.dat is not already there (it will download to Eph.dat.1 otherwise) and do

wget ftp://skytraq:skytraq@60.250.205.31/ephemeris/Eph.dat

Linux/Posix based, adjust as needed.

#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>

static char string[64];
static void readtonull(int fd)
{
    int l;
    char *b = string;
    for (;;) {
        l = read(fd, b, 1);
        if (l <= 0)
            continue;
        if (!*b++)
            break;
    }
    printf("%s\n", string);
}

static unsigned char ephbuf[256 * 1024];
static const unsigned char setagps[8] = { 0xa0, 0xa1, 0x00, 0x01, 0x35, 0x35, 0x0d, 0x0a };
static const unsigned char agpsresp[9] = { 0xa0, 0xa1, 0x00, 0x02, 0x83, 0x35, 0xb6, 0x0d, 0x0a };
static const unsigned char agpsena[9] = { 0xa0, 0xa1, 0x00, 0x02, 0x33, 0x01, 0x32, 0x0d, 0x0a };

int main(int argc, char *argv[])
{
    int fd;
    char *gpsdev = "/dev/ttyUSB0";
    unsigned i;

    fd = open(gpsdev, O_RDWR);
    if (fd < 0)
        return -10;

    struct termios tio;
    if ((tcgetattr(fd, &tio)) == -1)
        return -1;
    cfmakeraw(&tio);
    if ((tcsetattr(fd, TCSAFLUSH, &tio)) == -1)
        return -1;
    // add: find baud rate

    unsigned char *ephdata;
    long ephbytes;

    ephdata = ephbuf;
    int ofd = open("Eph.dat", O_RDONLY);
    if (ofd < 0)
        return -2;
    ephbytes = read(ofd, ephbuf, 256 * 1024);
    if (ephbytes < 65536)
        return -3;

    // checksum
    unsigned char csuma, csumb = 0;
    for (i = 0; i < 0x10000; i++)
        csumb += ephdata[i];
    csuma = csumb;
    for (; i < ephbytes; i++)
        csuma += ephdata[i];

    // AGPS download startup: send command, get ack - maybe put in loop?
    do { // flush input buffer
        i = read(fd, string, 64);
    } while( i == 64 );
    printf( "Startup\n" );
    write(fd, setagps, 8);
    i = 0;
    while ( i < 0 || string[0] != '\xa0')
        i = read(fd, string, 1);
    while (i < 64) {
        i += read(fd, &string[i], 64 - i);
        if (i > 0 && string[i-1] == 0x0a)
            break;
    }
    printf( "Venus Ready\n" );
    if (memcmp(&string[i - 9], agpsresp, 9))
        return -6;
    
    /* start the transmission */
    sprintf(string, "BINSIZE = %ld Checksum = %d Checksumb = %d ", ephbytes, csuma, csumb);
    printf("%s:", string);
    write(fd, string, strlen(string) + 1);
    readtonull(fd);

#define BLKSIZ 8192
    while (ephbytes > 0) {
        printf("%ld left:", ephbytes);
        write(fd, ephdata, ephbytes > BLKSIZ ? BLKSIZ : ephbytes);
        readtonull(fd);        // OK
        ephbytes -= BLKSIZ;
        ephdata += BLKSIZ;
    }
    // Status "END" or "Error2"
    readtonull(fd);            // END

    sleep(1);
    write(fd, agpsena, 9);
    // maybe get ack?
    sleep(1);
    close(fd);
    return 0;
}