configuration of adafruit ultimate gps from raspberry pi does not work - gps

Question

I connected my ultimate gps breakout to my raspberry pi using the USB to TTL serial cable. Using C code I can easily connect to and read NMEA sentences from the GPS. But when I write configuration commands such as PMTK220 to set the update rate, they are ignored. I should get back a PMTK_ACK reporting success or failure, but it is not forthcoming. The problem also occurs when I use terminal windows. ie. I run:
while (true) do cat -A /dev/ttyUSB0 ; done
in one terminal, and get a stream of $GPGGA, $GPGSA, $GPRMC etc messages. In another terminal I run:
echo "$PMTK220,200*2C\r\n" > /dev/ttyUSB0
The NMEA messages continue but there is no PMTK001 coming back. Any ideas?

Answer 1

Ok, here's some code (inspired by gpsd source) that demonstrates sending a configuration message and getting an acknowledgement:
#define _BSD_SOURCE
#include <stdio.h>
#include <stdarg.h>
#include <sys/ioctl.h>
#include <termios.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <semaphore.h>
#include <string.h>
#ifndef CRTSCTS
# ifdef CNEW_RTSCTS
# define CRTSCTS CNEW_RTSCTS
# else
# define CRTSCTS 0
# endif /* CNEW_RTSCTS */
#endif /* !CRTSCTS */
int main (int argc, char **argv) {
int const baudr = B9600, mode = O_RDWR | O_NONBLOCK | O_NOCTTY;
int const fd = open("/dev/ttyUSB0", mode);
assert (fd != -1);
ioctl(fd, (unsigned long)TIOCEXCL);
struct termios ttyOld, ttyNew;
assert(tcgetattr(fd, &ttyOld) != -1);
ttyNew = ttyOld;
ttyNew.c_cflag &= ~(CSIZE | PARENB | PARODD | CRTSCTS | CSTOPB);
ttyNew.c_cflag |= CREAD | CLOCAL | CS8;
ttyNew.c_iflag = ttyNew.c_oflag = 0;
ttyNew.c_lflag = ICANON;
cfsetispeed(&ttyNew, baudr);
cfsetospeed(&ttyNew, baudr);
assert(tcsetattr(fd, TCSANOW, &ttyNew) != -1);
tcflush(fd, TCIOFLUSH);
usleep(200000);
tcflush(fd, TCIOFLUSH);
int const oldfl = fcntl(fd, F_GETFL);
assert (oldfl != -1);
fcntl(fd, F_SETFL, oldfl & ~O_NONBLOCK);
tcdrain(fd);
printf("port opened baudr=%d mode=%d i=%d o=%d c=%d l=%d fl=%d\n", baudr, mode, ttyNew.c_iflag, ttyNew.c_oflag, ttyNew.c_cflag, ttyNew.c_lflag, oldfl);
unsigned char buf[2048];
int count = 0;
while(++count < 20) {
if (count == 4) {
char const *const cmd = "$PMTK220,200*2C\r\n";
int const n = strlen(cmd);
assert(write(fd, cmd, n) == n);
tcdrain(fd);
printf("wrote command %d: %s\n", n, cmd);
}
int const n = read(fd, buf, sizeof(buf));
buf[(n >= sizeof(buf)) ? (sizeof(buf) - 1) : n] = 0;
printf(buf);
}
tcsetattr(fd, TCSANOW, &ttyOld);
close(fd);
}
with results:
pi#pi01 ~/c $ ./test
port opened baudr=13 mode=2306 i=0 o=0 c=3261 l=2 fl=2050
$GPGGA,175748.089,,,,,0,00,,,M,,M,,*71
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPRMC,175748.089,V,,,,,0.00,0.00,260715,,,N*43
wrote command 17: $PMTK220,200*2C
$GPVTG,0.00,T,,M,0.00,N,0.00,K,N*32
$PMTK001,220,2*31
$GPGGA,175749.089,,,,,0,00,,,M,,M,,*70
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPGSV,1,1,01,11,,,31*7A
$GPRMC,175749.089,V,,,,,0.00,0.00,260715,,,N*42
$GPVTG,0.00,T,,M,0.00,N,0.00,K,N*32
$GPGGA,175750.089,,,,,0,00,,,M,,M,,*78
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPRMC,175750.089,V,,,,,0.00,0.00,260715,,,N*4A
$GPVTG,0.00,T,,M,0.00,N,0.00,K,N*32
$GPGGA,175751.089,,,,,0,00,,,M,,M,,*79
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPRMC,175751.089,V,,,,,0.00,0.00,260715,,,N*4B
$GPVTG,0.00,T,,M,0.00,N,0.00,K,N*32
$GPGGA,175752.089,,,,,0,00,,,M,,M,,*7A
pi#pi01 ~/c $