On Tuesday, May 6, 2014 at 1:42:34 PM UTC-5, Andrey wrote:
>
> Hello,
>
> I need to use an UART to communicate between beaglebone and the device. I
> have written the C code, but it has a few problems within it. I can't
> identify the cause of the problem or to be more precise don't know how fix
> it.
>
> The problem is of
> reading in the information and displaying it on a shell. I can easily
> write any information data set that I wish, but when I try to read in
> response it either doesn't read in, with the perror being displayed as
> (erno = EINTR [Interrupted function call]) meaning read () function
> returned negative number.
>
>
> fd = open(PORT, O_RDWR | O_NOCTTY | O_NONBLOCK);
>
> if (fd<0) {perror(PORT); exit(-1);}
>
>
>
I think Simon's post will get you on the right track. You are sending a
command over the UART and expecting a response back, you pretty much have
to block or read in a loop to give the external device time to reply.
I open the port (O_RDWR | O_NOCTTY | O_NDELAY) and use
options.c_cc[VMIN] = 0; // wait for each character
options.c_cc[VTIME] = 10; // wait time in 1/10s of a second
Which will time out after 1 second without response (a true error for my
device, or a signal that the response string is complete)
Here is a code snippet of the function I use to check the signal strength
of a cell phone modem:
int check_signal(int fd){
char buffer[256];
char *bufptr;
int bytes, n=0, nbytes=0;
char tstr[64];
if (write(fd, "AT+CSQ\r", 7) <7){
printf("%s *** ERROR! writing AT+CSQ command! ***\n",getDateTime(tstr));
FONA_DEAD=1; // send text message that FONA backup messaging system not
working
return(-1);
}else{
while ((bytes = read(fd, &buffer[nbytes], sizeof(buffer)-nbytes-1)) >
0){ nbytes += bytes; }
buffer[nbytes] = '\0'; // nul terminate the string
if(nbytes>0){
if(bufptr=strstr(buffer,"+CSQ:")){
sscanf(bufptr,"+CSQ: %i", &n);
//if(n==0) printf("sscanf fail: %s\n", buffer); // actually was
intermittently 0 signal strength with some antenna positions!
//printf("Signal Strength: %i\n",n);
return(n);
}else{
// getting occasional failures, try to tell if its time outs or
unexpected characters that weren't flushed after previous function
// if I don't see this message from the errors increase time out a bit
printf("%s *** check_signal() Unexpected Response!
***\n%s\n",getDateTime(tstr),buffer);
return(-2);
}
}
}
}
It sends a multi-byte command string and then waits in a loop for a
multi-byte response than ends with a timeout after the last character is
received. If you always get a fixed length
reply you can make the code more responsive by exiting after the last
character comes in, instead of waiting for the timeout.
I generally find it easier to put blocking code in separate threads than to
try to do non-blocking read/write in a main loop. YMMV.
The suggestion below to use select() is also a good option.
--
For more options, visit http://beagleboard.org/discuss
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