Z85233 Datasheet, PDF (63/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

SCC™/ESCC™ User’s Manual
SCC/ESCC Ancillary Support Circuitry
3.2 BAUD RATE GENERATOR (Continued)
The time-constant can be changed at any time, but the
new value does not take effect until the next load of the
counter (i.e., after zero count is reached).
No attempt is made to synchronize the loading of a new
time-constant with the clock used to drive the generator.
When the time-constant is to be changed, the generator
should be stopped first by writing WR14 D0=0. After loading
the new time constant, the BRG can be started again. This
ensures the loading of a correct time constant, but loading
does not take place until zero count or a reset occurs.
If neither the transmit clock nor the receive clock are pro-
grammed to come from the /TRxC pin, the output of the
baud rate generator may be made available for external
use on the /TRxC pin.
Note: This feature is very useful for diagnostic purposes.
By programming the output of the baud rate generator as
output on the /TRxC pin, the BRG is source and time test-
ed, and the programmed time constant verified.
The clock source for the baud rate generator is selected by
bit D1 of WR14. When this bit is set to 0, the BRG uses the
signal on the /RTxC pin as its clock, independent of wheth-
er the /RTxC pin is a simple input or part of the crystal os-
cillator circuit. When this bit is set to 1, the BRG is clocked
by the PCLK. To avoid metastable problems in the
counter, this bit should be changed only while the baud
rate generator is disabled, since arbitrarily narrow pulses
can be generated at the output of the multiplexer when it
changes status.
The BRG is enabled while bit D0 of WR14 is set to 1. It is
disabled while WR14 D0=0 and after a hardware reset (but
not a software reset). To prevent metastable problems
when the baud rate generator is first enabled, the enable
bit is synchronized to the baud rate generator clock. This
introduces an additional delay when the baud rate
generator is first enabled (Figure 3-2). The baud rate
generator is disabled immediately when bit D0 of WR14 is
set to 0, because the delay is only necessary on start-up.
The baud rate generator is enabled and disabled on the fly,
but this delay on start-up must be taken into consideration.
Figure 3-2. Baud Rate Generator Start Up
3-2
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