[Ftdi-usb-sio-devel] Setting non-standard baud rates

[Derek S. <ds...@cs...>]

Hi,

I have a device with a FT232 USB-serial converter that requires a 
non-standard baud rate of 250kbaud.  I am able to open the device and 
configure it using standard baud rates, but have not been successful in 
configuring it for the required 250kbaud.

The project webpage (http://ftdi-usb-sio.sourceforge.net/) indicates there 
are ways to set non-standard baudrates, but none of them have worked for 
me.  The approaches listed and the problems I have encountered are as 
follows:

The first suggestion is to "set tty->termios->c_cflag speed to B38400 and 
then set your real speed in tty->alt_speed".  I have not been able find a 
way to access to the tty structure (tty_struct) to set the "alt_speed" 
member variable.  I found references to a TIOCTTYGSTRUCT ioctl to retrieve 
a pointer to the tty_struct, but the #define for this is now commented out 
in /usr/include/asm/ioctls.h and a comment indicates it is for debugging 
only.  I found a reference in the Linux kernel archives suggesting that 
this ioctl was to be removed.  Also, I have not been able to find the 
header file for "tty_struct" in /usr/include/linux (the kernel source has 
it in a file called "tty.h").  Is there another way to set "alt_speed"?

The second suggestion is to set a custom divisor in the termios structure. 
However, there is no integer divisor that gives me the required baud rate
(even with the various different baud_base settings of HI, VHI, SHI, or 
WARP).

The only other solutions seem cheesy: recompile the driver with the 
custom buad rate coded in, or perhaps change the crystal frequency in the 
attached device so that it scales to a standard baud rate...

Any suggestions would be appreciated.  If there is no elegant way to set 
non-standard baud rates, could one be added?

thanks,
Derek