UART compare charts. Beaglebone - c

Question

I have a problem trying to compare the uart input data (from a GPS) with '$' in order to detect a new package. I am sure that the problem is in how I manipulate the charRead variable. I tried one thousand of things, but probably because of my inexperience I have not figured out what it is the problem.
The code compiles and the data is coming all the time, but once I load the code into the beaglebone, it gets stacked but and it doesn't enter in the "if (charRead =='$')".
Thank you in advance!
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <time.h>
#include <iostream>
#include <termios.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
#include <limits.h>
#include "Payload.h"
#define SLOTS "/sys/devices/bone_capemgr.9/slots"
#define CR 0x0d
#define SPACE 0x20
#define COMMA 0x2C
#define MAXSIZE 100
unsigned long time_data;
unsigned int button = 45;
int i,z =0, j=0, value;
int rx_length;
int main()
{
//uart4 configuration using termios
int fd;
//unsigned char *mess;
unsigned int value = 0;
gpio_export(button);
//Wait until the button is pushed
while (value != 1){
if (z==0){
printf("waiting\n");}
z++;
gpio_get_value(button, &value);}
//OPEN THE UART
//open uart4 for tx/rx, not controlling device
if((fd = open("/dev/ttyO4", O_RDONLY | O_NOCTTY|O_NONBLOCK)) < 0){
printf("Unable to open uart4 access.\n");
}
termios uart4;
cfsetospeed(&uart4, B9600); //Set the speed
//set attributes of uart4
uart4.c_iflag = 0;
uart4.c_oflag = 0;
uart4.c_lflag = 0;
tcsetattr(fd, TCSANOW, &uart4);
//----- CHECK FOR ANY RX BYTES -----
// Read up to 100 characters from the port if they are there
unsigned char stringRead[MAXSIZE];
unsigned char charRead;
do{
if (rx_length = read(fd, (void*)charRead, MAXSIZE)>0){
if (charRead =='$'){
i=0;
stringRead[i] = charRead; //Store in the first position of the char --> $
do {
rx_length = read(fd, (void*)charRead, MAXSIZE); //Read the next bit
if( (charRead != '\0') ) {
i++;
stringRead[i] = charRead; //write into stringRead
}
} while(charRead != 'CR'); //ASCII Carriage Return
stringRead[i+1] = charRead;
printf("%s", stringRead);
}}
if (rx_length==0){
//No data
}
gpio_get_value(button, &value);
}while (value!=0);
gpio_unexport(button);
close(fd);
return 0;
}

Answer 1

You're passing a cast of the variable value of charRead rather than a pointer to a memory location as the function read() expects void *.
read(fd, (void*)charRead, MAXSIZE)
You need to either read one character at a time:
read(fd, &charRead, 1)
Or change your reading logic to maximize amount read and data processing. I also recommend adding a bounds check on accessing stringRead.

Answer 2

// The following should handle the reading of a GPS NMEA message and display it
// I have not run the program, but compiling it was successful
// note:
// 1) the handling of the 'i' variable
// 2) the calls to reading the GPS input
// 3) the handling of error conditions
// 4) the simple logic flow
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <time.h>
//#include <iostream> // this is not C++ so this line not needed
#include <termios.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
#include <limits.h>
#include "Payload.h"
// #define SLOTS "/sys/devices/bone_capemgr.9/slots" // not used, raises compiler warning
#define CR (0x0D)
// #define SPACE (0x20) // not used, raises compiler warning
// #define COMMA (0x2C) // not used, raises compiler warning
#define MAXSIZE (100)
#define BUTTON_PORT (45)
// unsigned long time_data; // not used, raises compiler warning
// int j=0; // not used, raises compiler warning
// int value = 0; // not used, raises compiler warning about variable masking
int main()
{
int i;
int z = 0; // flag used to control execution flow
int rx_length; // return status value from read()
//uart4 configuration using termios
int fd; // file descriptor number
//unsigned char *mess; // not used, raises compiler warning
unsigned int value = 0;
gpio_export(BUTTON_PORT);
//Wait until the button is pushed
// burn mass CPU cycles, while waiting
while (0 == value)
{
if (z==0)
{
printf("waiting\n");
z++; // to stop re-entry to this 'if' block
}
// suggest using nsleep() to free up CPU
gpio_get_value(BUTTON_PORT, &value);
} // end while
//open uart4 for rx
if((fd = open("/dev/ttyO4", O_RDONLY | O_NOCTTY|O_NONBLOCK)) < 0)
{
perror("open failed for /dev/tty04");
exit(1);
}
// implied else, open successful
termios uart4;
cfsetospeed(&uart4, B9600); //Set the speed to match the GPS output
//set attributes of uart4
// Note: probably better to read the current termois values
// then modify them to the desired states
uart4.c_iflag = 0;
uart4.c_oflag = 0;
uart4.c_lflag = 0;
tcsetattr(fd, TCSANOW, &uart4);
//----- CHECK FOR ANY RX BYTES -----
// Read up to 100 characters from the port if they are there
unsigned char stringRead[MAXSIZE]; // will contain a GPS NMEA message
unsigned char charRead; // input buffer
do{
while(1)
{
rx_length = read(fd, &charRead, 1);
if( 0 == rx_length )
{ // this will execute a lot since fd set to non-blocking
; // do nothing, while hogging CPU cycles
// suggest using nsleep() to free CPU
}
else if( 0 > rx_length )
{
perror( "read failed" );
exit(1);
}
else if (charRead =='$')
{
stringRead[0] = charRead; //Store first char of NMEA GPS message
i=1; // index for second char of NMEA message from GPS
}
else
{
stringRead[i] = charRead; //Store char
i++; // index for next char into stringRead buffer
if( MAXSIZE <= i )
{ // then overrun input buffer
perror( "read- overrun input buffer, GPS message too long");
exit(2);
}
if( CR == charRead ) //ASCII Carriage Return - end of message
{ // then, got complete message
break; // exit read loop, so can process message
}
} // end if
} // end while
stringRead[i] = '\0'; // terminate string so it can be printed
printf("%s", stringRead);
// get button state via BUTTON_PORT(45)
gpio_get_value(BUTTON_PORT, &value);
} while (value!=0); // then read next gps message
gpio_unexport(BUTTON_PORT);
close(fd);
return 0;
}