Setting the divisor, speed accounting
To correct this accounting (but not always fix the problem) you may use "setserial" to change the baud_base to the actual maximal speed of your port such as 230.4k. Then if you set the speed (by your application or by stty) to 230.4k, a divisor of 1 will be used and you'll get the same speed as you set.
If you have very old software which will not allow you to tell it such a high speed (but your hardware has it enabled) then you might want to look into using the "spd_cust" parameter. This allows you to tell the application that the speed is 38,400 but the actual speed for this case is determined by the value of "divisor" which has also been set in setserial. I think it best to try to avoid using this kludge.
There are some brands of UARTs that uses a very high divisor to set high speeds. There isn't any satisfactory way to use "setserial" (say set "divisor 32770") to get such a speed since then setserial would then think that the speed is very low and disable the FIFO in the UART.
Crystal frequency is higher than baud_base
Note that the baud_base setting is usually much lower than the frequency of the crystal oscillator since the crystal frequency of say 1.8432 MHz is divided by 16 in the hardware to get the actual top speed of 115.2k. The reason the crystal frequency needs to be higher is so that this high crystal speed can generate clock ticks to take a number of samples of each bit to determine if it's a 1 or a 0.
Actually, the 1.8432 MHz "crystal frequency" may be obtained from a 18.432 MHz crystal oscillator by dividing by 10 before being fed to the UART. Other schemes are also possible as long as the UART performs properly.
