Linux UART transmitting Problems - c

Question

I'm trying to communicate with an external IC via UART/tty in C. I'm using a nanoPI neo2 (allwinner h5) and friendlyARM's own Linux OS (which is a modified Ubuntu core).
I'v tried to get it working, but the TX output of the pi is ?logical inverted?. You can see what I mean in the oscilloscope pictures. There you can see if I select the "Polarity Invert"- Mode the data is ok, but in normal mode it isn't working.
In the past, I got around that by setting the invert Bit of the MCU (e.g. PIC MCU), but the IC I'm using now doesn't support this feature.
http://wiki.friendlyarm.com/wiki/index.php/NanoPi_NEO2
oscilloscope picture 1
oscilloscope picture 2
c source
#include <stdio.h>
//#include <errno.h>
//#include <termios.h>
//#include <unistd.h>
#include <string.h>
#include <fcntl.h> /* File Control Definitions */
#include <termios.h>/* POSIX Terminal Control Definitions*/
#include <unistd.h> /* UNIX Standard Definitions */
#include <errno.h> /* ERROR Number Definitions */
#include <unistd.h>
#include <sys/types.h>
#include <sys/signal.h>
/* source : http://stackoverflow.com/a/6947758 */
int fd;
int set_interface_attribs (int fd, int speed, int parity) {
struct termios tty;
memset (&tty, 0, sizeof tty);
if (tcgetattr (fd, &tty) != 0) {
perror ("error from tcgetattr");
return -1;
}
cfsetospeed (&tty, speed);
cfsetispeed (&tty, speed);
tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8; // 8-bit chars
// disable IGNBRK for mismatched speed tests; otherwise receive break
// as \000 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_cc[VMIN] = 0; // read doesn't block
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
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) {
perror ("error from tcsetattr");
return -1;
}
return 0;
}
void set_blocking (int fd, int should_block) {
struct termios tty;
memset (&tty, 0, sizeof tty);
if (tcgetattr (fd, &tty) != 0)
{
perror ("error from tggetattr");
return;
}
tty.c_cc[VMIN] = should_block ? 1 : 0;
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
if (tcsetattr (fd, TCSANOW, &tty) != 0)
perror ("error setting term attributes");
}
int main()
{
char *portname = "/dev/ttyS0";
int fd = open (portname, O_RDWR | O_NOCTTY | O_SYNC);
if (fd < 0)
{
//perror ("error %d opening %s: %s", errno, portname, strerror (errno));
return;
}
set_interface_attribs (fd, B115200, 0); // set speed to 115,200 bps, 8n1 (no parity)
set_blocking (fd, 0); // set no blocking
write (fd, "hello!\n", 7); // send 7 character greeting
usleep ((7 + 25) * 100); // sleep enough to transmit the 7 plus
// receive 25: approx 100 uS per char transmit
char buf [100];
int n = read (fd, buf, sizeof buf); // read up to 100 characters if ready to read
}
Thanks in advance
Axel

Answer 1

The picture on the scope looks fine - it is an UART signal. The problem here is not programming-related, but failure to understand the difference between UART and RS-232.
You have picked RS-232 encoding on your scope, and RS-232 uses a different, inverted polarity compared to UART.
UART idles high voltage=Vdd, RS-232 idles at negative voltage < -3V.
On UART, a high voltage=Vdd bit means 1 and low voltage=0V means 0.
On RS-232, a high voltage > +3V bit means 0, and negative voltage < -3V means 1.
So when you try to decode a UART signal with RS-232 encoding, it works when you inverse the polarity. Apparently Rigol isn't picky about the actual voltage levels, since it doesn't understand that the signal is not RS-232 at all.