Quatech RS-422 user manual Clock Rate Multiplier, RR1 RR0

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 or four 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 and X4 jumpers (J4-J5) 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

Figure 12 --- Rate Register bit definition

At powerup and reset, the Options Register is initialized to 0. The DSCLP-200/300 will thus powerup in the x1 mode with software control of the clock rate multiplier enabled as long as the X2 and 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

230.4kbps 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.

The X4 clock multiplier settings require the presence of 16750 UARTs. Operation at X4 cannot

be guaranteed with 16550 UARTs.

DSCLP-200/300 User's Manual 13