Xycom XVME-400 Manual page 35

XVME-400/40l/490/491 Manual
October, 1989
3.4.2 Baud Rate Generator
The SCC contains a programmable baud rate generator whose output can be used as
internal timing sources. The baud rate generator's clock input may be programmed to
connect to either the RXC/RT signal or PCLK (WR14). A 16-bit time constant can be
programmed into WR13 (most significant byte) and WR12 (least significant byte) to select
the baud rate generator's output frequency. The following equations show the baud rate
generator's output frequency as a function of the time constant, and vice versa:
If baud rate generator input is RXC or RT (WR14:Dl=O):
Time Constant = (Frequency of RXC/(2 * CM * Baud Rate)) -2
Baud Rate = Frequency of RXC/(2 * CM * (Time Constant + 2))
If baud rate generator input is PCLK (WR14:Dl=l):
Time Constant
Baud Rate = 1.8432 MHz/(CM * (Time Constant + 2))
In the above equations, CM is the clock multiplier used by the transmitter and
(i.e., CM=1 6 for X 16 clock multiplier).
The output of the baud rate generator can be used as the transmitter clock source and/or
the receiver clock source, and it may also be sent to the TXC/TT output.
combination of these three may be used.
3.4.2.1
Programming the Baud Rate Generator
The following steps should be followed in the specified order to program the baud rate
generator:
Disable the baud rate generator (WR14:D0-0).
1)
Load WR12 and WR13 with the desired time constant.
2)
Select baud rate generator clock source:
3)
WR14:Dl = 0 for RXC/RT
WR14:Dl = 1 for PCLK
Enable the baud rate generator, WR14:D0=l.
4)
3.4.2.2
Time Constant Examples
The following tables show the time constants required for popular baud rates when PLCK
is used as the baud rate generator input. Two tables are shown, one for synchronous (1X
clock) and one for asynchronous (16X clock). These particular constants are shown for
illustration only. Any time constant may be used.
= (1.8432 MHz/(CM * Baud Rate)) -2
(PCLK = 3.6864 MHz)
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receiver
Any