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, X4, and X8 jumpers are all removed. 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
At powerup and reset, the Options Register is initialized to 0. The DSC-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 (or by applying the X2 jumper) and
setting a software application for 115.2 kbps.
Quatech DSC-100 User's Manual
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