Re: Serial port communication - MSB is always set

On Aug 11, 6:36 pm, "Anthony Fremont" <any...@xxxxxxxxxx> wrote:
Affan wrote:
Hi all,
I have spent some time on writing a simple code that works with the
serial port on my embedded system (Mica2 from crossbow) that has an
Atmel 128L. After debugging the code upmteeth time and with help from
http://www.easysw.com/~mike/serial/serial.html, i seem to have it
working with the correct baud rate being set.
However, each byte that I receive in my embedded processor seems to
have its MSB set: e.g. if I send 4 as the first byte, the received
byte is 128+4=132, if I send 9 it is 128+9=137. Is there anything I am
missing here? I get no write errors on my PC side, and I have tried it
on different computers, nothing seems to work. Has any body had this
issue before? Thank you.

Here is the code that I use to set it to 8N1 (I dont want to use any
parity).

int setSerial( char *p_szPlatform,
struct termios *p_pNewtio )
{
if (tcgetattr(input_stream, p_pNewtio) == -1)
{

printf("Error getting tty attributes %s(%d).\n",
strerror(errno), errno);
return -1;
}
//to setup 8N1
p_pNewtio->c_cflag &= ~PARENB;//make sure parity bit is not set
p_pNewtio->c_cflag &= ~CSTOPB;
p_pNewtio->c_cflag &= ~CSIZE;//mask character size bits
p_pNewtio->c_cflag |= CS8 | CLOCAL | CREAD;

cfsetispeed(p_pNewtio, BAUDRATE_MICA2);
cfsetospeed(p_pNewtio, BAUDRATE_MICA2);

p_pNewtio->c_iflag |= IGNPAR;
//chooising raw input
p_pNewtio->c_iflag &=~(ICANON|ECHO|ECHOE|ISIG);

/* Raw output_file */
p_pNewtio->c_oflag &=~OPOST;

return iRet;
}

///The code where I write to the serial port is as follows
tcflush(input_stream, TCIFLUSH);

if (tcsetattr(input_stream, TCSANOW, &newtio) == -1)
{
printf("Error setting tty attributes %s(%d).
\n",strerror(errno), errno);
}
printf("Input baud rate changed to %d\n", (int)
cfgetispeed(&newtio));
printf("Output baud rate changed to %d\n", (int)
cfgetospeed(&newtio));
//send a packet via the serial port to the mote to test if the serial
is correctly set or not
typedef struct _app_header
{
uint8_t len; //the phyHeader

uint8_t type;//just for test purposes
int16_t crc; //the crc reqd by the physical layer
} __attribute__ ((packed)) AppHeader;

AppHeader pkt;
uint8_t* buf= (uint8_t*)&pkt;

pkt.len = sizeof(pkt);
pkt.type= 9;//debugging
pkt.crc=0;

pkt.crc=update_crc(pkt.len, pkt.crc);
pkt.crc=update_crc(pkt.type, pkt.crc);

for(i=0; i<pkt.len; i++){
if(write(input_stream, &buf[i], 1) != 1)
perror("write: ");
printf("Byte Value: %d\n",(uint8_t)buf[i]);
}

Sounds like your a tiny bit off on your clocking. The MSB is sent just
before the stop bit. The stop bit is always one, so it sounds like your
micro is seeing the stop bit instead of the MSB.

Thank you.. I think this might be it.. I know the code that I am using
changes timing to an external clock with a slightly different clock
rate. Let me get to my test bed and I will try to look into the code
to make sure that the clocking is corrected.. But thank you for
guiding me in the right direction.. I think :)

Affan.

.

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