Hardware manual

(a positive integer). This "divisor" divides the "maximum speed" of the hardware resulting in a slower speed
(except a divisor of 1 obviously tells the hardware to run at maximum speed).
There are exceptions to the above since for certain serial port hardware, speeds above 115.2k are set by using
a very high divisor. Keep that exception in mind as you read the rest of this section. Normally, if you specify a
speed of 115.2k (in your communication program or by stty) then the serial driver sets the port hardware to
divisor 1 which sets the highest speed.
Besides using a very high divisor to set high speed, the conventional way to do it is as follows: If you happen
to have hardware with a maximum speed of say 230.4k (and the 230.4k speed has been enabled in the
hardware), then specifying 115.2k will result in divisor 1. For some hardware this will actually give you
230.4k. This is double the speed that you set. In fact, for any speed you set, the actual speed will be double. If
you had hardware that could run at 460.8k then the actual speed would be quadruple what you set. All the
above assumes that you don't use "setserial" to modify things.
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.
12.2 Higher Serial Throughput
If you are seeing slow throughput and serial port overruns on a system with (E)IDE disk drives, you can get
hdparm. This is a utility that can modify (E)IDE parameters, including unmasking other IRQs during a disk
IRQ. This will improve responsiveness and will help eliminate overruns. Be sure to read the man page very
carefully, since some drive/controller combinations don't like this and may corrupt the filesystem.
Also have a look at a utility called irqtune that will change the IRQ priority of a device, for example the
serial port that your modem is on. This may improve the serial throughput on your system. The irqtune
Serial HOWTO
How speed is set in hardware: the divisor and baud_base 53