Quatech DSCLP-100 - 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 11.
RR1 and RR0 can be set by writing to the Options Register if the X2 and X4
jumpers (J4-J5) are all removed. This card has the 16550 UART. If one of these
jumpers is applied, the RR1 and RR0 bits are forced to the appropriate value.
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 11 --- Rate Register bit definition
*x8 mode is only accessable via sotware control
At powerup and reset, the Options Register is initialized to 0. The
DSCLP-100 will thus powerup in the x1 mode with software control of the clock
rate multiplier enabled as long as the X2 or X4 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 (or by applying the X2 jumper) and setting a software application for 115.2
kbps.
13 DSCLP/SSCLP-100 User's Manual