Using other UARTs in Linux

[root@Linux /dev]# stty -F /dev/ttyS0 
speed 115200 baud;
-brkint -imaxbel

Note you can use stty to setup the baud rate to be just about any standard UART speed up to 460.8kbps. However, you cannot use stty to set the UART to 921.6kbps

Setting Baud Rate to 921.6kbps

Because 921,600 bps is not a common standard baud rate programs like stty do not typically support it. A quick work around to this problem is to use a simple C program to set the baud rate manually using the termOptions struct in Linux. The underlying driver knows how to set the baud rate correctly (assuming you're using a patched version of the uart drivers) so you just need to be able to request it.

Here is a simple C program that can do this for us:

 
#include <unistd.h>
#include <stdio.h>
#include <termios.h>
#include <sys/fcntl.h>
 
 
int main( int argc, char *argv[] )
{
	struct termios termOptions;
	char        port[1024];
	int         ttyFid;
 
	if( argc == 2 )
	{
		// Copy the port from the argument
		strcpy( port, argv[1] );
		printf( "Read port name: %s\n", port );
	}
	else
	{
		// No argument or incorrect number of arguments read:
		printf( "Usage: setSer /dev/ttySX\n" );
		return -1;
	}
 
	// Open the tty:
	ttyFid = open( port, O_RDWR );
	if (ttyFid == -1)
	{
		printf( "Error unable to open port: %s\n", port );
		return -1;
	}
 
	// Get the current options:
	tcgetattr( ttyFid, &termOptions );
	
	// Set the input/output speed to 921.6kbps
	cfsetispeed( &termOptions, B921600 );
	cfsetospeed( &termOptions, B921600 );
 
	// Now set the term options (set immediately)
	tcsetattr( ttyFid, TCSANOW, &termOptions );
 
	// All done
	printf( "Done, port speed set on: %s\n", port );
}

To compile the program you can use arm-linux-gcc (I had a few problems with arm-linux-g++ with this code):

arm-linux-gcc main.c -o setSer
arm-linux-strip setSer

Once you've compiled the program you will need to either add it to your file system (and load the file system onto your platform) or download it somehow. To use simply call the program with the tty you wish to set i.e. setSerial /dev/ttyS2.

Note, that this code works on x86 desktop platforms just as easily. Also right now there's not too much error checking for the actual setting of the termOptions, so code for reloading the actual termOptions (after pausing to give the driver a chance to set them) and comparing them to your set options might be useful for some people. Also this code doesn't actually close the port it's opened. This doesn't seem to cause any problems, but you might want to add close( ttyFid ); to the last line of the code (unless that doesn't work of course).