Quatech RS-232 - Manual (Page 13)

4.4.2 Clock Rate Multiplier
A standard RS-232 serial port operates at a clock speed of 1.8432
MHz. In order to achieve higher data rates, Quatech Enhanced Serial
Adapters can operate at two times, four times or even eight times this
standard clock speed. This is controlled by the clock rate multiplier bits in
the Options Register.
Software can determine the UART clock frequency by reading the
clock rate multiplier bits RR1 and RR0 in the Options Register as shown in
Figure 7. RR1 and RR0 can be set by writing to the Options Register.
Reading the Options Register will always return the clock rate multiplier
at which the board is operating.
921.6 kbaud 14.7456 MHz X8 1 1
460.8 kbaud 7.3728 MHz X4 0 1
230.4 kbaud 3.6864 MHz X2 1 0
115.2 kbaud 1.8432 MHz
X1
(default)
0 0
Maximum Data
Rate
UART Clock
Frequency
Clock Rate
Multiplier
RR0 RR1
Figure 7 --- Rate Register bit definition
At powerup and reset, the Options Register is initialized to 0. The
ESC(LP)-100 will thus powerup in the x1 mode with software control of
the clock rate multiplier enabled as long as the X2, X4, and X8 jumpers are
not installed.
Software can control high baud rates through a combination of
changing the clock rate multiplier and the UART baud rate divisor. For
example, a baud rate of 230.4 kbps could be achieved by setting the clock
rate multiplier to X2 mode and setting a software application for 115.2
kbps.
ESC(LP)-100 User's Manual 7