4.5.2 Clock Rate Multiplier
A standard 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 12. 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.
RR1 | RR0 | Clock Rate | UART Clock | Maximum Data | |
Multiplier | Frequency | Rate | |||
|
| ||||
0 | 0 | X1 | 1.8432 MHz | 115.2 kbaud | |
(default) | |||||
|
|
|
| ||
0 | 1 | X2 | 3.6864 MHz | 230.4 kbaud | |
1 | 0 | X4 | 7.3728 MHz | 460.8 kbaud | |
1 | 1 | X8 | 14.7456 MHz | 921.6 kbaud |
Figure 12 --- Rate Register bit definition
At powerup and reset, the Options Register is initialized to 0. The DSC-200/300 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 | 17 |
