environmental-sensor-uart/mkr680-read.c

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

int set_interface_attribs(int fd, int speed, int parity) {
    struct termios tty;
    if(tcgetattr(fd, &tty) != 0) {
        fprintf(stderr, "error %d from tcgetattr", errno);
        return -1;
    }

    cfsetospeed(&tty, speed);
    cfsetispeed(&tty, speed);

    // disable IGNBRK for mismatched speed tests; otherwise receive breakas \000 chars
    tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;    // 8-bit chars
    tty.c_iflag &= ~IGNBRK;                        // disable break processing
    tty.c_lflag = 0;                               // no signaling chars, no echo, no canonical processing
    tty.c_oflag = 0;                               // no remapping, no delays
    tty.c_iflag &= ~(IXON | IXOFF | IXANY);        // shut off xon/xoff ctrl
    tty.c_cflag |= (CLOCAL | CREAD);	           // ignore modem controls, enable reading
    tty.c_cflag &= ~(PARENB | PARODD);             // shut off parity
    tty.c_cflag |= parity;
    tty.c_cflag &= ~CSTOPB;
    tty.c_cflag &= ~CRTSCTS;

    if(tcsetattr(fd, TCSANOW, &tty) != 0) {
        fprintf(stderr, "error %d from tcsetattr", errno);
        return -1;
    }

    return 0;
}

int main(void) {
    char *portname = "/dev/ttyACM0";
    int fd = open(portname, O_RDWR | O_NOCTTY | O_NONBLOCK);
    if(fd < 0) {
        fprintf(stderr, "error %d opening %s: %s", errno, portname, strerror(errno));
        return 1;
    }

    char buf[256];
    set_interface_attribs(fd, B9600, 0);        // set speed to 9600 bps, 8n1 (no parity)
    usleep((68 /* rx */) * 1043);               // wait for bootup message
    int n = read(fd, buf, sizeof buf);          // read out the startup message

    write(fd, "R\n", 2);                        // send 2 character `read` command
    usleep((2 /* tx */ + 256 /* rx */) * 1043); // wait for tx and ~256 character rx from iot-mkr680
    int rxd = 0;                                // read up to 256 characters if they are available
    while(0 < (n = read(fd, buf + rxd, sizeof buf - rxd))) rxd += n;

    if(0 < rxd) fwrite(buf, rxd, 1, stdout);    // send entire buffer to standard output

    close(fd);
    return 0;
}


/*  IoTeam mkr680 startup firmware
 *
 *  The device has a UART tranciever configured 8n1 at 9600 baud.  Individualt characters are
 *  therefore 10-bit and about 960 bytes are sent per second, or it takes about 1043 uS per char
 *
 *  on bootup it prints (text between hypens):
 *  ---------------------------------------
 *      ITM-MKR680 Configuration utility.
 *      Enter '?' for help.
 *      
 *      I2C 0x10> _
 *  ---------------------------------------
 *
 *  If we type in 'R' and then a newline we get:
 *  ---------------------------------------
 *  Timestamp         : 288.01 sec
 *  Temperature (RAW) : 24.41~C (24.46~C)
 *  Pressure          : 97845.23 hPa
 *  Humidity (RAW)    : 36.62% (36.55%)
 *  Gas Resistance    : 96815.84 Ohm
 *  IAQ (accuracy)    : 25.00 (0)
 *  ---------------------------------------
 *
 *  we need to recieve about 224 characters, reading 256 should be more than enough
 *
 *  serial port code mostly copied from [wallyk](https://stackoverflow.com/6947413/how-to-open-read-and-write-from-serial-port-in-c)
 */

/*  bug
 *  2021 December 2nd
 *  when developing on lop aarch (daily driver i7) I get the bootup message once, reading immediately in main() sends 'R' and gets response data
 *  but on solar-aquarium debian (sluggish rpi1b) I get only the bootup message every time the main() runs
 *
 *  a suitable fix for now is just to wait for the bootup message, read it, then send the `read` command
 *
 *  2021 December 3rd
 *  okay, this is getting out of hand.  I installed `gnu screen` and checked the version of the firmware, afterwards running with the changes
 *  I all of the sudden started only reading to the degrees symbol of the temperature.  Either the rpi1b is slower than normal or running
 *  `gnu screen` changes tty settings on the device file.  I should reboot, plug in the itm-mkr680 and check before and after, but for now
 *  I am just going to run this program with a sleep called slow_rpib1_sleep
 *
 *  this sleep failed, counting the number of bytes read I get 64 bytes.  This is suspicious.
 *
 *  finally I think solution is that read is unbuffered, for whatever reason only 64 bytes were read, the ammount of time that passes due
 *  to the original `usleep` is more than enough time for whatever buffers to fill, maybe I should be emptying them, maybe I should switch
 *  to fopen/fread but for now, just call `read` multiple times and keep track of how much the buffer is filling up
 */