User`s manual

90 rabbit.com Supported Rabbit 4000 Baud Rates
The default equation for the divisor is:
If the divisor is not an integer value, that baud rate is not available for that frequency (identified by a “-” in
the table). If the divisor is above 255, that baud rate is not available without further BIOS modification
(identified by a “*” in the table). To allow that baud rate, you need to clock the desired serial port via timer
A1 (by default they run off the peripheral clock / 2), then scale down timer A to make the serial port divi-
sor fall below 256.
Timer A can be clocked by the peripheral clock (PCLK) in addition to the default, which is the peripheral
clock/2 (PCLK/2). Furthermore, the asynchronous serial port data rate can be 8x the clock in addition to
the default of 16x the clock. Therefore, in addition to the equation above, the following equations may be
used to find the asynchronous divisor for a given clock frequency.
Timer A clocked by PCLK/2, serial data rate = 16 x clock
Timer A clocked by PCLK, serial data rate = 16 x clock:
Timer A clocked by PCLK/2, serial data rate = 8 x clock:
Timer A clocked by PCLK, serial data rate = 8 x clock:
divisor
CPU frequency in Hz
32 baud rate
----------------------------------------------------- 1–=
divisor
CPU frequency in Hz
16 2 baud rate
--------------------------------------------------- 1–=
divisor
CPU frequency in Hz
16 baud rate
--------------------------------------------------- 1–=
divisor
CPU frequency in Hz
82 baud rate
--------------------------------------------------- 1–=
divisor
CPU frequency in Hz
8 baud rate
--------------------------------------------------- 1–=